Research ArticleControlled Release Kinetics in Hydroxy Double SaltsEffect of Host Anion Structure
Stephen Majoni12 and Jeanne M Hossenlopp1
1 Department of Chemistry Marquette University PO Box 1881 Milwaukee WI 53201-1881 USA2Applied Chemistry Department National University of Science and Technology PO Box AC 939 AscotBulawayo Zimbabwe
Correspondence should be addressed to Stephen Majoni majoni stephenyahoocom
Received 10 September 2013 Revised 15 December 2013 Accepted 19 December 2013 Published 16 January 2014
Academic Editor Jan Skov Pedersen
Copyright copy 2014 S Majoni and J M Hossenlopp This is an open access article distributed under the Creative CommonsAttribution License which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited
Nanodimensional layered metal hydroxides such as layered double hydroxides (LDHs) and hydroxy double salts (HDSs) canundergo anion exchange reactions releasing intercalated anions Because of this these metal hydroxides have found applicationsin controlled release delivery of bioactive species such as drugs and pesticides In this work isomers of hydroxycinnamate wereused as model compounds to systematically explore the effects of anion structure on the rate and extent of anion release in HDSsFollowing intercalation and subsequent release of the isomers it has been demonstrated that the nature and position of substituentgroups on intercalated anions have profound effects on the rate and extent of release The extent of release was correlated with themagnitude of dipole moments while the rate of reaction showed strong dependence on the extent of hydrogen bonding within thelayers The orthoisomer showed a more sustained and complete release as compared to the other isomers
1 Introduction
Nanodimensional layered double hydroxides (LDHs) andhydroxy double salts (HDSs) have been shown to undergoion exchange reactionswith a variety of inorganic and organicions [1ndash3]This ion exchange capability coupledwith the abil-ity to vary the intralayer metal ions has enabled fine-tuningof these materials for different applications which range fromcatalysis to isomer separation [3ndash6] LDHs and HDSs have abrucite Mg(OH)
2 type layer structure in which magnesium
ions are surrounded by six hydroxide ions in an approxi-mately octahedral geometry with exchangeable anions occu-pying the interlayer region [7] LDHs have a general formula[M2+1minus119909
M3+119909(OH)2]A119899minus119909119899sdot yH2O and HDSs can be rep-
where M2+ and Me2+ represent different divalent metal ionsIn both LDHs andHDSs119860119899minus is an exchangeable anionwhichbalances the charge and controls the interlayer separation and119889-spacing
Anion exchange ability potential for sustained releaseand biocompatibility of the LDHs and HDSs have made
them useful in the uptake storage and controlled release ofbioactive materials such as drugs and plant growth regulators[8ndash11] The rate of release of stored drugs and pesticidesdepends on the intralayer metal composition of the hostmaterials and the size of the intercalated drugs and pesticides[10 12] In addition to the effect of the layer metal ion com-position and size of intercalated drugs the structure of theintercalated drugs is expected to significantly affect their ratesof release In studies involving intercalation reactions it hasbeen observed that LDHs and HDSs exhibit selectivity whenintercalating geometric isomers [5 13ndash15] This selectivityhas been attributed to differences in the interaction of theisomers with the metal hydroxide layers due to differencesin properties such as dipole moments [13] Differences in thestrength of attraction between the layers and the interlayeranions due to differences in electrostatic interactions havebeen shown to affect release in LDHs [16 17]
While many studies have been carried out on theapplications of LDHs and HDSs in controlled release ofpharmaceuticals [9 10 12] and different drugs have beenshown to have different release rates [10 18] a systematic
Hindawi Publishing CorporationAdvances in Physical ChemistryVolume 2014 Article ID 710487 12 pageshttpdxdoiorg1011552014710487
2 Advances in Physical Chemistry
study on the effect of anion structure on the rate and extentof release is yet to be investigated In this study a modelsystem of hydroxycinnamate isomers will be used as modelcompounds to study the effects of structure on controlledrelease of anions from HDSs It is anticipated that this studywill help identify structural parameters that can be adjustedin order to enable fine-tuning of rates of release by alteringthe structure of drugs and pesticides
(98) (All isomers were predominantly trans) were obtainedfrom Aldrich Chemical Co Sodium chloride (100) andzinc oxide (100) were obtained from J T Baker Sodiumhydroxide (pellets 98) was obtained fromEMDChemicals
22 Synthesis of Precursor Zinc Copper Acetate Nanohy-brids Precursor inorganic-organic nanohybrid zinc copperhydroxy acetate (ZC-Ac) was prepared according to theliterature methods [19] 041 g of ZnO in 5mL of deion-ized (DI) water was added slowly to a solution of 100 gCu(CH
3COO)
2sdotH2O in 5mL of DI water over a period
of about 10 minutes The resultant mixture was stirredfrequently at room temperature for 24 hours The light blueinorganic-organic nanohybrid was filtered washed severaltimes with DI water and dried at room temperature
23 Intercalation of Isomers of Hydroxycinnamate (n-HCn)into ZC-Ac The nanohybrids containing hydroxycinnamateisomers were prepared from ZC-Ac by anion exchangethey are referred to herein as ZC-o-HCn ZC-m-HCnand ZC-p-HCn for compounds containing the orthoisomermetaisomer and paraisomer respectively The conditionsfor exchange were optimized for each isomer to ensurecomplete exchange production of single product phaseand minimization of product degradation ZC-o-HCn wasprepared by mixing 200 g of ZC-Ac with 1000 cm3 of a 01Mo-hydroxycinnamate (o-HCn) solution at room temperaturefor 24 hours with frequent stirring and the exchange reactionwas repeated two more times using the same conditionsZC-m-HCn was prepared by reacting ZC-Ac with 05M m-hydroxycinnamate (m-HCn) solution at 40∘C for 24 hourswith frequent stirring the exchange was carried out threetimes under the same condition each time 10M p-HCn wasused in the preparation of ZC-p-HCnwith the exchange reac-tion being done once at 40∘C and the higher concentrationwas necessary to obtain a single p-HCn phase
24 Characterization Fourier transform infrared (FTIR)spectra of the nanohybrids were obtained on a Perkin ElmerSpectrum 100 FT-IR spectrometer operated at a 2 cmminus1resolution in the 4000ndash650 cmminus1 spectral rangeThe obtainedspectra were an average of 16 scans The FTIR spectra were
recorded using a single reflection ATR accessory with a ZnSeprism (PIKE MIRacle from PIKE technology) Elementalanalysis was carried out by Huffman labs Colorado usingatomic emission spectroscopy interfaced with inductivelycoupled plasma (AES-ICP) for metal determination afterqualitative digestion of the nanohybrids The elements C andH were determined by quantitatively digesting the samplethrough oxidative combustion nitrogen was analysed usingthe Kjeldahl method Position and full width at half maxima(FWHM) for peaks were determined by fitting with Gaussianpeak-shape functions
Powder X-ray diffraction (PXRD) measurements wererecorded on a Rigaku Miniflex II diffractometer using CuK120572 (120582 = 154 A) radiation source at 30 kV and 15mA Thediffractometer was calibrated using silicon reference material(RSRP-43275G manufactured by Rigaku Corporation) Thepowder samples were pressed into the trough of glass sampleholders The patterns were recorded in the 2120579 range of 20∘ndash450∘ data acquisition was performed using a step size of00167∘ per second
25 Calculations of Chain Length and Dipole Moments Thechain lengths and dipole moments of carboxylic anions werecalculated utilizing the Gaussian 98 program [20] and werecarried out at the DFT (B3LYP) level of theory with 6ndash311++G(dp) basis set The chain lengths were calculated asthe interatomic distance between the center of the carboxyloxygen and the furthest hydrogen atom
26 Controlled Release Experiments A batch process wasused to study the controlled release of isomers of hydrox-ycinnamate via anion exchange with chloride ions being usedas the exchange anion The temperature dependence wasinvestigated in the temperature range from 30∘C to 60∘CThe exchange reactions were performed in a shaking waterbath with a temperature stability of plusmn02∘C and shaking speedof 300 strokes per minute Several reaction mixtures wereprepared by mixing 015 g of the nanohybrids with 15mLof a 10M sodium chloride solution and were mechanicallyshaken in the water bath for a specified time period Thereaction was quenched by filtration followed by washing theresidue several times with DI water Each experiment wasrepeated at least two times The solid samples recovered atvarious contact times were analyzed by PXRD analysis
The concentrations of released anions (119899-HCn) at pre-set times were monitored by UV-Vis spectroscopy on aPerkin Elmer Lambda 35 UV-Vis spectrophotometer atthe characteristic absorbance maximum for each isomerThe selected wavelengths were 270 nm for o-HCn 272 nmfor m-HCn and 286 nm for p-HCn Calibration curvesfor calculating hydroxycinnamate concentration in aqueoussolutions were constructed in the presence of sodium chlo-ride concentrations identical to those used in the exchangereactions Complete release of the hydroxycinnamate iso-mers from the nanohybrids was achieved by suspendingthe nanohybrids in 10M sodium carbonate solution Theconcentration of anions released at any given time 119905(119862
119905) and
the total amount intercalated (119862infin) were used to calculate
Advances in Physical Chemistry 3
Table 1 Summary of elemental analysis
Nanohybrid Elemental analysis () experimental (calculated)Zn Cu H C
Figure 1 Structure of the anions used in this study (a) 119900-hydroxycinnamate (119900-HCn) (b) 119898-hydroxycinnamate (119898-HCn) and (c) 119901-hydroxycinnamate (119901-HCn)
the extent of reaction (120572) at any given time using (1)119862infin
was obtained from complete exchange using carbonateanions and from formulae obtained from elemental analysis(Table S1 see Supplementary Material available online athttpdxdoiorg1011552014710487) Equation (1) was alsoused to obtain extent of ion exchange (extent of reaction atequilibrium) with 119862
119905in (1) now being 119862
119905infin(amount released
at equilibrium)Consider
120572 =119862119905
119862infin
(1)
3 Results and Discussion
31 Preparation of Hydroxycinnamate Nanohybrids Hydrox-ycinnamate exists in 3 geometric isomers as shown inFigure 1 Nanohybrids containing isomers of hydroxycinna-mate in the interlayer space were prepared from ZC-Ac byanion exchangeThe uptake of these anions and the completereplacement of acetate anions in the interlayer space wasconfirmed by elemental analysis PXRD and ATR-FTIRThe total amount of anions intercalated in the nanohybridswere obtained from both elemental analysis and completeexchange with carbonate anions The values obtained fromboth methods are within 10 as shown in Table S1 insupporting information
A summary of elemental analysis for the nanohybridsis shown in Table 1 From the formulae obtained from
elemental analyses the ratio of copper to zinc in the metalhydroxide layers of ZC-m-HCn (28) and ZC-p-HCn (25) iscomparable to that of the precursor ZC-Ac (28) nanohybrid[21] This may indicate that the exchange reaction in theformation of these compounds was topotactic The ratioin ZC-o-HCn nanohybrid (35) is much higher than inthe precursor material which may be an indication thatZC-o-HCn was formed via the dissolution-recrystallizationmechanism In this case the nature of the cations andanions determines the ratio of the metal ions in the layerswhich may have resulted in the higher copper content[7 22] XRD profiles indicate that no crystalline copperhydroxide was detected in the material The low amountof water in the gallery (from Table 1) is consistent withTGA results shown in Figure S1 in supporting informationas an example TGA results for the degradation of ZC-o-HCn indicate that there is 16 weight loss attributed towater loss and the formula in Table 1 indicates 17 watercontent
ZC-Ac used here exhibits a spacing between the planes(119889-spacing) of 943 plusmn 003 A [21] which is comparable to theliterature value of 946 A [19] The PXRD patterns for ZC-Ac and exchange products are shown in Figure 2(a) All thematerials show at least three equally spaced Bragg reflectionsat low 2 120579 values indicating that the materials are layered andpossess high range orderingThese basal reflectionswere usedto calculate the 119889-spacing of the materials using the Braggequation 119899120582 = 2119889 sin 120579 (where 119899 is the reflection order 120582 isthe wavelength of incident X-ray 119889 is the spacing between the
4 Advances in Physical Chemistry
10 20 30 400
7500
15000
22500
0
5000
10000
15000
0
1500
3000
4500
0
3000
6000
9000In
tens
ity (c
ps)
Inte
nsity
(cps
)In
tens
ity (c
ps)
Inte
nsity
(cps
)
2120579 (deg)
10 20 30 402120579 (deg)
10 20 30 402120579 (deg)
10 20 30 402120579 (deg)
ZC-Ac
ZC-o-HCn
ZC-m-HCn
ZC-p-HCnd = 2103 A
d = 2072 A
d = 1695 A
d = 943 A
(a)
4000 3500 3000 2500 2000 1500 1000
4000 3500 3000 2500 2000 1500 1000
4000 3500 3000 2500 2000 1500 1000
4000 3500 3000 2500 2000 1500 100080
90
100
80
90
100
70
80
90
100
70
80
90
100
T (
) T
()
T (
) T
()
ZC-Ac
ZC-o-HCn
ZC-m-HCn
ZC-p-HCn
Wavenumber (cmminus1)
Wavenumber (cmminus1)
Wavenumber (cmminus1)
Wavenumber (cmminus1)
(b)
Figure 2 (a) PXRD profiles for ZC-Ac and exchange products and (b) FTIR spectra for ZC-Ac and exchange products
planes in the lattice and 120579 is the angle between the incidentray and the scattering planes) The Bragg reflections fromthe precursor material are no longer present in the PXRDtraces of the exchange products indicating that the exchangewas complete As revealed in Figure 2(a) there is an increasein the basal spacing as the acetate anion is replaced by n-HCn ions in the interlayer space The increase in the basalspace is consistent with a smaller anion (acetate ion chainlength = 165 A) being replaced by largern-HCnanions (chainlengths = 863 A for o-HCn 864 A for m-HCn and 911 Afor p-HCn) Considering the chain lengths of the meta-and paraisomers and the size of the gallery height (about16 A if the layer thickness is approximated to be 5 A as incopper hydroxide) [23] the anions are likely to be arrangedin a slightly tilted bilayer orientation The interlayer spaceobserved in ZC-o-HCn is significantly smaller comparedwith the other nanohybrids It was expected that the 119889-spacings would be comparable considering the similarities inthe sizes of the isomers The smaller 119889-spacing in ZC-o-HCn
might be a result of the orthoisomer being in a more tiltedorientation as compared to the other isomers
IR spectra presented in Figure 2(b) show that whenacetate ions were replaced by n-HCn ions the peaks due toC=O vibrations of acetate ions in ZC-Ac (120592asym = 1563 cmminus1
and 120592sym = 1410 cmminus1) [21] were replaced by a series ofpeaks (1637ndash1642 cmminus1 C=C 1540ndash1551 cmminus1 120592asym C=Oand 1399ndash1428 cmminus1 120592sym C=O)The disappearance of vibra-tion peaks from acetate ions is consistent with results inferredfrom PXRD analysis The absence of the C=O stretchingvibration of protonated carboxylic groups which is around1700 cmminus1 confirms that the isomers are present in the ionizedformThe full assignment of the IR peaks is found in Table S2in supplementary information [24]
The hydroxyl stretching vibrational modes (in the region3000 cmminus1ndash3600 cmminus1) provide information about the inter-actions in the interlayer space [25] The structural proper-ties of hydroxyl groups within the interlayer space can be
Advances in Physical Chemistry 5
96
97
98
99
100
3450
T (
)
ZC-Ac
4000 3800 3600 3400 3200 3000 2800 2600
Wavenumber (cmminus1)
(a)
96
97
98
99
100
3131
3489
ZC-o-HCn
4000 3800 3600 3400 3200 3000 2800 2600
Wavenumber (cmminus1)
T (
)
(b)
4000 3800 3600 3400 3200 3000 2800 260094
96
98
100
3578
3513
ZC-m-HCn
Wavenumber (cmminus1)
T (
)
(c)
970
975
980
985
990
995
3497
3574
ZC-p-HCn
4000 3800 3600 3400 3200 3000 2800 2600
Wavenumber (cmminus1)
T (
)
(d)
Figure 3 FTIR spectra for ZC-Ac and exchange products showing the hydroxyl stretching region
correlated to the OH vibrational modes Broad absorptionbands are associated with stretching modes of hydrogenbonded hydroxyl groups and sharp bands associated withfree hydroxyl groups [26] The FTIR profiles of the hydroxylstretching region of ZC-Ac and the exchange products areshown in Figure 3 ZC-Ac has extensive interlayer hydrogenbonding due to intercalated water molecules as reportedin our earlier publication [21] Figure 3 shows that whenacetate ions were replaced by m-HCn ions in ZC-m-HCnthe broad peak (FWHM = 359 cmminus1) centered at 3450 cmminus1in ZC-Ac was replaced by 2 sharp peaks a strong peakat 3513 cmminus1 (FWHM = 148 cmminus1) and a weak peak at3574 cmminus1 (FWHM = 163 cmminus1) The two sharp peaks aredue to layer (3574 cmminus1) and m-HCn (3513 cmminus1) hydroxylgroups which are not involved in substantial hydrogenbonding [25] This assignment is based on our previousresults which showed that the peak at 3513 cmminus1 was absentin HDSs containing cinnamate anions (no OH substitutionon the benzene ring) [21] The shift of the peaks to higherwavenumbers is also indicative of hydroxyl groups which arenot substantially involved in hydrogen bonding ZC-o-HCnhas a very weak peak around 3600 cmminus1 and 2 broad peakscentered at 3489 cmminus1 (FWHM = 880 cmminus1) and 3131 cmminus1(FWHM = 376 cmminus1) The broadening of peaks and the shiftto lower wavenumbers as compared to vibrations in ZC-m-HCn and free OH stretching vibrations is consistent with
hydrogen bonded hydroxyl groups [27] The low intensity ofthe sharp band at 3600 cmminus1 is indicative of extensive hydro-gen bonding between layer OH groups and o-HCn hydroxylgroups resulting in low density of free layer OH groupsIn ZC-p-HCn there is a strong sharp peak at 3574 cmminus1(FWHM = 72 cmminus1) which is due to layer OH groups whichare not involved in significant hydrogen bonding and abroad peak at 3497 cmminus1 (FWHM = 119 cmminus1) due to p-HCnhydroxyl groups involved in hydrogen bonding
The absence of strong sharp peaks in ZC-o-HCn com-pared with those observed for ZC-m-HCn and ZC-p-HCnindicates that there is high level of hydrogen bonding in ZC-o-HCn relative to the other nanohybrids Since hydrogenbonding is directional the m-HCn anions are not orientedin a way that the OH groups are able to interact with eitherthe layer OH groups or groups from other anions Theamount of water in the nanohybrids is low as indicated fromthermogravimetry and elemental analyses Therefore havingeliminated interlayer water as a likely hydrogen bonding par-ticipant the significant hydrogen bonding observed in ZC-o-HCn andZC-p-HCn ismost likely intermolecular or betweenanion and layer hydroxyl groups In ZC-o-HCn the o-HCnhydroxyl group may be in an orientation such that they areclose to the layers enabling formation of hydrogen bondswiththe layer OH groups [28] The interaction between o-HCnand layer hydroxyl groups may have resulted in the o-HCn
6 Advances in Physical Chemistry
Table 2 Summary of dipole moments and release data
Anion Equilibrium isomer released at 40∘C () Dipole moment Calculated chargesCarbon Oxygen 1 Oxygen 2
Figure 4 PXRD profile of a representative ZC-Cl (kapellasite)obtained from exchange reactions (ZC-o-HCnClminus reaction) Millerhkl indices for selected Clminus reflections are given PDF no 56ndash71
anion being tilted closer to the layers which may explain thesmaller gallery height (as compared to that inZC-m-HCn andZC-p-HCn) observed from PXRD analysis In ZC-p-HCnOH groups may be oriented in a way so as to participate inintermolecular H-bonding between anions
32 Kinetic Analysis The release of n-HCn from nanohy-brids was achieved by ion exchange using Clminus as exchangeanions After specified times supernatants were collected andanalyzed for released anions and the residues were analyzedfor solid state transformations using PXRD Analysis of thesolid samples recovered from the reaction mixtures aftereach contact time indicated that the same chloride phasewas obtained for all the nanohybrids A representative PXRDprofile for the zinc copper chloride (ZC-Cl) obtained fromexchange reactions is shown in Figure 4 The chloride phasehas a 119889-spacing of 574 plusmn 001 A which is in close agreementwith the literature value of 573 A [29] and has been indexedas kapellasite Cu
3Zn(OH)
6Cl2(PDF no 56ndash71) [28 30] The
decrease of the interlayer space as the isomers are released isconsistent with large anions being replaced by smaller ions
The concentrations of released anions were evaluatedusing UV-Vis spectroscopy The fractions of released isomersas a function of time (release profiles) at 40∘C are shown inFigures 5(a)ndash5(c) for m-HCn o-HCn and p-HCn respec-tively The release profiles at other temperatures show thesame general trend and are presented in Figure S2 in thesupporting information Although the chemical structures of
the isomers are similar and the release medium is thesame the release profiles and hence release properties aresignificantly different (Figure 5) The equilibrium amountsreleased at 40∘C together with calculated dipole momentsand selected atomic charges are shown in Table 2 As indi-cated in Table 2 the equilibrium amounts released from thenanohybrids were significantly different ZC-o-HCn released100 of the intercalated ions while ZC-m-HCn releasedabout 40 and ZC-p-HCn released about 22 While ZC-o-HCn showed complete release at all temperatures the extentof reaction for the release ofm-HCn and p-HCn showed tem-perature dependence (increasing with temperature as shownin Figure S2) The extents of reactions at 40∘C are confirmedby XRD data in Figure S3 in supporting information
The differences in the extent of reaction may indicatedifferences in the affinity of the HDS for the isomers anddifferences in the thermodynamic equilibrium constants ofthe systemsThe extent of anion exchange depends on severalfactors which include the solvation enthalpy of the isomers(in the bulk liquid and in the gallery) the binding enthalpy(between themetal hydroxide layers and the isomers) and theinter-intramolecular interactions of the anions within thelayers [4 28 31]Themajor contributing factor is expected tobe the electrostatic interaction between the positively chargedmetal hydroxide layers and the negatively charged anionswith the carboxylate group of the isomers being in closecontact with the layers Since the isomers have the samecharge and comparable size differences in dipole moments(an indicator of the charge distribution) could have moreinfluence on the thermodynamics of the release processThe magnitude of dipole moments has been shown to becorrelatedwith selectivity in intercalation reactions of layereddouble hydroxides [4 28 31] The affinity of the metalhydroxide layer is higher for the isomer with the highestdipole moment [13] therefore this isomer is expected to beretained within the layers more than the other isomers
The equilibrium amounts of anion released from thenanohybrids follow the following order o-HCn gt m-HCngt p-HCn which is opposite to that of the magnitude ofdipole moments as shown in Table 2 The order of isomerselectivity in intercalation reactions is as expected thereverse of the order of equilibrium amount release observedhere Since p-HCn has the highest dipole moments the metalhydroxide layers are expected to have the greatest affinityfor p-HCn as compared to the other isomers [13] this couldexplain the order observed here Although factors whichinfluence the extent of release are expected to be complexdue to the processes involved in anion exchange reactions[32] the results obtained here infer that the magnitudeof dipole moments plays a significant role in determining
Advances in Physical Chemistry 7
0 4 8 12 16
0
10
20
30
40
Frac
tion
rele
ased
()
Time (hours)
(a)
0 5 10 150
20
40
60
80
Frac
tion
rele
ased
()
Time (hours)
(b)
0 1 2 3 4 5
0
4
8
12
16
20
Frac
tion
rele
ased
()
Time (hours)
(c)
Figure 5 Release profiles for (a) ZC-m-HCn (b) ZC-o-HCn and (c) ZC-p-HCn at 40∘C
the equilibrium constant and hence the amount released atequilibrium The complex nature of the exchange reactionsis highlighted when our previous results for the release ofcinnamate anion are considered [21] Cinnamate anion hasa dipole moment of 146 D which is lower than that of m-HCn but there was only 31 Cn released at equilibriumThis indicates a complex interplay of the involved factors asit has been shown that highly hydrophobic anions are easyto intercalate into the layers Due to the absence of hydroxylgroup on the benzene ring of Cn anion this isomer is morehydrophobic than the others and is therefore not readilydeintercalated into the polar aqueous solvent environment
From Figure 5 it can qualitatively be observed that therelease rates and the release profiles of the isomers aresignificantly different While the profiles for o-HCn releaseare sigmoidal those for the meta- and paraisomers aredeceleratory Reaction rates can be quantified by fittingexperimental data to reaction models which best describe(reproduce) the experimental data in the so-called model
fitting procedureThis procedure is applicable to singlemech-anism reactions in which the mechanism does not change asthe reaction progresses [33] We have shown previously thatisoconversional methods can be extended to anion exchangereactions in layered metal hydroxide as a strategy to identifywhen using model based approaches are appropriate [21] Aconstant 119864
119886as a function of extent of reaction indicates that
the reaction may effectively be characterized with a singlemechanism and in these cases model based procedures areapplicable [33]The integral isoconversional method (2) wasapplied to the kinetic data obtained here
ln 119905 = ln119892 (120572)
119860+119864119886
119877119879 (2)
In (2) 119864119886is the activation energy 120572 is the extent of
reaction 119860 is the preexponential factor 119905 is the time 119879 thetemperature 119877 is the molar gas constant and 119892(120572) is theintegral reactionmodelThe isoconversional analysis data forall the nanohybrids is shown in Figure 6 in which the 119864
119886
8 Advances in Physical Chemistry
02 04 06 08 1065
70
75
80
85ZC-o-HCn
Extent of reaction (120572)
Ea
(kJm
ol)
(a)
03 04 05 06 07 08 09
30
40
50
60ZC-p-HCn
Extent of reaction (120572)
Ea
(kJm
ol)
(b)
03 04 05 06 07 08 09 10
50
60
70
80ZC-m-HCn
Extent of reaction (120572)
Ea
(kJ m
olminus1)
(c)
Figure 6 Variation of effective 119864119886with 120572 for ZC-m-HCn ZC-o-HCn and ZC-p-HCn
values remains constant within experimental error over theentire conversion range for ZC-m-HCn and ZC-p-HCn
The 119864119886values for ZC-o-HCn are within 10 although
the changes at higher conversion appear significant comparedwith experimental uncertainty The linear decrease of the 119864
119886
at higher conversion above 120572 = 05 has been attributed toweakening of the electrostatic attraction between the anionsand the layers as the anions are progressively exchanged fromlayer to layer [34] The constant 119864
119886at low conversion may
be due to some solid state transformations occurring whichmight be responsible for the induction period observedin the PXRD analysis (Figure S3) Although the inductionperiod is short the process might still be governing theenergetics of the reaction even during the exchange periodresulting in the constant energy being observed up to extentof reaction of 05 Since 119864
119886values for exchange reactions at
the anion binding sites have been shown to range from 30to 70 kJmolminus1 depending on the nature and size of the guestanions [31 32 35] the reactionmay be controlled by chemical
reaction at the exchange site It is also important to note thatdiffusion of anions in a dense organicmatrix can have high119864
119886
values sometimes being more than 100 kJmolminus1 [36] as suchdiffusion limited process cannot be ruled out if the anions areclosely packed in the gallery
The 119864119886values for the release of m-HCn and p-HCn
are in the range of both reaction controlled process anddiffusion controlled processes [21] The fact that the 119864
119886does
not significantly change over the entire reaction may indicatethat the reaction is diffusion controlled The differences inthe mechanism between o-HCn release and the other twosystems may be due to different strength of interactions withthe layersThe presence of H-bonding between the layers andthe o-HCn anions may bring the anions closer to the layersresulting in an increased electrostatic attraction resulting inthe process being controlled by reaction at the exchangesite Since there was no significant variation of the 119864
119886with
conversion for m-HCn and p-HCn release and the energydifference for o-HCn release is too low to reflect a change in
Advances in Physical Chemistry 9
mechanism model based approach was used to obtain rateconstants for the exchange reactions The Avrami-Erofersquoevnucleation-growth model [37 38] was used here since ithas the advantage that it can be used to describe reactionsoccurring via different mechanisms making comparison ofthe processes possible since the order of reactions will be thesame In the model the extent of reaction (120572) depends on therate constant 119896 and a coefficient 119899 as shown in the followingequation
119886 (119905) = 1 minus 119890[minus(119896119905)
119899
] (3)
The coefficient 119899 (Avrami exponent) potentially containsinformation about the mechanism of the reaction [39] TheAvrami-Erofersquoev model has been applied successfully to solu-tion phase anion exchange reactions in layeredmaterials andother fluid phase reactions [2 10 40 41] we have also shownthat values of parameters obtained from analysis of bothsolid state transformations and solution phase processesemploying the Avrami model are comparable indicating thatthere is correlation between solid state transformation andfluid phase processes [21] The Avrami exponent is usuallyobtained by taking double logarithm of the Avrami-Erofersquoevequation obtaining (4) which was popularized by Sharp andHancock in 1972 [42] Consider
A plot of ln[minus ln(1 minus 120572)] as a function of ln 119905 (doublelogarithmic plot) gives a linear plot in which the value of 119899 isobtained from the slope and the value of 119896 is evaluated fromthe intercept
The extent of reaction versus time plots obtained for theZC-o-HCnClminus exchange reactions at different temperaturesis shown in Figure 7(a) The Avrami-Erofersquoev model wasapplied and correctly described the obtained experimentaldatawithin the120572 range of 015ndash10The validity of theAvrami-Erofersquoevmodel can be confirmedby the corresponding doublelogarithmic plots in Figure 7(b) where straight lines wereobtained with 1198772 values ranging from 0996 to 0999 Thedouble logarithmic plots are almost parallel indicating thatthe reaction proceeds with the same mechanism over thetemperature range used here The obtained 119899 value was closeto 10 consistent with a nucleation controlled mechanismwhich agrees well with the activation energies obtained fromisoconversional methods and discussed before
The release profiles together with the double logarithmicplots for the release of m-HCn and p-HCn are shown in thesupporting information Figures S4 and S5 respectively Thekinetics of m-HCn release also follows the Avrami-Erofersquoevmodel the data provided a good fit within the range 015 lt120572 lt 085 the fit to limited range of 120572 has been observedin other intercalation reactions Anomalously low values ofthe Avrami exponent observed here (shown in Table 3) havebeen reported in polymer and alloy crystallization and donot have physical mechanistic meaning [43 44] The Avramiexponent values obtained for the release of p-HCn are closeto 05 which is consistent with diffusion controlled reactions[10] Diffusion control of such a high 119864
119886highlights two
0 2 4 6 8 10 12 1400
02
04
06
08
10
Time (hours)
Exte
nt o
f rea
ctio
n (120572
)
300∘C400∘C500∘C600∘C
(a)
4 6 8 10
0
1
minus1
minus2
ln (t minus t0)
lnminusln(1
minus120572)
30∘C40∘C50∘C60∘C
(b)
Figure 7 Extent of reaction as a function of time (a) and doublelogarithmic plots (b) for the exchange reaction of Clminus anion andZC-o-HCn at various temperatures 60∘C (◻) 50∘C (Q) 40∘C (998771)and 30∘C (e)
issues (a) the interlayer space is densely packed and thediffusional resistance within the interlayer space approachesthat in polymers in which119864
119886values can be above 100 kJmolminus1
depending on the diffusing molecule [36] and (b) Kineticambiguity is common in model based approaches in thiscase the mechanistic interpretation from the model is not inline with the kinetic parameters obtained
FromTable 3 it can be concluded that the rate of release ofm-HCn from the nanohybrids is much faster compared with
10 Advances in Physical Chemistry
Table 3 A summary of kinetic parameters obtained at different temperatures
Nanohybrid Temperature (∘C) 119899 119896 sminus1 (lowast10minus3)
ZC-o-HCn
600 082 plusmn 002 17 plusmn 03
500 090 plusmn 002 07 plusmn 01
400 088 plusmn 001 029 plusmn 005
300 102 plusmn 001 010 plusmn 001
ZC-m-HCn
600 024 plusmn 001 31 plusmn 9
500 024 plusmn 001 20 plusmn 7
400 021 plusmn 002 7 plusmn 2
300 0225 plusmn 0007 35 plusmn 08
ZC-p-HCn
600 041 plusmn 001 29 plusmn 05
500 045 plusmn 002 16 plusmn 04
400 046 plusmn 001 10 plusmn 02
300 039 plusmn 001 05 plusmn 01
the other isomers As an example the rate constants at 40∘Cwere (7 plusmn 2) times 10minus3 sminus1 for m-HCn (10 plusmn 02) times 10minus3 sminus1 forp-HCn and (029 plusmn 005) times 10minus3 sminus1 for o-HCn The rate ofrelease follows the following order
119898ndashHCn gt 119901-HCn gt 119900-HCn
The release profile form-HCn at 40∘C (Figure 5(a)) showsthat there is a burst and rapid release of the intercalated anionin the first 10 minutes followed by a more sustained releasewith equilibrium being attained after 3 hours This burstrelease is observed for all the temperatures used here (FigureS2a in supporting information) and has been observed beforein anion exchange reactions in LDHs [10] The burst releaseobserved here is due to anion exchange as confirmed byPXRD results in Figure S3 The chloride phase representedby an asterisk in Figure S3 was significant at 30 seconds (theearliest we could sample) of ZC-m-HCn being in contact withthe chloride solution The initial slow and linear release inZC-o-HCn indicated in Figure 5(b) and confirmed by XRDprofiles in Figure S3 may be due to the increased stabilityof the HDS structure due to extensive hydrogen bonding asindicated by FTIR data in Figure 3 and the close arrangementof the anions implied by XRDprofiles in Figure 2(a)This firststage of release has a strong temperature dependence beingmore pronounced at low temperature and almost absent athigher temperatures (Figure S2b in supporting information)
Factors which may influence the rates of release includeinteractions between the metal hydroxide layers and theanions anion-anion interactions within the layers and in theexchange medium and solvent-anion interaction [10] It isexpected that these factors will have a complex effect on thereaction rates since they affect each isomer differently Thecomplex interplay of the abovementioned factors oftenmakeit difficult to come up with a simple relationship betweenthe rate of release and the aforementioned factors [10] It isimportant to note that these factors are expected to dependon the types and positions of chemical groups on the anionThepresence of hydrogen bondingwithin the interlayer spacereduces the Gibbs free energy of formation of the nanohy-brids and stabilizes the system [45]The rate of release of ionsthen depends on the differences in the energy of nanohybrid
and the barrier to reaction As indicated in Figure 3 there issignificant level of hydrogen bonding in ZC-o-HCn whichthen resulted in a close interaction with the layers Thehydrogen bond network in the nanohybrid and the closeinteraction of the anions with the layers stabilized the systemas compared to the other nanohybrids The nanohybrid inwhich these interactions are low is relatively less stable and isexpected to proceed faster to equilibrium this is consistentwith results obtained here The anions in the unstablelessstable nanohybrid prefer bulk solution (in which there isstabilization by solvation with water molecules) to galleryspace which may result in the burst and fast release observedin ZC-m-HCnThe modification of bioactive molecules withgroups capable of hydrogen bonding for example hydroxylgroups especially at positions which allow interactions withthe layers may enable tuning of the release behavior ofthese molecules from layered materials It is important tonote that the modification should be at positions whichdo not affect the bioactivity of the molecules Additionalexperiments including studies on other isomeric anions withgroups capable of forming hydrogen bonds and effect ofmetallayer composition will be helpful in further elucidating howthe position of substituents control the rate as well as theextent of reaction
4 Conclusions
Anion exchange reactions in hydroxy double salts are affectedby the structure of the intercalated anions and there is astrong interplay between thermodynamics and kinetics ofthe release reactions In the present study we have shownthat there is a significant effect of the position of substituentgroups on the rate and extent of release of isomers ofhydroxycinnamate While the release of the metaisomer wasinstantaneous and fast the orthoisomer release exhibiteda slower more sustained and complete release the orderof reaction rates was m-HCn gt p-HCn gt o-HCn withthe o-HCn nanohybrid showing an induction period Theposition of the hydroxyl groups resulted in differences inthe interlayer interactions of the anions with presence ofinterlayer hydrogen bonds affecting the rates of the reaction
Advances in Physical Chemistry 11
The formation of hydrogen bonds between the layers and theintercalated orthoisomer resulted in greater stabilization ofthe nanohybrid and resulted in the observed induction periodand slow release The extent of reaction was correlated withthe magnitude of dipole moments These results highlightthe potential of tuning the release behavior of intercalatedbioactive compounds which bring control into both theamount released per given time and the period for totalrelease
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
This work was supported by the National Science Foundation(CHE-0809751)
References
[1] S Yamanaka T Sako andM Hattori ldquoAnion exchange in basiccopper acetaterdquo Chemistry Letters pp 1869ndash1872 1989
[2] E Kandare and J M Hossenlopp ldquoHydroxy double salt anionexchange kinetics effects of precursor structure and anionsizetrdquo Journal of Physical Chemistry B vol 109 no 17 pp 8469ndash8475 2005
[3] P K Dutta and M Puri ldquoAnion exchange in lithium aluminatehydroxidesrdquo Journal of Physical Chemistry vol 93 no 1 pp376ndash381 1989
[4] B M Choudary V S Jaya B R Reddy M L Kantam M MRao and S S Madhavendra ldquoSynthesis characterization ionexchange and catalytic properties of nanobinary and ternarymetal oxyhydroxidesrdquo Chemistry of Materials vol 17 no 10pp 2740ndash2743 2005
[5] A M Fogg J S Dunn S-G Shyu D R Cary and D OrsquoHareldquoSelective ion-exchange intercalation of isomeric dicarboxylateanions into the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Chemistry of Materials vol 10 no 1 pp 351ndash355 1998[6] T Ikeda H Amoh and T Yasunaga ldquoStereoselective exchange
kinetics of L- and D-histidines for Cl- in the interlayerof a hydrotalcite-like compound by the chemical relaxationmethodrdquo Journal of the American Chemical Society vol 106 no20 pp 5772ndash5775 1984
[7] M Meyn K Beneke and G Lagaly ldquoAnion-exchange reactionsof hydroxy double saltsrdquo Inorganic Chemistry vol 32 no 7 pp1209ndash1215 1993
[8] V Ambrogi G Fardella G Grandolini L Perioli and M CTiralti ldquoIntercalation compounds of hydrotalcite-like anionicclays with anti-inflammatory agents II uptake of diclofenac fora controlled release formulationrdquo AAPS PharmSciTech vol 3no 3 p E26 2002
[9] M Z Bin Hussein Z Zainal A H Yahaya and D WV Foo ldquoControlled release of a plant growth regulator 120572-naphthaleneacetate from the lamella of Zn-Al-layered doublehydroxide nanocompositerdquo Journal of Controlled Release vol82 no 2-3 pp 417ndash427 2002
[10] A I Khan A Ragavan B Fong et al ldquoRecent developments inthe use of layered double hydroxides as host materials for the
storage and triggered release of functional anionsrdquo Industrialand Engineering Chemistry Research vol 48 no 23 pp 10196ndash10205 2009
[11] M Silion D Hritcu I M Jaba et al ldquoIn vitro and invivo behavior of ketoprofen intercalated into layered doublehydroxidesrdquo Journal of Materials Science vol 21 no 11 pp3009ndash3018 2010
[12] J-H Yang Y-S Han M Park T Park S-J Hwang andJ-H Choy ldquoNew inorganic-based drug delivery system ofindole-3-acetic acid-layered metal hydroxide nanohybrids withcontrolled release raterdquo Chemistry of Materials vol 19 no 10pp 2679ndash2685 2007
[13] L Lei R P Vijayan and D OrsquoHare ldquoPreferential anionexchange intercalation of pyridinecarboxylate and toluate iso-mers in the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Journal of Materials Chemistry vol 11 no 12 pp 3276ndash32802001
[14] B Lotsch F Millange R I Walton and D OrsquoHare ldquoSepara-tion of nucleoside monophosphates using preferential anionexchange intercalation in layered double hydroxidesrdquo Solid StateSciences vol 3 no 8 pp 883ndash886 2001
[15] H Tagaya N Sasaki H Morioka and J Kadokawa ldquoPrepara-tion of new inorganicmdashorganic layered compounds hydroxydouble salts and preferential intercalation of organic carboxylicacids into themrdquoMolecular Crystals and Liquid Crystals Scienceand Technology A vol 341 pp 413ndash418 2000
[16] P P Kumar A G Kalinichev and R J Kirkpatrick ldquoHydra-tion swelling interlayer structure and hydrogen bondingin organolayered double hydroxides insights from moleculardynamics simulation of citrate-intercalated hydrotalciterdquo Jour-nal of Physical Chemistry B vol 110 no 9 pp 3841ndash3844 2006
[17] S V Prasanna and P V Kamath ldquoAnion-exchange reactions oflayered double hydroxides interplay between coulombic andh-bonding interactionsrdquo Industrial and Engineering ChemistryResearch vol 48 no 13 pp 6315ndash6320 2009
[18] Z Gu A CThomas Z P Xu J H Campbell and G Q Lu ldquoInvitro sustained release of LMWH from MgAl-layered doublehydroxide nanohybridsrdquo Chemistry of Materials vol 20 no 11pp 3715ndash3722 2008
[19] J T Rajamathi S Britto and M Rajamathi ldquoSynthesis andanion exchange reactions of a layered Copper-Zincrdquo Journal ofChemical Sciences vol 117 no 6 pp 629ndash633 2005
[20] M Frisch G Trucks H Schlegel et al ldquoGaussian 98 RevisionA114rdquo 2002
[21] S Majoni and J M Hossenlopp ldquoAnion exchange kinetics ofnanodimensional layered metal hydroxides use of isoconver-sional analysisrdquo Journal of Physical Chemistry A vol 114 no 49pp 12858ndash12869 2010
[22] C Chouillet J-M Krafft C Louis and H Lauron-PernotldquoCharacterization of zinc hydroxynitrates by diffuse reflectanceinfrared spectroscopymdashstructural modifications during ther-mal treatmentrdquo Spectrochimica Acta A vol 60 no 3 pp 505ndash511 2004
[23] W Fujita and K Awaga ldquoMagnetic properties of Cu2(OH)
3(al-
kanecarboxylate) compounds drastic mod-ification withextension of the Alkyl chainrdquo Inorganic Chemistry vol 35 no7 pp 1915ndash1917 1996
[24] R Swisłocka M Kowczyk-Sadowy M Kalinowska and WLewandowski ldquoSpectroscopic (FT-IR FT-Raman1H and13CNMR) and theoretical studies of p-coumaric acid and alkalimetal p-coumaratesrdquo Spectroscopy vol 27 no 1 pp 35ndash48 2012
12 Advances in Physical Chemistry
[25] T Biswick W Jones A Pacuła E Serwicka and J PodobinskildquoThe role of anhydrous zinc nitrate in the thermal decomposi-tion of the zinc hydroxy nitrates Zn
5(OH)
8(NO3)2sdot2H2O and
ZnOHNO3sdotH2Ordquo Journal of Solid State Chemistry vol 180 no
4 pp 1171ndash1179 2007[26] S-H Park and E L Cheol ldquoSynthesis characterization and
magnetic properties of a novel disulfonate-pillared copperhydroxide Cu
2(OH)
3(DS4)
12 DS4 = 14-butanedisulfonaterdquo
Bulletin of the Korean Chemical Society vol 27 no 10 pp 1587ndash1592 2006
[27] P Rabu M Drillon and C Hornick ldquoStructural and spectro-scopic study of organic-inorganic transitionmetal based layeredmagnetsrdquo Analusis vol 28 no 2 pp 103ndash108 2000
[28] L Lei A Khan and D OrsquoHare ldquoSelective anion-exchangeintercalation of isomeric benzoate anions into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo Journal of Solid State
Chemistry vol 178 no 12 pp 3648ndash3654 2005[29] W Krause H-J Bernhardt R S W Braithwaite U Kolitsch
and R Pritchard ldquoKapellasite Cu3Zn(OH)
6Cl2 a new mineral
from Lavrion Greece and its crystal structurerdquo MineralogicalMagazine vol 70 no 3 pp 329ndash340 2006
[30] L C Andrews L R Bernstein C M Foris et al PowderDiffraction File Inorganic International Centre for DiffractionData Newtown Square Pennsylvania 2009
[31] A Ragavan A I Khan and D OrsquoHare ldquoIsomer selectiveion-exchange intercalation of nitrophenolates into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotxH
2Ordquo Journal of Materials
Chemistry vol 16 no 6 pp 602ndash608 2006[32] A Ragavan A Khan and D OrsquoHare ldquoSelective intercalation
of chlorophenoxyacetates into the layered double hydroxide[LiAl2(OH)
6]ClsdotxH
2Ordquo Journal of Materials Chemistry vol 16
no 42 pp 4155ndash4159 2006[33] S Vyazovkin and C A Wight ldquoModel-free and model-fitting
approaches to kinetic analysis of isothermal and nonisothermaldatardquoThermochimica Acta vol 340-341 pp 53ndash68 1999
[34] R E Johnsen F Krumeich and P Norby ldquoStructural andmicrostructural changes during anion exchange ofCoAl layereddouble hydroxides an in situ X-ray powder diffraction studyrdquoJournal of Applied Crystallography vol 43 no 3 pp 434ndash4472010
[35] G R Williams T G Dunbar A J Beer A M Fogg and DOrsquoHare ldquoIntercalation chemistry of the novel layered doublehydroxides [MAl
4(OH)
12](NO
3)2sdotyH2O (M=Zn Cu Ni and
Co) 1 new organic intercalates and reaction mechanismsrdquoJournal of Materials Chemistry vol 16 no 13 pp 1222ndash12302006
[36] T E M Ten Hulscher and G Cornelissen ldquoEffect of tempera-ture on sorption equilibrium and sorption kinetics of organicmicropollutantsmdasha reviewrdquo Chemosphere vol 32 no 4 pp609ndash626 1996
[37] M Avrami ldquoKinetics of phase change I general theoryrdquo TheJournal of Chemical Physics vol 7 no 12 pp 1103ndash1112 1939
[38] M Avrami ldquoKinetics of phase change II Transformation-timerelations for random distribution of nucleirdquo The Journal ofChemical Physics vol 8 no 2 pp 212ndash224 1940
[39] R J Francis S OrsquoBrien A M Fogg et al ldquoTime-resolved in-situ energy and angular dispersive X-ray diffraction studies ofthe formation of themicroporous gallophosphateULM-5 underhydrothermal conditionsrdquo Journal of the American ChemicalSociety vol 121 no 5 pp 1002ndash1015 1999
[40] Z Miladinovic J Zakrzewska B Kovacevic and G BacicldquoMonitoring of crystallization processes during synthesis of
zeolite A by in situ 27Al NMR spectroscopyrdquo Materials Chem-istry and Physics vol 104 no 2-3 pp 384ndash389 2007
[41] R Serna-Guerrero and A Sayari ldquoModeling adsorption of CO2
on amine-functionalized mesoporous silica 2 kinetics andbreakthrough curvesrdquo Chemical Engineering Journal vol 161no 1-2 pp 182ndash190 2010
[42] J D Hancock and J H Sharp ldquoMethod of comparing solid-state kinetic data and its application to the decomposition ofkaolinite brucite and BaCO
3rdquo Journal of the American Ceramic
Society vol 55 no 2 pp 74ndash77 1972[43] F Auriemma C De Rosa and R Triolo ldquoSlow crystallization
kinetics of poly(vinyl alcohol) in confined environment duringcryotropic gelation of aqueous solutionsrdquo Macromolecules vol39 no 26 pp 9429ndash9434 2006
[44] R V Ramanujan and Y R Zhang ldquoQuantitative transmissionelectronmicroscopy analysis of the nanocrystallization kineticsof softmagnetic alloysrdquoPhysical ReviewB vol 74 no 22 ArticleID 224408 2006
[45] A G Kalinichev P Padma Kumar and R James KirkpatrickldquoMolecular dynamics computer simulations of the effects ofhydrogen bonding on the properties of layered double hydrox-ides intercalated with organic acidsrdquo Philosophical Magazinevol 90 no 17-18 pp 2475ndash2488 2010
study on the effect of anion structure on the rate and extentof release is yet to be investigated In this study a modelsystem of hydroxycinnamate isomers will be used as modelcompounds to study the effects of structure on controlledrelease of anions from HDSs It is anticipated that this studywill help identify structural parameters that can be adjustedin order to enable fine-tuning of rates of release by alteringthe structure of drugs and pesticides
(98) (All isomers were predominantly trans) were obtainedfrom Aldrich Chemical Co Sodium chloride (100) andzinc oxide (100) were obtained from J T Baker Sodiumhydroxide (pellets 98) was obtained fromEMDChemicals
22 Synthesis of Precursor Zinc Copper Acetate Nanohy-brids Precursor inorganic-organic nanohybrid zinc copperhydroxy acetate (ZC-Ac) was prepared according to theliterature methods [19] 041 g of ZnO in 5mL of deion-ized (DI) water was added slowly to a solution of 100 gCu(CH
3COO)
2sdotH2O in 5mL of DI water over a period
of about 10 minutes The resultant mixture was stirredfrequently at room temperature for 24 hours The light blueinorganic-organic nanohybrid was filtered washed severaltimes with DI water and dried at room temperature
23 Intercalation of Isomers of Hydroxycinnamate (n-HCn)into ZC-Ac The nanohybrids containing hydroxycinnamateisomers were prepared from ZC-Ac by anion exchangethey are referred to herein as ZC-o-HCn ZC-m-HCnand ZC-p-HCn for compounds containing the orthoisomermetaisomer and paraisomer respectively The conditionsfor exchange were optimized for each isomer to ensurecomplete exchange production of single product phaseand minimization of product degradation ZC-o-HCn wasprepared by mixing 200 g of ZC-Ac with 1000 cm3 of a 01Mo-hydroxycinnamate (o-HCn) solution at room temperaturefor 24 hours with frequent stirring and the exchange reactionwas repeated two more times using the same conditionsZC-m-HCn was prepared by reacting ZC-Ac with 05M m-hydroxycinnamate (m-HCn) solution at 40∘C for 24 hourswith frequent stirring the exchange was carried out threetimes under the same condition each time 10M p-HCn wasused in the preparation of ZC-p-HCnwith the exchange reac-tion being done once at 40∘C and the higher concentrationwas necessary to obtain a single p-HCn phase
24 Characterization Fourier transform infrared (FTIR)spectra of the nanohybrids were obtained on a Perkin ElmerSpectrum 100 FT-IR spectrometer operated at a 2 cmminus1resolution in the 4000ndash650 cmminus1 spectral rangeThe obtainedspectra were an average of 16 scans The FTIR spectra were
recorded using a single reflection ATR accessory with a ZnSeprism (PIKE MIRacle from PIKE technology) Elementalanalysis was carried out by Huffman labs Colorado usingatomic emission spectroscopy interfaced with inductivelycoupled plasma (AES-ICP) for metal determination afterqualitative digestion of the nanohybrids The elements C andH were determined by quantitatively digesting the samplethrough oxidative combustion nitrogen was analysed usingthe Kjeldahl method Position and full width at half maxima(FWHM) for peaks were determined by fitting with Gaussianpeak-shape functions
Powder X-ray diffraction (PXRD) measurements wererecorded on a Rigaku Miniflex II diffractometer using CuK120572 (120582 = 154 A) radiation source at 30 kV and 15mA Thediffractometer was calibrated using silicon reference material(RSRP-43275G manufactured by Rigaku Corporation) Thepowder samples were pressed into the trough of glass sampleholders The patterns were recorded in the 2120579 range of 20∘ndash450∘ data acquisition was performed using a step size of00167∘ per second
25 Calculations of Chain Length and Dipole Moments Thechain lengths and dipole moments of carboxylic anions werecalculated utilizing the Gaussian 98 program [20] and werecarried out at the DFT (B3LYP) level of theory with 6ndash311++G(dp) basis set The chain lengths were calculated asthe interatomic distance between the center of the carboxyloxygen and the furthest hydrogen atom
26 Controlled Release Experiments A batch process wasused to study the controlled release of isomers of hydrox-ycinnamate via anion exchange with chloride ions being usedas the exchange anion The temperature dependence wasinvestigated in the temperature range from 30∘C to 60∘CThe exchange reactions were performed in a shaking waterbath with a temperature stability of plusmn02∘C and shaking speedof 300 strokes per minute Several reaction mixtures wereprepared by mixing 015 g of the nanohybrids with 15mLof a 10M sodium chloride solution and were mechanicallyshaken in the water bath for a specified time period Thereaction was quenched by filtration followed by washing theresidue several times with DI water Each experiment wasrepeated at least two times The solid samples recovered atvarious contact times were analyzed by PXRD analysis
The concentrations of released anions (119899-HCn) at pre-set times were monitored by UV-Vis spectroscopy on aPerkin Elmer Lambda 35 UV-Vis spectrophotometer atthe characteristic absorbance maximum for each isomerThe selected wavelengths were 270 nm for o-HCn 272 nmfor m-HCn and 286 nm for p-HCn Calibration curvesfor calculating hydroxycinnamate concentration in aqueoussolutions were constructed in the presence of sodium chlo-ride concentrations identical to those used in the exchangereactions Complete release of the hydroxycinnamate iso-mers from the nanohybrids was achieved by suspendingthe nanohybrids in 10M sodium carbonate solution Theconcentration of anions released at any given time 119905(119862
119905) and
the total amount intercalated (119862infin) were used to calculate
Advances in Physical Chemistry 3
Table 1 Summary of elemental analysis
Nanohybrid Elemental analysis () experimental (calculated)Zn Cu H C
Figure 1 Structure of the anions used in this study (a) 119900-hydroxycinnamate (119900-HCn) (b) 119898-hydroxycinnamate (119898-HCn) and (c) 119901-hydroxycinnamate (119901-HCn)
the extent of reaction (120572) at any given time using (1)119862infin
was obtained from complete exchange using carbonateanions and from formulae obtained from elemental analysis(Table S1 see Supplementary Material available online athttpdxdoiorg1011552014710487) Equation (1) was alsoused to obtain extent of ion exchange (extent of reaction atequilibrium) with 119862
119905in (1) now being 119862
119905infin(amount released
at equilibrium)Consider
120572 =119862119905
119862infin
(1)
3 Results and Discussion
31 Preparation of Hydroxycinnamate Nanohybrids Hydrox-ycinnamate exists in 3 geometric isomers as shown inFigure 1 Nanohybrids containing isomers of hydroxycinna-mate in the interlayer space were prepared from ZC-Ac byanion exchangeThe uptake of these anions and the completereplacement of acetate anions in the interlayer space wasconfirmed by elemental analysis PXRD and ATR-FTIRThe total amount of anions intercalated in the nanohybridswere obtained from both elemental analysis and completeexchange with carbonate anions The values obtained fromboth methods are within 10 as shown in Table S1 insupporting information
A summary of elemental analysis for the nanohybridsis shown in Table 1 From the formulae obtained from
elemental analyses the ratio of copper to zinc in the metalhydroxide layers of ZC-m-HCn (28) and ZC-p-HCn (25) iscomparable to that of the precursor ZC-Ac (28) nanohybrid[21] This may indicate that the exchange reaction in theformation of these compounds was topotactic The ratioin ZC-o-HCn nanohybrid (35) is much higher than inthe precursor material which may be an indication thatZC-o-HCn was formed via the dissolution-recrystallizationmechanism In this case the nature of the cations andanions determines the ratio of the metal ions in the layerswhich may have resulted in the higher copper content[7 22] XRD profiles indicate that no crystalline copperhydroxide was detected in the material The low amountof water in the gallery (from Table 1) is consistent withTGA results shown in Figure S1 in supporting informationas an example TGA results for the degradation of ZC-o-HCn indicate that there is 16 weight loss attributed towater loss and the formula in Table 1 indicates 17 watercontent
ZC-Ac used here exhibits a spacing between the planes(119889-spacing) of 943 plusmn 003 A [21] which is comparable to theliterature value of 946 A [19] The PXRD patterns for ZC-Ac and exchange products are shown in Figure 2(a) All thematerials show at least three equally spaced Bragg reflectionsat low 2 120579 values indicating that the materials are layered andpossess high range orderingThese basal reflectionswere usedto calculate the 119889-spacing of the materials using the Braggequation 119899120582 = 2119889 sin 120579 (where 119899 is the reflection order 120582 isthe wavelength of incident X-ray 119889 is the spacing between the
4 Advances in Physical Chemistry
10 20 30 400
7500
15000
22500
0
5000
10000
15000
0
1500
3000
4500
0
3000
6000
9000In
tens
ity (c
ps)
Inte
nsity
(cps
)In
tens
ity (c
ps)
Inte
nsity
(cps
)
2120579 (deg)
10 20 30 402120579 (deg)
10 20 30 402120579 (deg)
10 20 30 402120579 (deg)
ZC-Ac
ZC-o-HCn
ZC-m-HCn
ZC-p-HCnd = 2103 A
d = 2072 A
d = 1695 A
d = 943 A
(a)
4000 3500 3000 2500 2000 1500 1000
4000 3500 3000 2500 2000 1500 1000
4000 3500 3000 2500 2000 1500 1000
4000 3500 3000 2500 2000 1500 100080
90
100
80
90
100
70
80
90
100
70
80
90
100
T (
) T
()
T (
) T
()
ZC-Ac
ZC-o-HCn
ZC-m-HCn
ZC-p-HCn
Wavenumber (cmminus1)
Wavenumber (cmminus1)
Wavenumber (cmminus1)
Wavenumber (cmminus1)
(b)
Figure 2 (a) PXRD profiles for ZC-Ac and exchange products and (b) FTIR spectra for ZC-Ac and exchange products
planes in the lattice and 120579 is the angle between the incidentray and the scattering planes) The Bragg reflections fromthe precursor material are no longer present in the PXRDtraces of the exchange products indicating that the exchangewas complete As revealed in Figure 2(a) there is an increasein the basal spacing as the acetate anion is replaced by n-HCn ions in the interlayer space The increase in the basalspace is consistent with a smaller anion (acetate ion chainlength = 165 A) being replaced by largern-HCnanions (chainlengths = 863 A for o-HCn 864 A for m-HCn and 911 Afor p-HCn) Considering the chain lengths of the meta-and paraisomers and the size of the gallery height (about16 A if the layer thickness is approximated to be 5 A as incopper hydroxide) [23] the anions are likely to be arrangedin a slightly tilted bilayer orientation The interlayer spaceobserved in ZC-o-HCn is significantly smaller comparedwith the other nanohybrids It was expected that the 119889-spacings would be comparable considering the similarities inthe sizes of the isomers The smaller 119889-spacing in ZC-o-HCn
might be a result of the orthoisomer being in a more tiltedorientation as compared to the other isomers
IR spectra presented in Figure 2(b) show that whenacetate ions were replaced by n-HCn ions the peaks due toC=O vibrations of acetate ions in ZC-Ac (120592asym = 1563 cmminus1
and 120592sym = 1410 cmminus1) [21] were replaced by a series ofpeaks (1637ndash1642 cmminus1 C=C 1540ndash1551 cmminus1 120592asym C=Oand 1399ndash1428 cmminus1 120592sym C=O)The disappearance of vibra-tion peaks from acetate ions is consistent with results inferredfrom PXRD analysis The absence of the C=O stretchingvibration of protonated carboxylic groups which is around1700 cmminus1 confirms that the isomers are present in the ionizedformThe full assignment of the IR peaks is found in Table S2in supplementary information [24]
The hydroxyl stretching vibrational modes (in the region3000 cmminus1ndash3600 cmminus1) provide information about the inter-actions in the interlayer space [25] The structural proper-ties of hydroxyl groups within the interlayer space can be
Advances in Physical Chemistry 5
96
97
98
99
100
3450
T (
)
ZC-Ac
4000 3800 3600 3400 3200 3000 2800 2600
Wavenumber (cmminus1)
(a)
96
97
98
99
100
3131
3489
ZC-o-HCn
4000 3800 3600 3400 3200 3000 2800 2600
Wavenumber (cmminus1)
T (
)
(b)
4000 3800 3600 3400 3200 3000 2800 260094
96
98
100
3578
3513
ZC-m-HCn
Wavenumber (cmminus1)
T (
)
(c)
970
975
980
985
990
995
3497
3574
ZC-p-HCn
4000 3800 3600 3400 3200 3000 2800 2600
Wavenumber (cmminus1)
T (
)
(d)
Figure 3 FTIR spectra for ZC-Ac and exchange products showing the hydroxyl stretching region
correlated to the OH vibrational modes Broad absorptionbands are associated with stretching modes of hydrogenbonded hydroxyl groups and sharp bands associated withfree hydroxyl groups [26] The FTIR profiles of the hydroxylstretching region of ZC-Ac and the exchange products areshown in Figure 3 ZC-Ac has extensive interlayer hydrogenbonding due to intercalated water molecules as reportedin our earlier publication [21] Figure 3 shows that whenacetate ions were replaced by m-HCn ions in ZC-m-HCnthe broad peak (FWHM = 359 cmminus1) centered at 3450 cmminus1in ZC-Ac was replaced by 2 sharp peaks a strong peakat 3513 cmminus1 (FWHM = 148 cmminus1) and a weak peak at3574 cmminus1 (FWHM = 163 cmminus1) The two sharp peaks aredue to layer (3574 cmminus1) and m-HCn (3513 cmminus1) hydroxylgroups which are not involved in substantial hydrogenbonding [25] This assignment is based on our previousresults which showed that the peak at 3513 cmminus1 was absentin HDSs containing cinnamate anions (no OH substitutionon the benzene ring) [21] The shift of the peaks to higherwavenumbers is also indicative of hydroxyl groups which arenot substantially involved in hydrogen bonding ZC-o-HCnhas a very weak peak around 3600 cmminus1 and 2 broad peakscentered at 3489 cmminus1 (FWHM = 880 cmminus1) and 3131 cmminus1(FWHM = 376 cmminus1) The broadening of peaks and the shiftto lower wavenumbers as compared to vibrations in ZC-m-HCn and free OH stretching vibrations is consistent with
hydrogen bonded hydroxyl groups [27] The low intensity ofthe sharp band at 3600 cmminus1 is indicative of extensive hydro-gen bonding between layer OH groups and o-HCn hydroxylgroups resulting in low density of free layer OH groupsIn ZC-p-HCn there is a strong sharp peak at 3574 cmminus1(FWHM = 72 cmminus1) which is due to layer OH groups whichare not involved in significant hydrogen bonding and abroad peak at 3497 cmminus1 (FWHM = 119 cmminus1) due to p-HCnhydroxyl groups involved in hydrogen bonding
The absence of strong sharp peaks in ZC-o-HCn com-pared with those observed for ZC-m-HCn and ZC-p-HCnindicates that there is high level of hydrogen bonding in ZC-o-HCn relative to the other nanohybrids Since hydrogenbonding is directional the m-HCn anions are not orientedin a way that the OH groups are able to interact with eitherthe layer OH groups or groups from other anions Theamount of water in the nanohybrids is low as indicated fromthermogravimetry and elemental analyses Therefore havingeliminated interlayer water as a likely hydrogen bonding par-ticipant the significant hydrogen bonding observed in ZC-o-HCn andZC-p-HCn ismost likely intermolecular or betweenanion and layer hydroxyl groups In ZC-o-HCn the o-HCnhydroxyl group may be in an orientation such that they areclose to the layers enabling formation of hydrogen bondswiththe layer OH groups [28] The interaction between o-HCnand layer hydroxyl groups may have resulted in the o-HCn
6 Advances in Physical Chemistry
Table 2 Summary of dipole moments and release data
Anion Equilibrium isomer released at 40∘C () Dipole moment Calculated chargesCarbon Oxygen 1 Oxygen 2
Figure 4 PXRD profile of a representative ZC-Cl (kapellasite)obtained from exchange reactions (ZC-o-HCnClminus reaction) Millerhkl indices for selected Clminus reflections are given PDF no 56ndash71
anion being tilted closer to the layers which may explain thesmaller gallery height (as compared to that inZC-m-HCn andZC-p-HCn) observed from PXRD analysis In ZC-p-HCnOH groups may be oriented in a way so as to participate inintermolecular H-bonding between anions
32 Kinetic Analysis The release of n-HCn from nanohy-brids was achieved by ion exchange using Clminus as exchangeanions After specified times supernatants were collected andanalyzed for released anions and the residues were analyzedfor solid state transformations using PXRD Analysis of thesolid samples recovered from the reaction mixtures aftereach contact time indicated that the same chloride phasewas obtained for all the nanohybrids A representative PXRDprofile for the zinc copper chloride (ZC-Cl) obtained fromexchange reactions is shown in Figure 4 The chloride phasehas a 119889-spacing of 574 plusmn 001 A which is in close agreementwith the literature value of 573 A [29] and has been indexedas kapellasite Cu
3Zn(OH)
6Cl2(PDF no 56ndash71) [28 30] The
decrease of the interlayer space as the isomers are released isconsistent with large anions being replaced by smaller ions
The concentrations of released anions were evaluatedusing UV-Vis spectroscopy The fractions of released isomersas a function of time (release profiles) at 40∘C are shown inFigures 5(a)ndash5(c) for m-HCn o-HCn and p-HCn respec-tively The release profiles at other temperatures show thesame general trend and are presented in Figure S2 in thesupporting information Although the chemical structures of
the isomers are similar and the release medium is thesame the release profiles and hence release properties aresignificantly different (Figure 5) The equilibrium amountsreleased at 40∘C together with calculated dipole momentsand selected atomic charges are shown in Table 2 As indi-cated in Table 2 the equilibrium amounts released from thenanohybrids were significantly different ZC-o-HCn released100 of the intercalated ions while ZC-m-HCn releasedabout 40 and ZC-p-HCn released about 22 While ZC-o-HCn showed complete release at all temperatures the extentof reaction for the release ofm-HCn and p-HCn showed tem-perature dependence (increasing with temperature as shownin Figure S2) The extents of reactions at 40∘C are confirmedby XRD data in Figure S3 in supporting information
The differences in the extent of reaction may indicatedifferences in the affinity of the HDS for the isomers anddifferences in the thermodynamic equilibrium constants ofthe systemsThe extent of anion exchange depends on severalfactors which include the solvation enthalpy of the isomers(in the bulk liquid and in the gallery) the binding enthalpy(between themetal hydroxide layers and the isomers) and theinter-intramolecular interactions of the anions within thelayers [4 28 31]Themajor contributing factor is expected tobe the electrostatic interaction between the positively chargedmetal hydroxide layers and the negatively charged anionswith the carboxylate group of the isomers being in closecontact with the layers Since the isomers have the samecharge and comparable size differences in dipole moments(an indicator of the charge distribution) could have moreinfluence on the thermodynamics of the release processThe magnitude of dipole moments has been shown to becorrelatedwith selectivity in intercalation reactions of layereddouble hydroxides [4 28 31] The affinity of the metalhydroxide layer is higher for the isomer with the highestdipole moment [13] therefore this isomer is expected to beretained within the layers more than the other isomers
The equilibrium amounts of anion released from thenanohybrids follow the following order o-HCn gt m-HCngt p-HCn which is opposite to that of the magnitude ofdipole moments as shown in Table 2 The order of isomerselectivity in intercalation reactions is as expected thereverse of the order of equilibrium amount release observedhere Since p-HCn has the highest dipole moments the metalhydroxide layers are expected to have the greatest affinityfor p-HCn as compared to the other isomers [13] this couldexplain the order observed here Although factors whichinfluence the extent of release are expected to be complexdue to the processes involved in anion exchange reactions[32] the results obtained here infer that the magnitudeof dipole moments plays a significant role in determining
Advances in Physical Chemistry 7
0 4 8 12 16
0
10
20
30
40
Frac
tion
rele
ased
()
Time (hours)
(a)
0 5 10 150
20
40
60
80
Frac
tion
rele
ased
()
Time (hours)
(b)
0 1 2 3 4 5
0
4
8
12
16
20
Frac
tion
rele
ased
()
Time (hours)
(c)
Figure 5 Release profiles for (a) ZC-m-HCn (b) ZC-o-HCn and (c) ZC-p-HCn at 40∘C
the equilibrium constant and hence the amount released atequilibrium The complex nature of the exchange reactionsis highlighted when our previous results for the release ofcinnamate anion are considered [21] Cinnamate anion hasa dipole moment of 146 D which is lower than that of m-HCn but there was only 31 Cn released at equilibriumThis indicates a complex interplay of the involved factors asit has been shown that highly hydrophobic anions are easyto intercalate into the layers Due to the absence of hydroxylgroup on the benzene ring of Cn anion this isomer is morehydrophobic than the others and is therefore not readilydeintercalated into the polar aqueous solvent environment
From Figure 5 it can qualitatively be observed that therelease rates and the release profiles of the isomers aresignificantly different While the profiles for o-HCn releaseare sigmoidal those for the meta- and paraisomers aredeceleratory Reaction rates can be quantified by fittingexperimental data to reaction models which best describe(reproduce) the experimental data in the so-called model
fitting procedureThis procedure is applicable to singlemech-anism reactions in which the mechanism does not change asthe reaction progresses [33] We have shown previously thatisoconversional methods can be extended to anion exchangereactions in layered metal hydroxide as a strategy to identifywhen using model based approaches are appropriate [21] Aconstant 119864
119886as a function of extent of reaction indicates that
the reaction may effectively be characterized with a singlemechanism and in these cases model based procedures areapplicable [33]The integral isoconversional method (2) wasapplied to the kinetic data obtained here
ln 119905 = ln119892 (120572)
119860+119864119886
119877119879 (2)
In (2) 119864119886is the activation energy 120572 is the extent of
reaction 119860 is the preexponential factor 119905 is the time 119879 thetemperature 119877 is the molar gas constant and 119892(120572) is theintegral reactionmodelThe isoconversional analysis data forall the nanohybrids is shown in Figure 6 in which the 119864
119886
8 Advances in Physical Chemistry
02 04 06 08 1065
70
75
80
85ZC-o-HCn
Extent of reaction (120572)
Ea
(kJm
ol)
(a)
03 04 05 06 07 08 09
30
40
50
60ZC-p-HCn
Extent of reaction (120572)
Ea
(kJm
ol)
(b)
03 04 05 06 07 08 09 10
50
60
70
80ZC-m-HCn
Extent of reaction (120572)
Ea
(kJ m
olminus1)
(c)
Figure 6 Variation of effective 119864119886with 120572 for ZC-m-HCn ZC-o-HCn and ZC-p-HCn
values remains constant within experimental error over theentire conversion range for ZC-m-HCn and ZC-p-HCn
The 119864119886values for ZC-o-HCn are within 10 although
the changes at higher conversion appear significant comparedwith experimental uncertainty The linear decrease of the 119864
119886
at higher conversion above 120572 = 05 has been attributed toweakening of the electrostatic attraction between the anionsand the layers as the anions are progressively exchanged fromlayer to layer [34] The constant 119864
119886at low conversion may
be due to some solid state transformations occurring whichmight be responsible for the induction period observedin the PXRD analysis (Figure S3) Although the inductionperiod is short the process might still be governing theenergetics of the reaction even during the exchange periodresulting in the constant energy being observed up to extentof reaction of 05 Since 119864
119886values for exchange reactions at
the anion binding sites have been shown to range from 30to 70 kJmolminus1 depending on the nature and size of the guestanions [31 32 35] the reactionmay be controlled by chemical
reaction at the exchange site It is also important to note thatdiffusion of anions in a dense organicmatrix can have high119864
119886
values sometimes being more than 100 kJmolminus1 [36] as suchdiffusion limited process cannot be ruled out if the anions areclosely packed in the gallery
The 119864119886values for the release of m-HCn and p-HCn
are in the range of both reaction controlled process anddiffusion controlled processes [21] The fact that the 119864
119886does
not significantly change over the entire reaction may indicatethat the reaction is diffusion controlled The differences inthe mechanism between o-HCn release and the other twosystems may be due to different strength of interactions withthe layersThe presence of H-bonding between the layers andthe o-HCn anions may bring the anions closer to the layersresulting in an increased electrostatic attraction resulting inthe process being controlled by reaction at the exchangesite Since there was no significant variation of the 119864
119886with
conversion for m-HCn and p-HCn release and the energydifference for o-HCn release is too low to reflect a change in
Advances in Physical Chemistry 9
mechanism model based approach was used to obtain rateconstants for the exchange reactions The Avrami-Erofersquoevnucleation-growth model [37 38] was used here since ithas the advantage that it can be used to describe reactionsoccurring via different mechanisms making comparison ofthe processes possible since the order of reactions will be thesame In the model the extent of reaction (120572) depends on therate constant 119896 and a coefficient 119899 as shown in the followingequation
119886 (119905) = 1 minus 119890[minus(119896119905)
119899
] (3)
The coefficient 119899 (Avrami exponent) potentially containsinformation about the mechanism of the reaction [39] TheAvrami-Erofersquoev model has been applied successfully to solu-tion phase anion exchange reactions in layeredmaterials andother fluid phase reactions [2 10 40 41] we have also shownthat values of parameters obtained from analysis of bothsolid state transformations and solution phase processesemploying the Avrami model are comparable indicating thatthere is correlation between solid state transformation andfluid phase processes [21] The Avrami exponent is usuallyobtained by taking double logarithm of the Avrami-Erofersquoevequation obtaining (4) which was popularized by Sharp andHancock in 1972 [42] Consider
A plot of ln[minus ln(1 minus 120572)] as a function of ln 119905 (doublelogarithmic plot) gives a linear plot in which the value of 119899 isobtained from the slope and the value of 119896 is evaluated fromthe intercept
The extent of reaction versus time plots obtained for theZC-o-HCnClminus exchange reactions at different temperaturesis shown in Figure 7(a) The Avrami-Erofersquoev model wasapplied and correctly described the obtained experimentaldatawithin the120572 range of 015ndash10The validity of theAvrami-Erofersquoevmodel can be confirmedby the corresponding doublelogarithmic plots in Figure 7(b) where straight lines wereobtained with 1198772 values ranging from 0996 to 0999 Thedouble logarithmic plots are almost parallel indicating thatthe reaction proceeds with the same mechanism over thetemperature range used here The obtained 119899 value was closeto 10 consistent with a nucleation controlled mechanismwhich agrees well with the activation energies obtained fromisoconversional methods and discussed before
The release profiles together with the double logarithmicplots for the release of m-HCn and p-HCn are shown in thesupporting information Figures S4 and S5 respectively Thekinetics of m-HCn release also follows the Avrami-Erofersquoevmodel the data provided a good fit within the range 015 lt120572 lt 085 the fit to limited range of 120572 has been observedin other intercalation reactions Anomalously low values ofthe Avrami exponent observed here (shown in Table 3) havebeen reported in polymer and alloy crystallization and donot have physical mechanistic meaning [43 44] The Avramiexponent values obtained for the release of p-HCn are closeto 05 which is consistent with diffusion controlled reactions[10] Diffusion control of such a high 119864
119886highlights two
0 2 4 6 8 10 12 1400
02
04
06
08
10
Time (hours)
Exte
nt o
f rea
ctio
n (120572
)
300∘C400∘C500∘C600∘C
(a)
4 6 8 10
0
1
minus1
minus2
ln (t minus t0)
lnminusln(1
minus120572)
30∘C40∘C50∘C60∘C
(b)
Figure 7 Extent of reaction as a function of time (a) and doublelogarithmic plots (b) for the exchange reaction of Clminus anion andZC-o-HCn at various temperatures 60∘C (◻) 50∘C (Q) 40∘C (998771)and 30∘C (e)
issues (a) the interlayer space is densely packed and thediffusional resistance within the interlayer space approachesthat in polymers in which119864
119886values can be above 100 kJmolminus1
depending on the diffusing molecule [36] and (b) Kineticambiguity is common in model based approaches in thiscase the mechanistic interpretation from the model is not inline with the kinetic parameters obtained
FromTable 3 it can be concluded that the rate of release ofm-HCn from the nanohybrids is much faster compared with
10 Advances in Physical Chemistry
Table 3 A summary of kinetic parameters obtained at different temperatures
Nanohybrid Temperature (∘C) 119899 119896 sminus1 (lowast10minus3)
ZC-o-HCn
600 082 plusmn 002 17 plusmn 03
500 090 plusmn 002 07 plusmn 01
400 088 plusmn 001 029 plusmn 005
300 102 plusmn 001 010 plusmn 001
ZC-m-HCn
600 024 plusmn 001 31 plusmn 9
500 024 plusmn 001 20 plusmn 7
400 021 plusmn 002 7 plusmn 2
300 0225 plusmn 0007 35 plusmn 08
ZC-p-HCn
600 041 plusmn 001 29 plusmn 05
500 045 plusmn 002 16 plusmn 04
400 046 plusmn 001 10 plusmn 02
300 039 plusmn 001 05 plusmn 01
the other isomers As an example the rate constants at 40∘Cwere (7 plusmn 2) times 10minus3 sminus1 for m-HCn (10 plusmn 02) times 10minus3 sminus1 forp-HCn and (029 plusmn 005) times 10minus3 sminus1 for o-HCn The rate ofrelease follows the following order
119898ndashHCn gt 119901-HCn gt 119900-HCn
The release profile form-HCn at 40∘C (Figure 5(a)) showsthat there is a burst and rapid release of the intercalated anionin the first 10 minutes followed by a more sustained releasewith equilibrium being attained after 3 hours This burstrelease is observed for all the temperatures used here (FigureS2a in supporting information) and has been observed beforein anion exchange reactions in LDHs [10] The burst releaseobserved here is due to anion exchange as confirmed byPXRD results in Figure S3 The chloride phase representedby an asterisk in Figure S3 was significant at 30 seconds (theearliest we could sample) of ZC-m-HCn being in contact withthe chloride solution The initial slow and linear release inZC-o-HCn indicated in Figure 5(b) and confirmed by XRDprofiles in Figure S3 may be due to the increased stabilityof the HDS structure due to extensive hydrogen bonding asindicated by FTIR data in Figure 3 and the close arrangementof the anions implied by XRDprofiles in Figure 2(a)This firststage of release has a strong temperature dependence beingmore pronounced at low temperature and almost absent athigher temperatures (Figure S2b in supporting information)
Factors which may influence the rates of release includeinteractions between the metal hydroxide layers and theanions anion-anion interactions within the layers and in theexchange medium and solvent-anion interaction [10] It isexpected that these factors will have a complex effect on thereaction rates since they affect each isomer differently Thecomplex interplay of the abovementioned factors oftenmakeit difficult to come up with a simple relationship betweenthe rate of release and the aforementioned factors [10] It isimportant to note that these factors are expected to dependon the types and positions of chemical groups on the anionThepresence of hydrogen bondingwithin the interlayer spacereduces the Gibbs free energy of formation of the nanohy-brids and stabilizes the system [45]The rate of release of ionsthen depends on the differences in the energy of nanohybrid
and the barrier to reaction As indicated in Figure 3 there issignificant level of hydrogen bonding in ZC-o-HCn whichthen resulted in a close interaction with the layers Thehydrogen bond network in the nanohybrid and the closeinteraction of the anions with the layers stabilized the systemas compared to the other nanohybrids The nanohybrid inwhich these interactions are low is relatively less stable and isexpected to proceed faster to equilibrium this is consistentwith results obtained here The anions in the unstablelessstable nanohybrid prefer bulk solution (in which there isstabilization by solvation with water molecules) to galleryspace which may result in the burst and fast release observedin ZC-m-HCnThe modification of bioactive molecules withgroups capable of hydrogen bonding for example hydroxylgroups especially at positions which allow interactions withthe layers may enable tuning of the release behavior ofthese molecules from layered materials It is important tonote that the modification should be at positions whichdo not affect the bioactivity of the molecules Additionalexperiments including studies on other isomeric anions withgroups capable of forming hydrogen bonds and effect ofmetallayer composition will be helpful in further elucidating howthe position of substituents control the rate as well as theextent of reaction
4 Conclusions
Anion exchange reactions in hydroxy double salts are affectedby the structure of the intercalated anions and there is astrong interplay between thermodynamics and kinetics ofthe release reactions In the present study we have shownthat there is a significant effect of the position of substituentgroups on the rate and extent of release of isomers ofhydroxycinnamate While the release of the metaisomer wasinstantaneous and fast the orthoisomer release exhibiteda slower more sustained and complete release the orderof reaction rates was m-HCn gt p-HCn gt o-HCn withthe o-HCn nanohybrid showing an induction period Theposition of the hydroxyl groups resulted in differences inthe interlayer interactions of the anions with presence ofinterlayer hydrogen bonds affecting the rates of the reaction
Advances in Physical Chemistry 11
The formation of hydrogen bonds between the layers and theintercalated orthoisomer resulted in greater stabilization ofthe nanohybrid and resulted in the observed induction periodand slow release The extent of reaction was correlated withthe magnitude of dipole moments These results highlightthe potential of tuning the release behavior of intercalatedbioactive compounds which bring control into both theamount released per given time and the period for totalrelease
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
This work was supported by the National Science Foundation(CHE-0809751)
References
[1] S Yamanaka T Sako andM Hattori ldquoAnion exchange in basiccopper acetaterdquo Chemistry Letters pp 1869ndash1872 1989
[2] E Kandare and J M Hossenlopp ldquoHydroxy double salt anionexchange kinetics effects of precursor structure and anionsizetrdquo Journal of Physical Chemistry B vol 109 no 17 pp 8469ndash8475 2005
[3] P K Dutta and M Puri ldquoAnion exchange in lithium aluminatehydroxidesrdquo Journal of Physical Chemistry vol 93 no 1 pp376ndash381 1989
[4] B M Choudary V S Jaya B R Reddy M L Kantam M MRao and S S Madhavendra ldquoSynthesis characterization ionexchange and catalytic properties of nanobinary and ternarymetal oxyhydroxidesrdquo Chemistry of Materials vol 17 no 10pp 2740ndash2743 2005
[5] A M Fogg J S Dunn S-G Shyu D R Cary and D OrsquoHareldquoSelective ion-exchange intercalation of isomeric dicarboxylateanions into the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Chemistry of Materials vol 10 no 1 pp 351ndash355 1998[6] T Ikeda H Amoh and T Yasunaga ldquoStereoselective exchange
kinetics of L- and D-histidines for Cl- in the interlayerof a hydrotalcite-like compound by the chemical relaxationmethodrdquo Journal of the American Chemical Society vol 106 no20 pp 5772ndash5775 1984
[7] M Meyn K Beneke and G Lagaly ldquoAnion-exchange reactionsof hydroxy double saltsrdquo Inorganic Chemistry vol 32 no 7 pp1209ndash1215 1993
[8] V Ambrogi G Fardella G Grandolini L Perioli and M CTiralti ldquoIntercalation compounds of hydrotalcite-like anionicclays with anti-inflammatory agents II uptake of diclofenac fora controlled release formulationrdquo AAPS PharmSciTech vol 3no 3 p E26 2002
[9] M Z Bin Hussein Z Zainal A H Yahaya and D WV Foo ldquoControlled release of a plant growth regulator 120572-naphthaleneacetate from the lamella of Zn-Al-layered doublehydroxide nanocompositerdquo Journal of Controlled Release vol82 no 2-3 pp 417ndash427 2002
[10] A I Khan A Ragavan B Fong et al ldquoRecent developments inthe use of layered double hydroxides as host materials for the
storage and triggered release of functional anionsrdquo Industrialand Engineering Chemistry Research vol 48 no 23 pp 10196ndash10205 2009
[11] M Silion D Hritcu I M Jaba et al ldquoIn vitro and invivo behavior of ketoprofen intercalated into layered doublehydroxidesrdquo Journal of Materials Science vol 21 no 11 pp3009ndash3018 2010
[12] J-H Yang Y-S Han M Park T Park S-J Hwang andJ-H Choy ldquoNew inorganic-based drug delivery system ofindole-3-acetic acid-layered metal hydroxide nanohybrids withcontrolled release raterdquo Chemistry of Materials vol 19 no 10pp 2679ndash2685 2007
[13] L Lei R P Vijayan and D OrsquoHare ldquoPreferential anionexchange intercalation of pyridinecarboxylate and toluate iso-mers in the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Journal of Materials Chemistry vol 11 no 12 pp 3276ndash32802001
[14] B Lotsch F Millange R I Walton and D OrsquoHare ldquoSepara-tion of nucleoside monophosphates using preferential anionexchange intercalation in layered double hydroxidesrdquo Solid StateSciences vol 3 no 8 pp 883ndash886 2001
[15] H Tagaya N Sasaki H Morioka and J Kadokawa ldquoPrepara-tion of new inorganicmdashorganic layered compounds hydroxydouble salts and preferential intercalation of organic carboxylicacids into themrdquoMolecular Crystals and Liquid Crystals Scienceand Technology A vol 341 pp 413ndash418 2000
[16] P P Kumar A G Kalinichev and R J Kirkpatrick ldquoHydra-tion swelling interlayer structure and hydrogen bondingin organolayered double hydroxides insights from moleculardynamics simulation of citrate-intercalated hydrotalciterdquo Jour-nal of Physical Chemistry B vol 110 no 9 pp 3841ndash3844 2006
[17] S V Prasanna and P V Kamath ldquoAnion-exchange reactions oflayered double hydroxides interplay between coulombic andh-bonding interactionsrdquo Industrial and Engineering ChemistryResearch vol 48 no 13 pp 6315ndash6320 2009
[18] Z Gu A CThomas Z P Xu J H Campbell and G Q Lu ldquoInvitro sustained release of LMWH from MgAl-layered doublehydroxide nanohybridsrdquo Chemistry of Materials vol 20 no 11pp 3715ndash3722 2008
[19] J T Rajamathi S Britto and M Rajamathi ldquoSynthesis andanion exchange reactions of a layered Copper-Zincrdquo Journal ofChemical Sciences vol 117 no 6 pp 629ndash633 2005
[20] M Frisch G Trucks H Schlegel et al ldquoGaussian 98 RevisionA114rdquo 2002
[21] S Majoni and J M Hossenlopp ldquoAnion exchange kinetics ofnanodimensional layered metal hydroxides use of isoconver-sional analysisrdquo Journal of Physical Chemistry A vol 114 no 49pp 12858ndash12869 2010
[22] C Chouillet J-M Krafft C Louis and H Lauron-PernotldquoCharacterization of zinc hydroxynitrates by diffuse reflectanceinfrared spectroscopymdashstructural modifications during ther-mal treatmentrdquo Spectrochimica Acta A vol 60 no 3 pp 505ndash511 2004
[23] W Fujita and K Awaga ldquoMagnetic properties of Cu2(OH)
3(al-
kanecarboxylate) compounds drastic mod-ification withextension of the Alkyl chainrdquo Inorganic Chemistry vol 35 no7 pp 1915ndash1917 1996
[24] R Swisłocka M Kowczyk-Sadowy M Kalinowska and WLewandowski ldquoSpectroscopic (FT-IR FT-Raman1H and13CNMR) and theoretical studies of p-coumaric acid and alkalimetal p-coumaratesrdquo Spectroscopy vol 27 no 1 pp 35ndash48 2012
12 Advances in Physical Chemistry
[25] T Biswick W Jones A Pacuła E Serwicka and J PodobinskildquoThe role of anhydrous zinc nitrate in the thermal decomposi-tion of the zinc hydroxy nitrates Zn
5(OH)
8(NO3)2sdot2H2O and
ZnOHNO3sdotH2Ordquo Journal of Solid State Chemistry vol 180 no
4 pp 1171ndash1179 2007[26] S-H Park and E L Cheol ldquoSynthesis characterization and
magnetic properties of a novel disulfonate-pillared copperhydroxide Cu
2(OH)
3(DS4)
12 DS4 = 14-butanedisulfonaterdquo
Bulletin of the Korean Chemical Society vol 27 no 10 pp 1587ndash1592 2006
[27] P Rabu M Drillon and C Hornick ldquoStructural and spectro-scopic study of organic-inorganic transitionmetal based layeredmagnetsrdquo Analusis vol 28 no 2 pp 103ndash108 2000
[28] L Lei A Khan and D OrsquoHare ldquoSelective anion-exchangeintercalation of isomeric benzoate anions into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo Journal of Solid State
Chemistry vol 178 no 12 pp 3648ndash3654 2005[29] W Krause H-J Bernhardt R S W Braithwaite U Kolitsch
and R Pritchard ldquoKapellasite Cu3Zn(OH)
6Cl2 a new mineral
from Lavrion Greece and its crystal structurerdquo MineralogicalMagazine vol 70 no 3 pp 329ndash340 2006
[30] L C Andrews L R Bernstein C M Foris et al PowderDiffraction File Inorganic International Centre for DiffractionData Newtown Square Pennsylvania 2009
[31] A Ragavan A I Khan and D OrsquoHare ldquoIsomer selectiveion-exchange intercalation of nitrophenolates into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotxH
2Ordquo Journal of Materials
Chemistry vol 16 no 6 pp 602ndash608 2006[32] A Ragavan A Khan and D OrsquoHare ldquoSelective intercalation
of chlorophenoxyacetates into the layered double hydroxide[LiAl2(OH)
6]ClsdotxH
2Ordquo Journal of Materials Chemistry vol 16
no 42 pp 4155ndash4159 2006[33] S Vyazovkin and C A Wight ldquoModel-free and model-fitting
approaches to kinetic analysis of isothermal and nonisothermaldatardquoThermochimica Acta vol 340-341 pp 53ndash68 1999
[34] R E Johnsen F Krumeich and P Norby ldquoStructural andmicrostructural changes during anion exchange ofCoAl layereddouble hydroxides an in situ X-ray powder diffraction studyrdquoJournal of Applied Crystallography vol 43 no 3 pp 434ndash4472010
[35] G R Williams T G Dunbar A J Beer A M Fogg and DOrsquoHare ldquoIntercalation chemistry of the novel layered doublehydroxides [MAl
4(OH)
12](NO
3)2sdotyH2O (M=Zn Cu Ni and
Co) 1 new organic intercalates and reaction mechanismsrdquoJournal of Materials Chemistry vol 16 no 13 pp 1222ndash12302006
[36] T E M Ten Hulscher and G Cornelissen ldquoEffect of tempera-ture on sorption equilibrium and sorption kinetics of organicmicropollutantsmdasha reviewrdquo Chemosphere vol 32 no 4 pp609ndash626 1996
[37] M Avrami ldquoKinetics of phase change I general theoryrdquo TheJournal of Chemical Physics vol 7 no 12 pp 1103ndash1112 1939
[38] M Avrami ldquoKinetics of phase change II Transformation-timerelations for random distribution of nucleirdquo The Journal ofChemical Physics vol 8 no 2 pp 212ndash224 1940
[39] R J Francis S OrsquoBrien A M Fogg et al ldquoTime-resolved in-situ energy and angular dispersive X-ray diffraction studies ofthe formation of themicroporous gallophosphateULM-5 underhydrothermal conditionsrdquo Journal of the American ChemicalSociety vol 121 no 5 pp 1002ndash1015 1999
[40] Z Miladinovic J Zakrzewska B Kovacevic and G BacicldquoMonitoring of crystallization processes during synthesis of
zeolite A by in situ 27Al NMR spectroscopyrdquo Materials Chem-istry and Physics vol 104 no 2-3 pp 384ndash389 2007
[41] R Serna-Guerrero and A Sayari ldquoModeling adsorption of CO2
on amine-functionalized mesoporous silica 2 kinetics andbreakthrough curvesrdquo Chemical Engineering Journal vol 161no 1-2 pp 182ndash190 2010
[42] J D Hancock and J H Sharp ldquoMethod of comparing solid-state kinetic data and its application to the decomposition ofkaolinite brucite and BaCO
3rdquo Journal of the American Ceramic
Society vol 55 no 2 pp 74ndash77 1972[43] F Auriemma C De Rosa and R Triolo ldquoSlow crystallization
kinetics of poly(vinyl alcohol) in confined environment duringcryotropic gelation of aqueous solutionsrdquo Macromolecules vol39 no 26 pp 9429ndash9434 2006
[44] R V Ramanujan and Y R Zhang ldquoQuantitative transmissionelectronmicroscopy analysis of the nanocrystallization kineticsof softmagnetic alloysrdquoPhysical ReviewB vol 74 no 22 ArticleID 224408 2006
[45] A G Kalinichev P Padma Kumar and R James KirkpatrickldquoMolecular dynamics computer simulations of the effects ofhydrogen bonding on the properties of layered double hydrox-ides intercalated with organic acidsrdquo Philosophical Magazinevol 90 no 17-18 pp 2475ndash2488 2010
Figure 1 Structure of the anions used in this study (a) 119900-hydroxycinnamate (119900-HCn) (b) 119898-hydroxycinnamate (119898-HCn) and (c) 119901-hydroxycinnamate (119901-HCn)
the extent of reaction (120572) at any given time using (1)119862infin
was obtained from complete exchange using carbonateanions and from formulae obtained from elemental analysis(Table S1 see Supplementary Material available online athttpdxdoiorg1011552014710487) Equation (1) was alsoused to obtain extent of ion exchange (extent of reaction atequilibrium) with 119862
119905in (1) now being 119862
119905infin(amount released
at equilibrium)Consider
120572 =119862119905
119862infin
(1)
3 Results and Discussion
31 Preparation of Hydroxycinnamate Nanohybrids Hydrox-ycinnamate exists in 3 geometric isomers as shown inFigure 1 Nanohybrids containing isomers of hydroxycinna-mate in the interlayer space were prepared from ZC-Ac byanion exchangeThe uptake of these anions and the completereplacement of acetate anions in the interlayer space wasconfirmed by elemental analysis PXRD and ATR-FTIRThe total amount of anions intercalated in the nanohybridswere obtained from both elemental analysis and completeexchange with carbonate anions The values obtained fromboth methods are within 10 as shown in Table S1 insupporting information
A summary of elemental analysis for the nanohybridsis shown in Table 1 From the formulae obtained from
elemental analyses the ratio of copper to zinc in the metalhydroxide layers of ZC-m-HCn (28) and ZC-p-HCn (25) iscomparable to that of the precursor ZC-Ac (28) nanohybrid[21] This may indicate that the exchange reaction in theformation of these compounds was topotactic The ratioin ZC-o-HCn nanohybrid (35) is much higher than inthe precursor material which may be an indication thatZC-o-HCn was formed via the dissolution-recrystallizationmechanism In this case the nature of the cations andanions determines the ratio of the metal ions in the layerswhich may have resulted in the higher copper content[7 22] XRD profiles indicate that no crystalline copperhydroxide was detected in the material The low amountof water in the gallery (from Table 1) is consistent withTGA results shown in Figure S1 in supporting informationas an example TGA results for the degradation of ZC-o-HCn indicate that there is 16 weight loss attributed towater loss and the formula in Table 1 indicates 17 watercontent
ZC-Ac used here exhibits a spacing between the planes(119889-spacing) of 943 plusmn 003 A [21] which is comparable to theliterature value of 946 A [19] The PXRD patterns for ZC-Ac and exchange products are shown in Figure 2(a) All thematerials show at least three equally spaced Bragg reflectionsat low 2 120579 values indicating that the materials are layered andpossess high range orderingThese basal reflectionswere usedto calculate the 119889-spacing of the materials using the Braggequation 119899120582 = 2119889 sin 120579 (where 119899 is the reflection order 120582 isthe wavelength of incident X-ray 119889 is the spacing between the
4 Advances in Physical Chemistry
10 20 30 400
7500
15000
22500
0
5000
10000
15000
0
1500
3000
4500
0
3000
6000
9000In
tens
ity (c
ps)
Inte
nsity
(cps
)In
tens
ity (c
ps)
Inte
nsity
(cps
)
2120579 (deg)
10 20 30 402120579 (deg)
10 20 30 402120579 (deg)
10 20 30 402120579 (deg)
ZC-Ac
ZC-o-HCn
ZC-m-HCn
ZC-p-HCnd = 2103 A
d = 2072 A
d = 1695 A
d = 943 A
(a)
4000 3500 3000 2500 2000 1500 1000
4000 3500 3000 2500 2000 1500 1000
4000 3500 3000 2500 2000 1500 1000
4000 3500 3000 2500 2000 1500 100080
90
100
80
90
100
70
80
90
100
70
80
90
100
T (
) T
()
T (
) T
()
ZC-Ac
ZC-o-HCn
ZC-m-HCn
ZC-p-HCn
Wavenumber (cmminus1)
Wavenumber (cmminus1)
Wavenumber (cmminus1)
Wavenumber (cmminus1)
(b)
Figure 2 (a) PXRD profiles for ZC-Ac and exchange products and (b) FTIR spectra for ZC-Ac and exchange products
planes in the lattice and 120579 is the angle between the incidentray and the scattering planes) The Bragg reflections fromthe precursor material are no longer present in the PXRDtraces of the exchange products indicating that the exchangewas complete As revealed in Figure 2(a) there is an increasein the basal spacing as the acetate anion is replaced by n-HCn ions in the interlayer space The increase in the basalspace is consistent with a smaller anion (acetate ion chainlength = 165 A) being replaced by largern-HCnanions (chainlengths = 863 A for o-HCn 864 A for m-HCn and 911 Afor p-HCn) Considering the chain lengths of the meta-and paraisomers and the size of the gallery height (about16 A if the layer thickness is approximated to be 5 A as incopper hydroxide) [23] the anions are likely to be arrangedin a slightly tilted bilayer orientation The interlayer spaceobserved in ZC-o-HCn is significantly smaller comparedwith the other nanohybrids It was expected that the 119889-spacings would be comparable considering the similarities inthe sizes of the isomers The smaller 119889-spacing in ZC-o-HCn
might be a result of the orthoisomer being in a more tiltedorientation as compared to the other isomers
IR spectra presented in Figure 2(b) show that whenacetate ions were replaced by n-HCn ions the peaks due toC=O vibrations of acetate ions in ZC-Ac (120592asym = 1563 cmminus1
and 120592sym = 1410 cmminus1) [21] were replaced by a series ofpeaks (1637ndash1642 cmminus1 C=C 1540ndash1551 cmminus1 120592asym C=Oand 1399ndash1428 cmminus1 120592sym C=O)The disappearance of vibra-tion peaks from acetate ions is consistent with results inferredfrom PXRD analysis The absence of the C=O stretchingvibration of protonated carboxylic groups which is around1700 cmminus1 confirms that the isomers are present in the ionizedformThe full assignment of the IR peaks is found in Table S2in supplementary information [24]
The hydroxyl stretching vibrational modes (in the region3000 cmminus1ndash3600 cmminus1) provide information about the inter-actions in the interlayer space [25] The structural proper-ties of hydroxyl groups within the interlayer space can be
Advances in Physical Chemistry 5
96
97
98
99
100
3450
T (
)
ZC-Ac
4000 3800 3600 3400 3200 3000 2800 2600
Wavenumber (cmminus1)
(a)
96
97
98
99
100
3131
3489
ZC-o-HCn
4000 3800 3600 3400 3200 3000 2800 2600
Wavenumber (cmminus1)
T (
)
(b)
4000 3800 3600 3400 3200 3000 2800 260094
96
98
100
3578
3513
ZC-m-HCn
Wavenumber (cmminus1)
T (
)
(c)
970
975
980
985
990
995
3497
3574
ZC-p-HCn
4000 3800 3600 3400 3200 3000 2800 2600
Wavenumber (cmminus1)
T (
)
(d)
Figure 3 FTIR spectra for ZC-Ac and exchange products showing the hydroxyl stretching region
correlated to the OH vibrational modes Broad absorptionbands are associated with stretching modes of hydrogenbonded hydroxyl groups and sharp bands associated withfree hydroxyl groups [26] The FTIR profiles of the hydroxylstretching region of ZC-Ac and the exchange products areshown in Figure 3 ZC-Ac has extensive interlayer hydrogenbonding due to intercalated water molecules as reportedin our earlier publication [21] Figure 3 shows that whenacetate ions were replaced by m-HCn ions in ZC-m-HCnthe broad peak (FWHM = 359 cmminus1) centered at 3450 cmminus1in ZC-Ac was replaced by 2 sharp peaks a strong peakat 3513 cmminus1 (FWHM = 148 cmminus1) and a weak peak at3574 cmminus1 (FWHM = 163 cmminus1) The two sharp peaks aredue to layer (3574 cmminus1) and m-HCn (3513 cmminus1) hydroxylgroups which are not involved in substantial hydrogenbonding [25] This assignment is based on our previousresults which showed that the peak at 3513 cmminus1 was absentin HDSs containing cinnamate anions (no OH substitutionon the benzene ring) [21] The shift of the peaks to higherwavenumbers is also indicative of hydroxyl groups which arenot substantially involved in hydrogen bonding ZC-o-HCnhas a very weak peak around 3600 cmminus1 and 2 broad peakscentered at 3489 cmminus1 (FWHM = 880 cmminus1) and 3131 cmminus1(FWHM = 376 cmminus1) The broadening of peaks and the shiftto lower wavenumbers as compared to vibrations in ZC-m-HCn and free OH stretching vibrations is consistent with
hydrogen bonded hydroxyl groups [27] The low intensity ofthe sharp band at 3600 cmminus1 is indicative of extensive hydro-gen bonding between layer OH groups and o-HCn hydroxylgroups resulting in low density of free layer OH groupsIn ZC-p-HCn there is a strong sharp peak at 3574 cmminus1(FWHM = 72 cmminus1) which is due to layer OH groups whichare not involved in significant hydrogen bonding and abroad peak at 3497 cmminus1 (FWHM = 119 cmminus1) due to p-HCnhydroxyl groups involved in hydrogen bonding
The absence of strong sharp peaks in ZC-o-HCn com-pared with those observed for ZC-m-HCn and ZC-p-HCnindicates that there is high level of hydrogen bonding in ZC-o-HCn relative to the other nanohybrids Since hydrogenbonding is directional the m-HCn anions are not orientedin a way that the OH groups are able to interact with eitherthe layer OH groups or groups from other anions Theamount of water in the nanohybrids is low as indicated fromthermogravimetry and elemental analyses Therefore havingeliminated interlayer water as a likely hydrogen bonding par-ticipant the significant hydrogen bonding observed in ZC-o-HCn andZC-p-HCn ismost likely intermolecular or betweenanion and layer hydroxyl groups In ZC-o-HCn the o-HCnhydroxyl group may be in an orientation such that they areclose to the layers enabling formation of hydrogen bondswiththe layer OH groups [28] The interaction between o-HCnand layer hydroxyl groups may have resulted in the o-HCn
6 Advances in Physical Chemistry
Table 2 Summary of dipole moments and release data
Anion Equilibrium isomer released at 40∘C () Dipole moment Calculated chargesCarbon Oxygen 1 Oxygen 2
Figure 4 PXRD profile of a representative ZC-Cl (kapellasite)obtained from exchange reactions (ZC-o-HCnClminus reaction) Millerhkl indices for selected Clminus reflections are given PDF no 56ndash71
anion being tilted closer to the layers which may explain thesmaller gallery height (as compared to that inZC-m-HCn andZC-p-HCn) observed from PXRD analysis In ZC-p-HCnOH groups may be oriented in a way so as to participate inintermolecular H-bonding between anions
32 Kinetic Analysis The release of n-HCn from nanohy-brids was achieved by ion exchange using Clminus as exchangeanions After specified times supernatants were collected andanalyzed for released anions and the residues were analyzedfor solid state transformations using PXRD Analysis of thesolid samples recovered from the reaction mixtures aftereach contact time indicated that the same chloride phasewas obtained for all the nanohybrids A representative PXRDprofile for the zinc copper chloride (ZC-Cl) obtained fromexchange reactions is shown in Figure 4 The chloride phasehas a 119889-spacing of 574 plusmn 001 A which is in close agreementwith the literature value of 573 A [29] and has been indexedas kapellasite Cu
3Zn(OH)
6Cl2(PDF no 56ndash71) [28 30] The
decrease of the interlayer space as the isomers are released isconsistent with large anions being replaced by smaller ions
The concentrations of released anions were evaluatedusing UV-Vis spectroscopy The fractions of released isomersas a function of time (release profiles) at 40∘C are shown inFigures 5(a)ndash5(c) for m-HCn o-HCn and p-HCn respec-tively The release profiles at other temperatures show thesame general trend and are presented in Figure S2 in thesupporting information Although the chemical structures of
the isomers are similar and the release medium is thesame the release profiles and hence release properties aresignificantly different (Figure 5) The equilibrium amountsreleased at 40∘C together with calculated dipole momentsand selected atomic charges are shown in Table 2 As indi-cated in Table 2 the equilibrium amounts released from thenanohybrids were significantly different ZC-o-HCn released100 of the intercalated ions while ZC-m-HCn releasedabout 40 and ZC-p-HCn released about 22 While ZC-o-HCn showed complete release at all temperatures the extentof reaction for the release ofm-HCn and p-HCn showed tem-perature dependence (increasing with temperature as shownin Figure S2) The extents of reactions at 40∘C are confirmedby XRD data in Figure S3 in supporting information
The differences in the extent of reaction may indicatedifferences in the affinity of the HDS for the isomers anddifferences in the thermodynamic equilibrium constants ofthe systemsThe extent of anion exchange depends on severalfactors which include the solvation enthalpy of the isomers(in the bulk liquid and in the gallery) the binding enthalpy(between themetal hydroxide layers and the isomers) and theinter-intramolecular interactions of the anions within thelayers [4 28 31]Themajor contributing factor is expected tobe the electrostatic interaction between the positively chargedmetal hydroxide layers and the negatively charged anionswith the carboxylate group of the isomers being in closecontact with the layers Since the isomers have the samecharge and comparable size differences in dipole moments(an indicator of the charge distribution) could have moreinfluence on the thermodynamics of the release processThe magnitude of dipole moments has been shown to becorrelatedwith selectivity in intercalation reactions of layereddouble hydroxides [4 28 31] The affinity of the metalhydroxide layer is higher for the isomer with the highestdipole moment [13] therefore this isomer is expected to beretained within the layers more than the other isomers
The equilibrium amounts of anion released from thenanohybrids follow the following order o-HCn gt m-HCngt p-HCn which is opposite to that of the magnitude ofdipole moments as shown in Table 2 The order of isomerselectivity in intercalation reactions is as expected thereverse of the order of equilibrium amount release observedhere Since p-HCn has the highest dipole moments the metalhydroxide layers are expected to have the greatest affinityfor p-HCn as compared to the other isomers [13] this couldexplain the order observed here Although factors whichinfluence the extent of release are expected to be complexdue to the processes involved in anion exchange reactions[32] the results obtained here infer that the magnitudeof dipole moments plays a significant role in determining
Advances in Physical Chemistry 7
0 4 8 12 16
0
10
20
30
40
Frac
tion
rele
ased
()
Time (hours)
(a)
0 5 10 150
20
40
60
80
Frac
tion
rele
ased
()
Time (hours)
(b)
0 1 2 3 4 5
0
4
8
12
16
20
Frac
tion
rele
ased
()
Time (hours)
(c)
Figure 5 Release profiles for (a) ZC-m-HCn (b) ZC-o-HCn and (c) ZC-p-HCn at 40∘C
the equilibrium constant and hence the amount released atequilibrium The complex nature of the exchange reactionsis highlighted when our previous results for the release ofcinnamate anion are considered [21] Cinnamate anion hasa dipole moment of 146 D which is lower than that of m-HCn but there was only 31 Cn released at equilibriumThis indicates a complex interplay of the involved factors asit has been shown that highly hydrophobic anions are easyto intercalate into the layers Due to the absence of hydroxylgroup on the benzene ring of Cn anion this isomer is morehydrophobic than the others and is therefore not readilydeintercalated into the polar aqueous solvent environment
From Figure 5 it can qualitatively be observed that therelease rates and the release profiles of the isomers aresignificantly different While the profiles for o-HCn releaseare sigmoidal those for the meta- and paraisomers aredeceleratory Reaction rates can be quantified by fittingexperimental data to reaction models which best describe(reproduce) the experimental data in the so-called model
fitting procedureThis procedure is applicable to singlemech-anism reactions in which the mechanism does not change asthe reaction progresses [33] We have shown previously thatisoconversional methods can be extended to anion exchangereactions in layered metal hydroxide as a strategy to identifywhen using model based approaches are appropriate [21] Aconstant 119864
119886as a function of extent of reaction indicates that
the reaction may effectively be characterized with a singlemechanism and in these cases model based procedures areapplicable [33]The integral isoconversional method (2) wasapplied to the kinetic data obtained here
ln 119905 = ln119892 (120572)
119860+119864119886
119877119879 (2)
In (2) 119864119886is the activation energy 120572 is the extent of
reaction 119860 is the preexponential factor 119905 is the time 119879 thetemperature 119877 is the molar gas constant and 119892(120572) is theintegral reactionmodelThe isoconversional analysis data forall the nanohybrids is shown in Figure 6 in which the 119864
119886
8 Advances in Physical Chemistry
02 04 06 08 1065
70
75
80
85ZC-o-HCn
Extent of reaction (120572)
Ea
(kJm
ol)
(a)
03 04 05 06 07 08 09
30
40
50
60ZC-p-HCn
Extent of reaction (120572)
Ea
(kJm
ol)
(b)
03 04 05 06 07 08 09 10
50
60
70
80ZC-m-HCn
Extent of reaction (120572)
Ea
(kJ m
olminus1)
(c)
Figure 6 Variation of effective 119864119886with 120572 for ZC-m-HCn ZC-o-HCn and ZC-p-HCn
values remains constant within experimental error over theentire conversion range for ZC-m-HCn and ZC-p-HCn
The 119864119886values for ZC-o-HCn are within 10 although
the changes at higher conversion appear significant comparedwith experimental uncertainty The linear decrease of the 119864
119886
at higher conversion above 120572 = 05 has been attributed toweakening of the electrostatic attraction between the anionsand the layers as the anions are progressively exchanged fromlayer to layer [34] The constant 119864
119886at low conversion may
be due to some solid state transformations occurring whichmight be responsible for the induction period observedin the PXRD analysis (Figure S3) Although the inductionperiod is short the process might still be governing theenergetics of the reaction even during the exchange periodresulting in the constant energy being observed up to extentof reaction of 05 Since 119864
119886values for exchange reactions at
the anion binding sites have been shown to range from 30to 70 kJmolminus1 depending on the nature and size of the guestanions [31 32 35] the reactionmay be controlled by chemical
reaction at the exchange site It is also important to note thatdiffusion of anions in a dense organicmatrix can have high119864
119886
values sometimes being more than 100 kJmolminus1 [36] as suchdiffusion limited process cannot be ruled out if the anions areclosely packed in the gallery
The 119864119886values for the release of m-HCn and p-HCn
are in the range of both reaction controlled process anddiffusion controlled processes [21] The fact that the 119864
119886does
not significantly change over the entire reaction may indicatethat the reaction is diffusion controlled The differences inthe mechanism between o-HCn release and the other twosystems may be due to different strength of interactions withthe layersThe presence of H-bonding between the layers andthe o-HCn anions may bring the anions closer to the layersresulting in an increased electrostatic attraction resulting inthe process being controlled by reaction at the exchangesite Since there was no significant variation of the 119864
119886with
conversion for m-HCn and p-HCn release and the energydifference for o-HCn release is too low to reflect a change in
Advances in Physical Chemistry 9
mechanism model based approach was used to obtain rateconstants for the exchange reactions The Avrami-Erofersquoevnucleation-growth model [37 38] was used here since ithas the advantage that it can be used to describe reactionsoccurring via different mechanisms making comparison ofthe processes possible since the order of reactions will be thesame In the model the extent of reaction (120572) depends on therate constant 119896 and a coefficient 119899 as shown in the followingequation
119886 (119905) = 1 minus 119890[minus(119896119905)
119899
] (3)
The coefficient 119899 (Avrami exponent) potentially containsinformation about the mechanism of the reaction [39] TheAvrami-Erofersquoev model has been applied successfully to solu-tion phase anion exchange reactions in layeredmaterials andother fluid phase reactions [2 10 40 41] we have also shownthat values of parameters obtained from analysis of bothsolid state transformations and solution phase processesemploying the Avrami model are comparable indicating thatthere is correlation between solid state transformation andfluid phase processes [21] The Avrami exponent is usuallyobtained by taking double logarithm of the Avrami-Erofersquoevequation obtaining (4) which was popularized by Sharp andHancock in 1972 [42] Consider
A plot of ln[minus ln(1 minus 120572)] as a function of ln 119905 (doublelogarithmic plot) gives a linear plot in which the value of 119899 isobtained from the slope and the value of 119896 is evaluated fromthe intercept
The extent of reaction versus time plots obtained for theZC-o-HCnClminus exchange reactions at different temperaturesis shown in Figure 7(a) The Avrami-Erofersquoev model wasapplied and correctly described the obtained experimentaldatawithin the120572 range of 015ndash10The validity of theAvrami-Erofersquoevmodel can be confirmedby the corresponding doublelogarithmic plots in Figure 7(b) where straight lines wereobtained with 1198772 values ranging from 0996 to 0999 Thedouble logarithmic plots are almost parallel indicating thatthe reaction proceeds with the same mechanism over thetemperature range used here The obtained 119899 value was closeto 10 consistent with a nucleation controlled mechanismwhich agrees well with the activation energies obtained fromisoconversional methods and discussed before
The release profiles together with the double logarithmicplots for the release of m-HCn and p-HCn are shown in thesupporting information Figures S4 and S5 respectively Thekinetics of m-HCn release also follows the Avrami-Erofersquoevmodel the data provided a good fit within the range 015 lt120572 lt 085 the fit to limited range of 120572 has been observedin other intercalation reactions Anomalously low values ofthe Avrami exponent observed here (shown in Table 3) havebeen reported in polymer and alloy crystallization and donot have physical mechanistic meaning [43 44] The Avramiexponent values obtained for the release of p-HCn are closeto 05 which is consistent with diffusion controlled reactions[10] Diffusion control of such a high 119864
119886highlights two
0 2 4 6 8 10 12 1400
02
04
06
08
10
Time (hours)
Exte
nt o
f rea
ctio
n (120572
)
300∘C400∘C500∘C600∘C
(a)
4 6 8 10
0
1
minus1
minus2
ln (t minus t0)
lnminusln(1
minus120572)
30∘C40∘C50∘C60∘C
(b)
Figure 7 Extent of reaction as a function of time (a) and doublelogarithmic plots (b) for the exchange reaction of Clminus anion andZC-o-HCn at various temperatures 60∘C (◻) 50∘C (Q) 40∘C (998771)and 30∘C (e)
issues (a) the interlayer space is densely packed and thediffusional resistance within the interlayer space approachesthat in polymers in which119864
119886values can be above 100 kJmolminus1
depending on the diffusing molecule [36] and (b) Kineticambiguity is common in model based approaches in thiscase the mechanistic interpretation from the model is not inline with the kinetic parameters obtained
FromTable 3 it can be concluded that the rate of release ofm-HCn from the nanohybrids is much faster compared with
10 Advances in Physical Chemistry
Table 3 A summary of kinetic parameters obtained at different temperatures
Nanohybrid Temperature (∘C) 119899 119896 sminus1 (lowast10minus3)
ZC-o-HCn
600 082 plusmn 002 17 plusmn 03
500 090 plusmn 002 07 plusmn 01
400 088 plusmn 001 029 plusmn 005
300 102 plusmn 001 010 plusmn 001
ZC-m-HCn
600 024 plusmn 001 31 plusmn 9
500 024 plusmn 001 20 plusmn 7
400 021 plusmn 002 7 plusmn 2
300 0225 plusmn 0007 35 plusmn 08
ZC-p-HCn
600 041 plusmn 001 29 plusmn 05
500 045 plusmn 002 16 plusmn 04
400 046 plusmn 001 10 plusmn 02
300 039 plusmn 001 05 plusmn 01
the other isomers As an example the rate constants at 40∘Cwere (7 plusmn 2) times 10minus3 sminus1 for m-HCn (10 plusmn 02) times 10minus3 sminus1 forp-HCn and (029 plusmn 005) times 10minus3 sminus1 for o-HCn The rate ofrelease follows the following order
119898ndashHCn gt 119901-HCn gt 119900-HCn
The release profile form-HCn at 40∘C (Figure 5(a)) showsthat there is a burst and rapid release of the intercalated anionin the first 10 minutes followed by a more sustained releasewith equilibrium being attained after 3 hours This burstrelease is observed for all the temperatures used here (FigureS2a in supporting information) and has been observed beforein anion exchange reactions in LDHs [10] The burst releaseobserved here is due to anion exchange as confirmed byPXRD results in Figure S3 The chloride phase representedby an asterisk in Figure S3 was significant at 30 seconds (theearliest we could sample) of ZC-m-HCn being in contact withthe chloride solution The initial slow and linear release inZC-o-HCn indicated in Figure 5(b) and confirmed by XRDprofiles in Figure S3 may be due to the increased stabilityof the HDS structure due to extensive hydrogen bonding asindicated by FTIR data in Figure 3 and the close arrangementof the anions implied by XRDprofiles in Figure 2(a)This firststage of release has a strong temperature dependence beingmore pronounced at low temperature and almost absent athigher temperatures (Figure S2b in supporting information)
Factors which may influence the rates of release includeinteractions between the metal hydroxide layers and theanions anion-anion interactions within the layers and in theexchange medium and solvent-anion interaction [10] It isexpected that these factors will have a complex effect on thereaction rates since they affect each isomer differently Thecomplex interplay of the abovementioned factors oftenmakeit difficult to come up with a simple relationship betweenthe rate of release and the aforementioned factors [10] It isimportant to note that these factors are expected to dependon the types and positions of chemical groups on the anionThepresence of hydrogen bondingwithin the interlayer spacereduces the Gibbs free energy of formation of the nanohy-brids and stabilizes the system [45]The rate of release of ionsthen depends on the differences in the energy of nanohybrid
and the barrier to reaction As indicated in Figure 3 there issignificant level of hydrogen bonding in ZC-o-HCn whichthen resulted in a close interaction with the layers Thehydrogen bond network in the nanohybrid and the closeinteraction of the anions with the layers stabilized the systemas compared to the other nanohybrids The nanohybrid inwhich these interactions are low is relatively less stable and isexpected to proceed faster to equilibrium this is consistentwith results obtained here The anions in the unstablelessstable nanohybrid prefer bulk solution (in which there isstabilization by solvation with water molecules) to galleryspace which may result in the burst and fast release observedin ZC-m-HCnThe modification of bioactive molecules withgroups capable of hydrogen bonding for example hydroxylgroups especially at positions which allow interactions withthe layers may enable tuning of the release behavior ofthese molecules from layered materials It is important tonote that the modification should be at positions whichdo not affect the bioactivity of the molecules Additionalexperiments including studies on other isomeric anions withgroups capable of forming hydrogen bonds and effect ofmetallayer composition will be helpful in further elucidating howthe position of substituents control the rate as well as theextent of reaction
4 Conclusions
Anion exchange reactions in hydroxy double salts are affectedby the structure of the intercalated anions and there is astrong interplay between thermodynamics and kinetics ofthe release reactions In the present study we have shownthat there is a significant effect of the position of substituentgroups on the rate and extent of release of isomers ofhydroxycinnamate While the release of the metaisomer wasinstantaneous and fast the orthoisomer release exhibiteda slower more sustained and complete release the orderof reaction rates was m-HCn gt p-HCn gt o-HCn withthe o-HCn nanohybrid showing an induction period Theposition of the hydroxyl groups resulted in differences inthe interlayer interactions of the anions with presence ofinterlayer hydrogen bonds affecting the rates of the reaction
Advances in Physical Chemistry 11
The formation of hydrogen bonds between the layers and theintercalated orthoisomer resulted in greater stabilization ofthe nanohybrid and resulted in the observed induction periodand slow release The extent of reaction was correlated withthe magnitude of dipole moments These results highlightthe potential of tuning the release behavior of intercalatedbioactive compounds which bring control into both theamount released per given time and the period for totalrelease
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
This work was supported by the National Science Foundation(CHE-0809751)
References
[1] S Yamanaka T Sako andM Hattori ldquoAnion exchange in basiccopper acetaterdquo Chemistry Letters pp 1869ndash1872 1989
[2] E Kandare and J M Hossenlopp ldquoHydroxy double salt anionexchange kinetics effects of precursor structure and anionsizetrdquo Journal of Physical Chemistry B vol 109 no 17 pp 8469ndash8475 2005
[3] P K Dutta and M Puri ldquoAnion exchange in lithium aluminatehydroxidesrdquo Journal of Physical Chemistry vol 93 no 1 pp376ndash381 1989
[4] B M Choudary V S Jaya B R Reddy M L Kantam M MRao and S S Madhavendra ldquoSynthesis characterization ionexchange and catalytic properties of nanobinary and ternarymetal oxyhydroxidesrdquo Chemistry of Materials vol 17 no 10pp 2740ndash2743 2005
[5] A M Fogg J S Dunn S-G Shyu D R Cary and D OrsquoHareldquoSelective ion-exchange intercalation of isomeric dicarboxylateanions into the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Chemistry of Materials vol 10 no 1 pp 351ndash355 1998[6] T Ikeda H Amoh and T Yasunaga ldquoStereoselective exchange
kinetics of L- and D-histidines for Cl- in the interlayerof a hydrotalcite-like compound by the chemical relaxationmethodrdquo Journal of the American Chemical Society vol 106 no20 pp 5772ndash5775 1984
[7] M Meyn K Beneke and G Lagaly ldquoAnion-exchange reactionsof hydroxy double saltsrdquo Inorganic Chemistry vol 32 no 7 pp1209ndash1215 1993
[8] V Ambrogi G Fardella G Grandolini L Perioli and M CTiralti ldquoIntercalation compounds of hydrotalcite-like anionicclays with anti-inflammatory agents II uptake of diclofenac fora controlled release formulationrdquo AAPS PharmSciTech vol 3no 3 p E26 2002
[9] M Z Bin Hussein Z Zainal A H Yahaya and D WV Foo ldquoControlled release of a plant growth regulator 120572-naphthaleneacetate from the lamella of Zn-Al-layered doublehydroxide nanocompositerdquo Journal of Controlled Release vol82 no 2-3 pp 417ndash427 2002
[10] A I Khan A Ragavan B Fong et al ldquoRecent developments inthe use of layered double hydroxides as host materials for the
storage and triggered release of functional anionsrdquo Industrialand Engineering Chemistry Research vol 48 no 23 pp 10196ndash10205 2009
[11] M Silion D Hritcu I M Jaba et al ldquoIn vitro and invivo behavior of ketoprofen intercalated into layered doublehydroxidesrdquo Journal of Materials Science vol 21 no 11 pp3009ndash3018 2010
[12] J-H Yang Y-S Han M Park T Park S-J Hwang andJ-H Choy ldquoNew inorganic-based drug delivery system ofindole-3-acetic acid-layered metal hydroxide nanohybrids withcontrolled release raterdquo Chemistry of Materials vol 19 no 10pp 2679ndash2685 2007
[13] L Lei R P Vijayan and D OrsquoHare ldquoPreferential anionexchange intercalation of pyridinecarboxylate and toluate iso-mers in the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Journal of Materials Chemistry vol 11 no 12 pp 3276ndash32802001
[14] B Lotsch F Millange R I Walton and D OrsquoHare ldquoSepara-tion of nucleoside monophosphates using preferential anionexchange intercalation in layered double hydroxidesrdquo Solid StateSciences vol 3 no 8 pp 883ndash886 2001
[15] H Tagaya N Sasaki H Morioka and J Kadokawa ldquoPrepara-tion of new inorganicmdashorganic layered compounds hydroxydouble salts and preferential intercalation of organic carboxylicacids into themrdquoMolecular Crystals and Liquid Crystals Scienceand Technology A vol 341 pp 413ndash418 2000
[16] P P Kumar A G Kalinichev and R J Kirkpatrick ldquoHydra-tion swelling interlayer structure and hydrogen bondingin organolayered double hydroxides insights from moleculardynamics simulation of citrate-intercalated hydrotalciterdquo Jour-nal of Physical Chemistry B vol 110 no 9 pp 3841ndash3844 2006
[17] S V Prasanna and P V Kamath ldquoAnion-exchange reactions oflayered double hydroxides interplay between coulombic andh-bonding interactionsrdquo Industrial and Engineering ChemistryResearch vol 48 no 13 pp 6315ndash6320 2009
[18] Z Gu A CThomas Z P Xu J H Campbell and G Q Lu ldquoInvitro sustained release of LMWH from MgAl-layered doublehydroxide nanohybridsrdquo Chemistry of Materials vol 20 no 11pp 3715ndash3722 2008
[19] J T Rajamathi S Britto and M Rajamathi ldquoSynthesis andanion exchange reactions of a layered Copper-Zincrdquo Journal ofChemical Sciences vol 117 no 6 pp 629ndash633 2005
[20] M Frisch G Trucks H Schlegel et al ldquoGaussian 98 RevisionA114rdquo 2002
[21] S Majoni and J M Hossenlopp ldquoAnion exchange kinetics ofnanodimensional layered metal hydroxides use of isoconver-sional analysisrdquo Journal of Physical Chemistry A vol 114 no 49pp 12858ndash12869 2010
[22] C Chouillet J-M Krafft C Louis and H Lauron-PernotldquoCharacterization of zinc hydroxynitrates by diffuse reflectanceinfrared spectroscopymdashstructural modifications during ther-mal treatmentrdquo Spectrochimica Acta A vol 60 no 3 pp 505ndash511 2004
[23] W Fujita and K Awaga ldquoMagnetic properties of Cu2(OH)
3(al-
kanecarboxylate) compounds drastic mod-ification withextension of the Alkyl chainrdquo Inorganic Chemistry vol 35 no7 pp 1915ndash1917 1996
[24] R Swisłocka M Kowczyk-Sadowy M Kalinowska and WLewandowski ldquoSpectroscopic (FT-IR FT-Raman1H and13CNMR) and theoretical studies of p-coumaric acid and alkalimetal p-coumaratesrdquo Spectroscopy vol 27 no 1 pp 35ndash48 2012
12 Advances in Physical Chemistry
[25] T Biswick W Jones A Pacuła E Serwicka and J PodobinskildquoThe role of anhydrous zinc nitrate in the thermal decomposi-tion of the zinc hydroxy nitrates Zn
5(OH)
8(NO3)2sdot2H2O and
ZnOHNO3sdotH2Ordquo Journal of Solid State Chemistry vol 180 no
4 pp 1171ndash1179 2007[26] S-H Park and E L Cheol ldquoSynthesis characterization and
magnetic properties of a novel disulfonate-pillared copperhydroxide Cu
2(OH)
3(DS4)
12 DS4 = 14-butanedisulfonaterdquo
Bulletin of the Korean Chemical Society vol 27 no 10 pp 1587ndash1592 2006
[27] P Rabu M Drillon and C Hornick ldquoStructural and spectro-scopic study of organic-inorganic transitionmetal based layeredmagnetsrdquo Analusis vol 28 no 2 pp 103ndash108 2000
[28] L Lei A Khan and D OrsquoHare ldquoSelective anion-exchangeintercalation of isomeric benzoate anions into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo Journal of Solid State
Chemistry vol 178 no 12 pp 3648ndash3654 2005[29] W Krause H-J Bernhardt R S W Braithwaite U Kolitsch
and R Pritchard ldquoKapellasite Cu3Zn(OH)
6Cl2 a new mineral
from Lavrion Greece and its crystal structurerdquo MineralogicalMagazine vol 70 no 3 pp 329ndash340 2006
[30] L C Andrews L R Bernstein C M Foris et al PowderDiffraction File Inorganic International Centre for DiffractionData Newtown Square Pennsylvania 2009
[31] A Ragavan A I Khan and D OrsquoHare ldquoIsomer selectiveion-exchange intercalation of nitrophenolates into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotxH
2Ordquo Journal of Materials
Chemistry vol 16 no 6 pp 602ndash608 2006[32] A Ragavan A Khan and D OrsquoHare ldquoSelective intercalation
of chlorophenoxyacetates into the layered double hydroxide[LiAl2(OH)
6]ClsdotxH
2Ordquo Journal of Materials Chemistry vol 16
no 42 pp 4155ndash4159 2006[33] S Vyazovkin and C A Wight ldquoModel-free and model-fitting
approaches to kinetic analysis of isothermal and nonisothermaldatardquoThermochimica Acta vol 340-341 pp 53ndash68 1999
[34] R E Johnsen F Krumeich and P Norby ldquoStructural andmicrostructural changes during anion exchange ofCoAl layereddouble hydroxides an in situ X-ray powder diffraction studyrdquoJournal of Applied Crystallography vol 43 no 3 pp 434ndash4472010
[35] G R Williams T G Dunbar A J Beer A M Fogg and DOrsquoHare ldquoIntercalation chemistry of the novel layered doublehydroxides [MAl
4(OH)
12](NO
3)2sdotyH2O (M=Zn Cu Ni and
Co) 1 new organic intercalates and reaction mechanismsrdquoJournal of Materials Chemistry vol 16 no 13 pp 1222ndash12302006
[36] T E M Ten Hulscher and G Cornelissen ldquoEffect of tempera-ture on sorption equilibrium and sorption kinetics of organicmicropollutantsmdasha reviewrdquo Chemosphere vol 32 no 4 pp609ndash626 1996
[37] M Avrami ldquoKinetics of phase change I general theoryrdquo TheJournal of Chemical Physics vol 7 no 12 pp 1103ndash1112 1939
[38] M Avrami ldquoKinetics of phase change II Transformation-timerelations for random distribution of nucleirdquo The Journal ofChemical Physics vol 8 no 2 pp 212ndash224 1940
[39] R J Francis S OrsquoBrien A M Fogg et al ldquoTime-resolved in-situ energy and angular dispersive X-ray diffraction studies ofthe formation of themicroporous gallophosphateULM-5 underhydrothermal conditionsrdquo Journal of the American ChemicalSociety vol 121 no 5 pp 1002ndash1015 1999
[40] Z Miladinovic J Zakrzewska B Kovacevic and G BacicldquoMonitoring of crystallization processes during synthesis of
zeolite A by in situ 27Al NMR spectroscopyrdquo Materials Chem-istry and Physics vol 104 no 2-3 pp 384ndash389 2007
[41] R Serna-Guerrero and A Sayari ldquoModeling adsorption of CO2
on amine-functionalized mesoporous silica 2 kinetics andbreakthrough curvesrdquo Chemical Engineering Journal vol 161no 1-2 pp 182ndash190 2010
[42] J D Hancock and J H Sharp ldquoMethod of comparing solid-state kinetic data and its application to the decomposition ofkaolinite brucite and BaCO
3rdquo Journal of the American Ceramic
Society vol 55 no 2 pp 74ndash77 1972[43] F Auriemma C De Rosa and R Triolo ldquoSlow crystallization
kinetics of poly(vinyl alcohol) in confined environment duringcryotropic gelation of aqueous solutionsrdquo Macromolecules vol39 no 26 pp 9429ndash9434 2006
[44] R V Ramanujan and Y R Zhang ldquoQuantitative transmissionelectronmicroscopy analysis of the nanocrystallization kineticsof softmagnetic alloysrdquoPhysical ReviewB vol 74 no 22 ArticleID 224408 2006
[45] A G Kalinichev P Padma Kumar and R James KirkpatrickldquoMolecular dynamics computer simulations of the effects ofhydrogen bonding on the properties of layered double hydrox-ides intercalated with organic acidsrdquo Philosophical Magazinevol 90 no 17-18 pp 2475ndash2488 2010
Figure 2 (a) PXRD profiles for ZC-Ac and exchange products and (b) FTIR spectra for ZC-Ac and exchange products
planes in the lattice and 120579 is the angle between the incidentray and the scattering planes) The Bragg reflections fromthe precursor material are no longer present in the PXRDtraces of the exchange products indicating that the exchangewas complete As revealed in Figure 2(a) there is an increasein the basal spacing as the acetate anion is replaced by n-HCn ions in the interlayer space The increase in the basalspace is consistent with a smaller anion (acetate ion chainlength = 165 A) being replaced by largern-HCnanions (chainlengths = 863 A for o-HCn 864 A for m-HCn and 911 Afor p-HCn) Considering the chain lengths of the meta-and paraisomers and the size of the gallery height (about16 A if the layer thickness is approximated to be 5 A as incopper hydroxide) [23] the anions are likely to be arrangedin a slightly tilted bilayer orientation The interlayer spaceobserved in ZC-o-HCn is significantly smaller comparedwith the other nanohybrids It was expected that the 119889-spacings would be comparable considering the similarities inthe sizes of the isomers The smaller 119889-spacing in ZC-o-HCn
might be a result of the orthoisomer being in a more tiltedorientation as compared to the other isomers
IR spectra presented in Figure 2(b) show that whenacetate ions were replaced by n-HCn ions the peaks due toC=O vibrations of acetate ions in ZC-Ac (120592asym = 1563 cmminus1
and 120592sym = 1410 cmminus1) [21] were replaced by a series ofpeaks (1637ndash1642 cmminus1 C=C 1540ndash1551 cmminus1 120592asym C=Oand 1399ndash1428 cmminus1 120592sym C=O)The disappearance of vibra-tion peaks from acetate ions is consistent with results inferredfrom PXRD analysis The absence of the C=O stretchingvibration of protonated carboxylic groups which is around1700 cmminus1 confirms that the isomers are present in the ionizedformThe full assignment of the IR peaks is found in Table S2in supplementary information [24]
The hydroxyl stretching vibrational modes (in the region3000 cmminus1ndash3600 cmminus1) provide information about the inter-actions in the interlayer space [25] The structural proper-ties of hydroxyl groups within the interlayer space can be
Advances in Physical Chemistry 5
96
97
98
99
100
3450
T (
)
ZC-Ac
4000 3800 3600 3400 3200 3000 2800 2600
Wavenumber (cmminus1)
(a)
96
97
98
99
100
3131
3489
ZC-o-HCn
4000 3800 3600 3400 3200 3000 2800 2600
Wavenumber (cmminus1)
T (
)
(b)
4000 3800 3600 3400 3200 3000 2800 260094
96
98
100
3578
3513
ZC-m-HCn
Wavenumber (cmminus1)
T (
)
(c)
970
975
980
985
990
995
3497
3574
ZC-p-HCn
4000 3800 3600 3400 3200 3000 2800 2600
Wavenumber (cmminus1)
T (
)
(d)
Figure 3 FTIR spectra for ZC-Ac and exchange products showing the hydroxyl stretching region
correlated to the OH vibrational modes Broad absorptionbands are associated with stretching modes of hydrogenbonded hydroxyl groups and sharp bands associated withfree hydroxyl groups [26] The FTIR profiles of the hydroxylstretching region of ZC-Ac and the exchange products areshown in Figure 3 ZC-Ac has extensive interlayer hydrogenbonding due to intercalated water molecules as reportedin our earlier publication [21] Figure 3 shows that whenacetate ions were replaced by m-HCn ions in ZC-m-HCnthe broad peak (FWHM = 359 cmminus1) centered at 3450 cmminus1in ZC-Ac was replaced by 2 sharp peaks a strong peakat 3513 cmminus1 (FWHM = 148 cmminus1) and a weak peak at3574 cmminus1 (FWHM = 163 cmminus1) The two sharp peaks aredue to layer (3574 cmminus1) and m-HCn (3513 cmminus1) hydroxylgroups which are not involved in substantial hydrogenbonding [25] This assignment is based on our previousresults which showed that the peak at 3513 cmminus1 was absentin HDSs containing cinnamate anions (no OH substitutionon the benzene ring) [21] The shift of the peaks to higherwavenumbers is also indicative of hydroxyl groups which arenot substantially involved in hydrogen bonding ZC-o-HCnhas a very weak peak around 3600 cmminus1 and 2 broad peakscentered at 3489 cmminus1 (FWHM = 880 cmminus1) and 3131 cmminus1(FWHM = 376 cmminus1) The broadening of peaks and the shiftto lower wavenumbers as compared to vibrations in ZC-m-HCn and free OH stretching vibrations is consistent with
hydrogen bonded hydroxyl groups [27] The low intensity ofthe sharp band at 3600 cmminus1 is indicative of extensive hydro-gen bonding between layer OH groups and o-HCn hydroxylgroups resulting in low density of free layer OH groupsIn ZC-p-HCn there is a strong sharp peak at 3574 cmminus1(FWHM = 72 cmminus1) which is due to layer OH groups whichare not involved in significant hydrogen bonding and abroad peak at 3497 cmminus1 (FWHM = 119 cmminus1) due to p-HCnhydroxyl groups involved in hydrogen bonding
The absence of strong sharp peaks in ZC-o-HCn com-pared with those observed for ZC-m-HCn and ZC-p-HCnindicates that there is high level of hydrogen bonding in ZC-o-HCn relative to the other nanohybrids Since hydrogenbonding is directional the m-HCn anions are not orientedin a way that the OH groups are able to interact with eitherthe layer OH groups or groups from other anions Theamount of water in the nanohybrids is low as indicated fromthermogravimetry and elemental analyses Therefore havingeliminated interlayer water as a likely hydrogen bonding par-ticipant the significant hydrogen bonding observed in ZC-o-HCn andZC-p-HCn ismost likely intermolecular or betweenanion and layer hydroxyl groups In ZC-o-HCn the o-HCnhydroxyl group may be in an orientation such that they areclose to the layers enabling formation of hydrogen bondswiththe layer OH groups [28] The interaction between o-HCnand layer hydroxyl groups may have resulted in the o-HCn
6 Advances in Physical Chemistry
Table 2 Summary of dipole moments and release data
Anion Equilibrium isomer released at 40∘C () Dipole moment Calculated chargesCarbon Oxygen 1 Oxygen 2
Figure 4 PXRD profile of a representative ZC-Cl (kapellasite)obtained from exchange reactions (ZC-o-HCnClminus reaction) Millerhkl indices for selected Clminus reflections are given PDF no 56ndash71
anion being tilted closer to the layers which may explain thesmaller gallery height (as compared to that inZC-m-HCn andZC-p-HCn) observed from PXRD analysis In ZC-p-HCnOH groups may be oriented in a way so as to participate inintermolecular H-bonding between anions
32 Kinetic Analysis The release of n-HCn from nanohy-brids was achieved by ion exchange using Clminus as exchangeanions After specified times supernatants were collected andanalyzed for released anions and the residues were analyzedfor solid state transformations using PXRD Analysis of thesolid samples recovered from the reaction mixtures aftereach contact time indicated that the same chloride phasewas obtained for all the nanohybrids A representative PXRDprofile for the zinc copper chloride (ZC-Cl) obtained fromexchange reactions is shown in Figure 4 The chloride phasehas a 119889-spacing of 574 plusmn 001 A which is in close agreementwith the literature value of 573 A [29] and has been indexedas kapellasite Cu
3Zn(OH)
6Cl2(PDF no 56ndash71) [28 30] The
decrease of the interlayer space as the isomers are released isconsistent with large anions being replaced by smaller ions
The concentrations of released anions were evaluatedusing UV-Vis spectroscopy The fractions of released isomersas a function of time (release profiles) at 40∘C are shown inFigures 5(a)ndash5(c) for m-HCn o-HCn and p-HCn respec-tively The release profiles at other temperatures show thesame general trend and are presented in Figure S2 in thesupporting information Although the chemical structures of
the isomers are similar and the release medium is thesame the release profiles and hence release properties aresignificantly different (Figure 5) The equilibrium amountsreleased at 40∘C together with calculated dipole momentsand selected atomic charges are shown in Table 2 As indi-cated in Table 2 the equilibrium amounts released from thenanohybrids were significantly different ZC-o-HCn released100 of the intercalated ions while ZC-m-HCn releasedabout 40 and ZC-p-HCn released about 22 While ZC-o-HCn showed complete release at all temperatures the extentof reaction for the release ofm-HCn and p-HCn showed tem-perature dependence (increasing with temperature as shownin Figure S2) The extents of reactions at 40∘C are confirmedby XRD data in Figure S3 in supporting information
The differences in the extent of reaction may indicatedifferences in the affinity of the HDS for the isomers anddifferences in the thermodynamic equilibrium constants ofthe systemsThe extent of anion exchange depends on severalfactors which include the solvation enthalpy of the isomers(in the bulk liquid and in the gallery) the binding enthalpy(between themetal hydroxide layers and the isomers) and theinter-intramolecular interactions of the anions within thelayers [4 28 31]Themajor contributing factor is expected tobe the electrostatic interaction between the positively chargedmetal hydroxide layers and the negatively charged anionswith the carboxylate group of the isomers being in closecontact with the layers Since the isomers have the samecharge and comparable size differences in dipole moments(an indicator of the charge distribution) could have moreinfluence on the thermodynamics of the release processThe magnitude of dipole moments has been shown to becorrelatedwith selectivity in intercalation reactions of layereddouble hydroxides [4 28 31] The affinity of the metalhydroxide layer is higher for the isomer with the highestdipole moment [13] therefore this isomer is expected to beretained within the layers more than the other isomers
The equilibrium amounts of anion released from thenanohybrids follow the following order o-HCn gt m-HCngt p-HCn which is opposite to that of the magnitude ofdipole moments as shown in Table 2 The order of isomerselectivity in intercalation reactions is as expected thereverse of the order of equilibrium amount release observedhere Since p-HCn has the highest dipole moments the metalhydroxide layers are expected to have the greatest affinityfor p-HCn as compared to the other isomers [13] this couldexplain the order observed here Although factors whichinfluence the extent of release are expected to be complexdue to the processes involved in anion exchange reactions[32] the results obtained here infer that the magnitudeof dipole moments plays a significant role in determining
Advances in Physical Chemistry 7
0 4 8 12 16
0
10
20
30
40
Frac
tion
rele
ased
()
Time (hours)
(a)
0 5 10 150
20
40
60
80
Frac
tion
rele
ased
()
Time (hours)
(b)
0 1 2 3 4 5
0
4
8
12
16
20
Frac
tion
rele
ased
()
Time (hours)
(c)
Figure 5 Release profiles for (a) ZC-m-HCn (b) ZC-o-HCn and (c) ZC-p-HCn at 40∘C
the equilibrium constant and hence the amount released atequilibrium The complex nature of the exchange reactionsis highlighted when our previous results for the release ofcinnamate anion are considered [21] Cinnamate anion hasa dipole moment of 146 D which is lower than that of m-HCn but there was only 31 Cn released at equilibriumThis indicates a complex interplay of the involved factors asit has been shown that highly hydrophobic anions are easyto intercalate into the layers Due to the absence of hydroxylgroup on the benzene ring of Cn anion this isomer is morehydrophobic than the others and is therefore not readilydeintercalated into the polar aqueous solvent environment
From Figure 5 it can qualitatively be observed that therelease rates and the release profiles of the isomers aresignificantly different While the profiles for o-HCn releaseare sigmoidal those for the meta- and paraisomers aredeceleratory Reaction rates can be quantified by fittingexperimental data to reaction models which best describe(reproduce) the experimental data in the so-called model
fitting procedureThis procedure is applicable to singlemech-anism reactions in which the mechanism does not change asthe reaction progresses [33] We have shown previously thatisoconversional methods can be extended to anion exchangereactions in layered metal hydroxide as a strategy to identifywhen using model based approaches are appropriate [21] Aconstant 119864
119886as a function of extent of reaction indicates that
the reaction may effectively be characterized with a singlemechanism and in these cases model based procedures areapplicable [33]The integral isoconversional method (2) wasapplied to the kinetic data obtained here
ln 119905 = ln119892 (120572)
119860+119864119886
119877119879 (2)
In (2) 119864119886is the activation energy 120572 is the extent of
reaction 119860 is the preexponential factor 119905 is the time 119879 thetemperature 119877 is the molar gas constant and 119892(120572) is theintegral reactionmodelThe isoconversional analysis data forall the nanohybrids is shown in Figure 6 in which the 119864
119886
8 Advances in Physical Chemistry
02 04 06 08 1065
70
75
80
85ZC-o-HCn
Extent of reaction (120572)
Ea
(kJm
ol)
(a)
03 04 05 06 07 08 09
30
40
50
60ZC-p-HCn
Extent of reaction (120572)
Ea
(kJm
ol)
(b)
03 04 05 06 07 08 09 10
50
60
70
80ZC-m-HCn
Extent of reaction (120572)
Ea
(kJ m
olminus1)
(c)
Figure 6 Variation of effective 119864119886with 120572 for ZC-m-HCn ZC-o-HCn and ZC-p-HCn
values remains constant within experimental error over theentire conversion range for ZC-m-HCn and ZC-p-HCn
The 119864119886values for ZC-o-HCn are within 10 although
the changes at higher conversion appear significant comparedwith experimental uncertainty The linear decrease of the 119864
119886
at higher conversion above 120572 = 05 has been attributed toweakening of the electrostatic attraction between the anionsand the layers as the anions are progressively exchanged fromlayer to layer [34] The constant 119864
119886at low conversion may
be due to some solid state transformations occurring whichmight be responsible for the induction period observedin the PXRD analysis (Figure S3) Although the inductionperiod is short the process might still be governing theenergetics of the reaction even during the exchange periodresulting in the constant energy being observed up to extentof reaction of 05 Since 119864
119886values for exchange reactions at
the anion binding sites have been shown to range from 30to 70 kJmolminus1 depending on the nature and size of the guestanions [31 32 35] the reactionmay be controlled by chemical
reaction at the exchange site It is also important to note thatdiffusion of anions in a dense organicmatrix can have high119864
119886
values sometimes being more than 100 kJmolminus1 [36] as suchdiffusion limited process cannot be ruled out if the anions areclosely packed in the gallery
The 119864119886values for the release of m-HCn and p-HCn
are in the range of both reaction controlled process anddiffusion controlled processes [21] The fact that the 119864
119886does
not significantly change over the entire reaction may indicatethat the reaction is diffusion controlled The differences inthe mechanism between o-HCn release and the other twosystems may be due to different strength of interactions withthe layersThe presence of H-bonding between the layers andthe o-HCn anions may bring the anions closer to the layersresulting in an increased electrostatic attraction resulting inthe process being controlled by reaction at the exchangesite Since there was no significant variation of the 119864
119886with
conversion for m-HCn and p-HCn release and the energydifference for o-HCn release is too low to reflect a change in
Advances in Physical Chemistry 9
mechanism model based approach was used to obtain rateconstants for the exchange reactions The Avrami-Erofersquoevnucleation-growth model [37 38] was used here since ithas the advantage that it can be used to describe reactionsoccurring via different mechanisms making comparison ofthe processes possible since the order of reactions will be thesame In the model the extent of reaction (120572) depends on therate constant 119896 and a coefficient 119899 as shown in the followingequation
119886 (119905) = 1 minus 119890[minus(119896119905)
119899
] (3)
The coefficient 119899 (Avrami exponent) potentially containsinformation about the mechanism of the reaction [39] TheAvrami-Erofersquoev model has been applied successfully to solu-tion phase anion exchange reactions in layeredmaterials andother fluid phase reactions [2 10 40 41] we have also shownthat values of parameters obtained from analysis of bothsolid state transformations and solution phase processesemploying the Avrami model are comparable indicating thatthere is correlation between solid state transformation andfluid phase processes [21] The Avrami exponent is usuallyobtained by taking double logarithm of the Avrami-Erofersquoevequation obtaining (4) which was popularized by Sharp andHancock in 1972 [42] Consider
A plot of ln[minus ln(1 minus 120572)] as a function of ln 119905 (doublelogarithmic plot) gives a linear plot in which the value of 119899 isobtained from the slope and the value of 119896 is evaluated fromthe intercept
The extent of reaction versus time plots obtained for theZC-o-HCnClminus exchange reactions at different temperaturesis shown in Figure 7(a) The Avrami-Erofersquoev model wasapplied and correctly described the obtained experimentaldatawithin the120572 range of 015ndash10The validity of theAvrami-Erofersquoevmodel can be confirmedby the corresponding doublelogarithmic plots in Figure 7(b) where straight lines wereobtained with 1198772 values ranging from 0996 to 0999 Thedouble logarithmic plots are almost parallel indicating thatthe reaction proceeds with the same mechanism over thetemperature range used here The obtained 119899 value was closeto 10 consistent with a nucleation controlled mechanismwhich agrees well with the activation energies obtained fromisoconversional methods and discussed before
The release profiles together with the double logarithmicplots for the release of m-HCn and p-HCn are shown in thesupporting information Figures S4 and S5 respectively Thekinetics of m-HCn release also follows the Avrami-Erofersquoevmodel the data provided a good fit within the range 015 lt120572 lt 085 the fit to limited range of 120572 has been observedin other intercalation reactions Anomalously low values ofthe Avrami exponent observed here (shown in Table 3) havebeen reported in polymer and alloy crystallization and donot have physical mechanistic meaning [43 44] The Avramiexponent values obtained for the release of p-HCn are closeto 05 which is consistent with diffusion controlled reactions[10] Diffusion control of such a high 119864
119886highlights two
0 2 4 6 8 10 12 1400
02
04
06
08
10
Time (hours)
Exte
nt o
f rea
ctio
n (120572
)
300∘C400∘C500∘C600∘C
(a)
4 6 8 10
0
1
minus1
minus2
ln (t minus t0)
lnminusln(1
minus120572)
30∘C40∘C50∘C60∘C
(b)
Figure 7 Extent of reaction as a function of time (a) and doublelogarithmic plots (b) for the exchange reaction of Clminus anion andZC-o-HCn at various temperatures 60∘C (◻) 50∘C (Q) 40∘C (998771)and 30∘C (e)
issues (a) the interlayer space is densely packed and thediffusional resistance within the interlayer space approachesthat in polymers in which119864
119886values can be above 100 kJmolminus1
depending on the diffusing molecule [36] and (b) Kineticambiguity is common in model based approaches in thiscase the mechanistic interpretation from the model is not inline with the kinetic parameters obtained
FromTable 3 it can be concluded that the rate of release ofm-HCn from the nanohybrids is much faster compared with
10 Advances in Physical Chemistry
Table 3 A summary of kinetic parameters obtained at different temperatures
Nanohybrid Temperature (∘C) 119899 119896 sminus1 (lowast10minus3)
ZC-o-HCn
600 082 plusmn 002 17 plusmn 03
500 090 plusmn 002 07 plusmn 01
400 088 plusmn 001 029 plusmn 005
300 102 plusmn 001 010 plusmn 001
ZC-m-HCn
600 024 plusmn 001 31 plusmn 9
500 024 plusmn 001 20 plusmn 7
400 021 plusmn 002 7 plusmn 2
300 0225 plusmn 0007 35 plusmn 08
ZC-p-HCn
600 041 plusmn 001 29 plusmn 05
500 045 plusmn 002 16 plusmn 04
400 046 plusmn 001 10 plusmn 02
300 039 plusmn 001 05 plusmn 01
the other isomers As an example the rate constants at 40∘Cwere (7 plusmn 2) times 10minus3 sminus1 for m-HCn (10 plusmn 02) times 10minus3 sminus1 forp-HCn and (029 plusmn 005) times 10minus3 sminus1 for o-HCn The rate ofrelease follows the following order
119898ndashHCn gt 119901-HCn gt 119900-HCn
The release profile form-HCn at 40∘C (Figure 5(a)) showsthat there is a burst and rapid release of the intercalated anionin the first 10 minutes followed by a more sustained releasewith equilibrium being attained after 3 hours This burstrelease is observed for all the temperatures used here (FigureS2a in supporting information) and has been observed beforein anion exchange reactions in LDHs [10] The burst releaseobserved here is due to anion exchange as confirmed byPXRD results in Figure S3 The chloride phase representedby an asterisk in Figure S3 was significant at 30 seconds (theearliest we could sample) of ZC-m-HCn being in contact withthe chloride solution The initial slow and linear release inZC-o-HCn indicated in Figure 5(b) and confirmed by XRDprofiles in Figure S3 may be due to the increased stabilityof the HDS structure due to extensive hydrogen bonding asindicated by FTIR data in Figure 3 and the close arrangementof the anions implied by XRDprofiles in Figure 2(a)This firststage of release has a strong temperature dependence beingmore pronounced at low temperature and almost absent athigher temperatures (Figure S2b in supporting information)
Factors which may influence the rates of release includeinteractions between the metal hydroxide layers and theanions anion-anion interactions within the layers and in theexchange medium and solvent-anion interaction [10] It isexpected that these factors will have a complex effect on thereaction rates since they affect each isomer differently Thecomplex interplay of the abovementioned factors oftenmakeit difficult to come up with a simple relationship betweenthe rate of release and the aforementioned factors [10] It isimportant to note that these factors are expected to dependon the types and positions of chemical groups on the anionThepresence of hydrogen bondingwithin the interlayer spacereduces the Gibbs free energy of formation of the nanohy-brids and stabilizes the system [45]The rate of release of ionsthen depends on the differences in the energy of nanohybrid
and the barrier to reaction As indicated in Figure 3 there issignificant level of hydrogen bonding in ZC-o-HCn whichthen resulted in a close interaction with the layers Thehydrogen bond network in the nanohybrid and the closeinteraction of the anions with the layers stabilized the systemas compared to the other nanohybrids The nanohybrid inwhich these interactions are low is relatively less stable and isexpected to proceed faster to equilibrium this is consistentwith results obtained here The anions in the unstablelessstable nanohybrid prefer bulk solution (in which there isstabilization by solvation with water molecules) to galleryspace which may result in the burst and fast release observedin ZC-m-HCnThe modification of bioactive molecules withgroups capable of hydrogen bonding for example hydroxylgroups especially at positions which allow interactions withthe layers may enable tuning of the release behavior ofthese molecules from layered materials It is important tonote that the modification should be at positions whichdo not affect the bioactivity of the molecules Additionalexperiments including studies on other isomeric anions withgroups capable of forming hydrogen bonds and effect ofmetallayer composition will be helpful in further elucidating howthe position of substituents control the rate as well as theextent of reaction
4 Conclusions
Anion exchange reactions in hydroxy double salts are affectedby the structure of the intercalated anions and there is astrong interplay between thermodynamics and kinetics ofthe release reactions In the present study we have shownthat there is a significant effect of the position of substituentgroups on the rate and extent of release of isomers ofhydroxycinnamate While the release of the metaisomer wasinstantaneous and fast the orthoisomer release exhibiteda slower more sustained and complete release the orderof reaction rates was m-HCn gt p-HCn gt o-HCn withthe o-HCn nanohybrid showing an induction period Theposition of the hydroxyl groups resulted in differences inthe interlayer interactions of the anions with presence ofinterlayer hydrogen bonds affecting the rates of the reaction
Advances in Physical Chemistry 11
The formation of hydrogen bonds between the layers and theintercalated orthoisomer resulted in greater stabilization ofthe nanohybrid and resulted in the observed induction periodand slow release The extent of reaction was correlated withthe magnitude of dipole moments These results highlightthe potential of tuning the release behavior of intercalatedbioactive compounds which bring control into both theamount released per given time and the period for totalrelease
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
This work was supported by the National Science Foundation(CHE-0809751)
References
[1] S Yamanaka T Sako andM Hattori ldquoAnion exchange in basiccopper acetaterdquo Chemistry Letters pp 1869ndash1872 1989
[2] E Kandare and J M Hossenlopp ldquoHydroxy double salt anionexchange kinetics effects of precursor structure and anionsizetrdquo Journal of Physical Chemistry B vol 109 no 17 pp 8469ndash8475 2005
[3] P K Dutta and M Puri ldquoAnion exchange in lithium aluminatehydroxidesrdquo Journal of Physical Chemistry vol 93 no 1 pp376ndash381 1989
[4] B M Choudary V S Jaya B R Reddy M L Kantam M MRao and S S Madhavendra ldquoSynthesis characterization ionexchange and catalytic properties of nanobinary and ternarymetal oxyhydroxidesrdquo Chemistry of Materials vol 17 no 10pp 2740ndash2743 2005
[5] A M Fogg J S Dunn S-G Shyu D R Cary and D OrsquoHareldquoSelective ion-exchange intercalation of isomeric dicarboxylateanions into the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Chemistry of Materials vol 10 no 1 pp 351ndash355 1998[6] T Ikeda H Amoh and T Yasunaga ldquoStereoselective exchange
kinetics of L- and D-histidines for Cl- in the interlayerof a hydrotalcite-like compound by the chemical relaxationmethodrdquo Journal of the American Chemical Society vol 106 no20 pp 5772ndash5775 1984
[7] M Meyn K Beneke and G Lagaly ldquoAnion-exchange reactionsof hydroxy double saltsrdquo Inorganic Chemistry vol 32 no 7 pp1209ndash1215 1993
[8] V Ambrogi G Fardella G Grandolini L Perioli and M CTiralti ldquoIntercalation compounds of hydrotalcite-like anionicclays with anti-inflammatory agents II uptake of diclofenac fora controlled release formulationrdquo AAPS PharmSciTech vol 3no 3 p E26 2002
[9] M Z Bin Hussein Z Zainal A H Yahaya and D WV Foo ldquoControlled release of a plant growth regulator 120572-naphthaleneacetate from the lamella of Zn-Al-layered doublehydroxide nanocompositerdquo Journal of Controlled Release vol82 no 2-3 pp 417ndash427 2002
[10] A I Khan A Ragavan B Fong et al ldquoRecent developments inthe use of layered double hydroxides as host materials for the
storage and triggered release of functional anionsrdquo Industrialand Engineering Chemistry Research vol 48 no 23 pp 10196ndash10205 2009
[11] M Silion D Hritcu I M Jaba et al ldquoIn vitro and invivo behavior of ketoprofen intercalated into layered doublehydroxidesrdquo Journal of Materials Science vol 21 no 11 pp3009ndash3018 2010
[12] J-H Yang Y-S Han M Park T Park S-J Hwang andJ-H Choy ldquoNew inorganic-based drug delivery system ofindole-3-acetic acid-layered metal hydroxide nanohybrids withcontrolled release raterdquo Chemistry of Materials vol 19 no 10pp 2679ndash2685 2007
[13] L Lei R P Vijayan and D OrsquoHare ldquoPreferential anionexchange intercalation of pyridinecarboxylate and toluate iso-mers in the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Journal of Materials Chemistry vol 11 no 12 pp 3276ndash32802001
[14] B Lotsch F Millange R I Walton and D OrsquoHare ldquoSepara-tion of nucleoside monophosphates using preferential anionexchange intercalation in layered double hydroxidesrdquo Solid StateSciences vol 3 no 8 pp 883ndash886 2001
[15] H Tagaya N Sasaki H Morioka and J Kadokawa ldquoPrepara-tion of new inorganicmdashorganic layered compounds hydroxydouble salts and preferential intercalation of organic carboxylicacids into themrdquoMolecular Crystals and Liquid Crystals Scienceand Technology A vol 341 pp 413ndash418 2000
[16] P P Kumar A G Kalinichev and R J Kirkpatrick ldquoHydra-tion swelling interlayer structure and hydrogen bondingin organolayered double hydroxides insights from moleculardynamics simulation of citrate-intercalated hydrotalciterdquo Jour-nal of Physical Chemistry B vol 110 no 9 pp 3841ndash3844 2006
[17] S V Prasanna and P V Kamath ldquoAnion-exchange reactions oflayered double hydroxides interplay between coulombic andh-bonding interactionsrdquo Industrial and Engineering ChemistryResearch vol 48 no 13 pp 6315ndash6320 2009
[18] Z Gu A CThomas Z P Xu J H Campbell and G Q Lu ldquoInvitro sustained release of LMWH from MgAl-layered doublehydroxide nanohybridsrdquo Chemistry of Materials vol 20 no 11pp 3715ndash3722 2008
[19] J T Rajamathi S Britto and M Rajamathi ldquoSynthesis andanion exchange reactions of a layered Copper-Zincrdquo Journal ofChemical Sciences vol 117 no 6 pp 629ndash633 2005
[20] M Frisch G Trucks H Schlegel et al ldquoGaussian 98 RevisionA114rdquo 2002
[21] S Majoni and J M Hossenlopp ldquoAnion exchange kinetics ofnanodimensional layered metal hydroxides use of isoconver-sional analysisrdquo Journal of Physical Chemistry A vol 114 no 49pp 12858ndash12869 2010
[22] C Chouillet J-M Krafft C Louis and H Lauron-PernotldquoCharacterization of zinc hydroxynitrates by diffuse reflectanceinfrared spectroscopymdashstructural modifications during ther-mal treatmentrdquo Spectrochimica Acta A vol 60 no 3 pp 505ndash511 2004
[23] W Fujita and K Awaga ldquoMagnetic properties of Cu2(OH)
3(al-
kanecarboxylate) compounds drastic mod-ification withextension of the Alkyl chainrdquo Inorganic Chemistry vol 35 no7 pp 1915ndash1917 1996
[24] R Swisłocka M Kowczyk-Sadowy M Kalinowska and WLewandowski ldquoSpectroscopic (FT-IR FT-Raman1H and13CNMR) and theoretical studies of p-coumaric acid and alkalimetal p-coumaratesrdquo Spectroscopy vol 27 no 1 pp 35ndash48 2012
12 Advances in Physical Chemistry
[25] T Biswick W Jones A Pacuła E Serwicka and J PodobinskildquoThe role of anhydrous zinc nitrate in the thermal decomposi-tion of the zinc hydroxy nitrates Zn
5(OH)
8(NO3)2sdot2H2O and
ZnOHNO3sdotH2Ordquo Journal of Solid State Chemistry vol 180 no
4 pp 1171ndash1179 2007[26] S-H Park and E L Cheol ldquoSynthesis characterization and
magnetic properties of a novel disulfonate-pillared copperhydroxide Cu
2(OH)
3(DS4)
12 DS4 = 14-butanedisulfonaterdquo
Bulletin of the Korean Chemical Society vol 27 no 10 pp 1587ndash1592 2006
[27] P Rabu M Drillon and C Hornick ldquoStructural and spectro-scopic study of organic-inorganic transitionmetal based layeredmagnetsrdquo Analusis vol 28 no 2 pp 103ndash108 2000
[28] L Lei A Khan and D OrsquoHare ldquoSelective anion-exchangeintercalation of isomeric benzoate anions into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo Journal of Solid State
Chemistry vol 178 no 12 pp 3648ndash3654 2005[29] W Krause H-J Bernhardt R S W Braithwaite U Kolitsch
and R Pritchard ldquoKapellasite Cu3Zn(OH)
6Cl2 a new mineral
from Lavrion Greece and its crystal structurerdquo MineralogicalMagazine vol 70 no 3 pp 329ndash340 2006
[30] L C Andrews L R Bernstein C M Foris et al PowderDiffraction File Inorganic International Centre for DiffractionData Newtown Square Pennsylvania 2009
[31] A Ragavan A I Khan and D OrsquoHare ldquoIsomer selectiveion-exchange intercalation of nitrophenolates into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotxH
2Ordquo Journal of Materials
Chemistry vol 16 no 6 pp 602ndash608 2006[32] A Ragavan A Khan and D OrsquoHare ldquoSelective intercalation
of chlorophenoxyacetates into the layered double hydroxide[LiAl2(OH)
6]ClsdotxH
2Ordquo Journal of Materials Chemistry vol 16
no 42 pp 4155ndash4159 2006[33] S Vyazovkin and C A Wight ldquoModel-free and model-fitting
approaches to kinetic analysis of isothermal and nonisothermaldatardquoThermochimica Acta vol 340-341 pp 53ndash68 1999
[34] R E Johnsen F Krumeich and P Norby ldquoStructural andmicrostructural changes during anion exchange ofCoAl layereddouble hydroxides an in situ X-ray powder diffraction studyrdquoJournal of Applied Crystallography vol 43 no 3 pp 434ndash4472010
[35] G R Williams T G Dunbar A J Beer A M Fogg and DOrsquoHare ldquoIntercalation chemistry of the novel layered doublehydroxides [MAl
4(OH)
12](NO
3)2sdotyH2O (M=Zn Cu Ni and
Co) 1 new organic intercalates and reaction mechanismsrdquoJournal of Materials Chemistry vol 16 no 13 pp 1222ndash12302006
[36] T E M Ten Hulscher and G Cornelissen ldquoEffect of tempera-ture on sorption equilibrium and sorption kinetics of organicmicropollutantsmdasha reviewrdquo Chemosphere vol 32 no 4 pp609ndash626 1996
[37] M Avrami ldquoKinetics of phase change I general theoryrdquo TheJournal of Chemical Physics vol 7 no 12 pp 1103ndash1112 1939
[38] M Avrami ldquoKinetics of phase change II Transformation-timerelations for random distribution of nucleirdquo The Journal ofChemical Physics vol 8 no 2 pp 212ndash224 1940
[39] R J Francis S OrsquoBrien A M Fogg et al ldquoTime-resolved in-situ energy and angular dispersive X-ray diffraction studies ofthe formation of themicroporous gallophosphateULM-5 underhydrothermal conditionsrdquo Journal of the American ChemicalSociety vol 121 no 5 pp 1002ndash1015 1999
[40] Z Miladinovic J Zakrzewska B Kovacevic and G BacicldquoMonitoring of crystallization processes during synthesis of
zeolite A by in situ 27Al NMR spectroscopyrdquo Materials Chem-istry and Physics vol 104 no 2-3 pp 384ndash389 2007
[41] R Serna-Guerrero and A Sayari ldquoModeling adsorption of CO2
on amine-functionalized mesoporous silica 2 kinetics andbreakthrough curvesrdquo Chemical Engineering Journal vol 161no 1-2 pp 182ndash190 2010
[42] J D Hancock and J H Sharp ldquoMethod of comparing solid-state kinetic data and its application to the decomposition ofkaolinite brucite and BaCO
3rdquo Journal of the American Ceramic
Society vol 55 no 2 pp 74ndash77 1972[43] F Auriemma C De Rosa and R Triolo ldquoSlow crystallization
kinetics of poly(vinyl alcohol) in confined environment duringcryotropic gelation of aqueous solutionsrdquo Macromolecules vol39 no 26 pp 9429ndash9434 2006
[44] R V Ramanujan and Y R Zhang ldquoQuantitative transmissionelectronmicroscopy analysis of the nanocrystallization kineticsof softmagnetic alloysrdquoPhysical ReviewB vol 74 no 22 ArticleID 224408 2006
[45] A G Kalinichev P Padma Kumar and R James KirkpatrickldquoMolecular dynamics computer simulations of the effects ofhydrogen bonding on the properties of layered double hydrox-ides intercalated with organic acidsrdquo Philosophical Magazinevol 90 no 17-18 pp 2475ndash2488 2010
Figure 3 FTIR spectra for ZC-Ac and exchange products showing the hydroxyl stretching region
correlated to the OH vibrational modes Broad absorptionbands are associated with stretching modes of hydrogenbonded hydroxyl groups and sharp bands associated withfree hydroxyl groups [26] The FTIR profiles of the hydroxylstretching region of ZC-Ac and the exchange products areshown in Figure 3 ZC-Ac has extensive interlayer hydrogenbonding due to intercalated water molecules as reportedin our earlier publication [21] Figure 3 shows that whenacetate ions were replaced by m-HCn ions in ZC-m-HCnthe broad peak (FWHM = 359 cmminus1) centered at 3450 cmminus1in ZC-Ac was replaced by 2 sharp peaks a strong peakat 3513 cmminus1 (FWHM = 148 cmminus1) and a weak peak at3574 cmminus1 (FWHM = 163 cmminus1) The two sharp peaks aredue to layer (3574 cmminus1) and m-HCn (3513 cmminus1) hydroxylgroups which are not involved in substantial hydrogenbonding [25] This assignment is based on our previousresults which showed that the peak at 3513 cmminus1 was absentin HDSs containing cinnamate anions (no OH substitutionon the benzene ring) [21] The shift of the peaks to higherwavenumbers is also indicative of hydroxyl groups which arenot substantially involved in hydrogen bonding ZC-o-HCnhas a very weak peak around 3600 cmminus1 and 2 broad peakscentered at 3489 cmminus1 (FWHM = 880 cmminus1) and 3131 cmminus1(FWHM = 376 cmminus1) The broadening of peaks and the shiftto lower wavenumbers as compared to vibrations in ZC-m-HCn and free OH stretching vibrations is consistent with
hydrogen bonded hydroxyl groups [27] The low intensity ofthe sharp band at 3600 cmminus1 is indicative of extensive hydro-gen bonding between layer OH groups and o-HCn hydroxylgroups resulting in low density of free layer OH groupsIn ZC-p-HCn there is a strong sharp peak at 3574 cmminus1(FWHM = 72 cmminus1) which is due to layer OH groups whichare not involved in significant hydrogen bonding and abroad peak at 3497 cmminus1 (FWHM = 119 cmminus1) due to p-HCnhydroxyl groups involved in hydrogen bonding
The absence of strong sharp peaks in ZC-o-HCn com-pared with those observed for ZC-m-HCn and ZC-p-HCnindicates that there is high level of hydrogen bonding in ZC-o-HCn relative to the other nanohybrids Since hydrogenbonding is directional the m-HCn anions are not orientedin a way that the OH groups are able to interact with eitherthe layer OH groups or groups from other anions Theamount of water in the nanohybrids is low as indicated fromthermogravimetry and elemental analyses Therefore havingeliminated interlayer water as a likely hydrogen bonding par-ticipant the significant hydrogen bonding observed in ZC-o-HCn andZC-p-HCn ismost likely intermolecular or betweenanion and layer hydroxyl groups In ZC-o-HCn the o-HCnhydroxyl group may be in an orientation such that they areclose to the layers enabling formation of hydrogen bondswiththe layer OH groups [28] The interaction between o-HCnand layer hydroxyl groups may have resulted in the o-HCn
6 Advances in Physical Chemistry
Table 2 Summary of dipole moments and release data
Anion Equilibrium isomer released at 40∘C () Dipole moment Calculated chargesCarbon Oxygen 1 Oxygen 2
Figure 4 PXRD profile of a representative ZC-Cl (kapellasite)obtained from exchange reactions (ZC-o-HCnClminus reaction) Millerhkl indices for selected Clminus reflections are given PDF no 56ndash71
anion being tilted closer to the layers which may explain thesmaller gallery height (as compared to that inZC-m-HCn andZC-p-HCn) observed from PXRD analysis In ZC-p-HCnOH groups may be oriented in a way so as to participate inintermolecular H-bonding between anions
32 Kinetic Analysis The release of n-HCn from nanohy-brids was achieved by ion exchange using Clminus as exchangeanions After specified times supernatants were collected andanalyzed for released anions and the residues were analyzedfor solid state transformations using PXRD Analysis of thesolid samples recovered from the reaction mixtures aftereach contact time indicated that the same chloride phasewas obtained for all the nanohybrids A representative PXRDprofile for the zinc copper chloride (ZC-Cl) obtained fromexchange reactions is shown in Figure 4 The chloride phasehas a 119889-spacing of 574 plusmn 001 A which is in close agreementwith the literature value of 573 A [29] and has been indexedas kapellasite Cu
3Zn(OH)
6Cl2(PDF no 56ndash71) [28 30] The
decrease of the interlayer space as the isomers are released isconsistent with large anions being replaced by smaller ions
The concentrations of released anions were evaluatedusing UV-Vis spectroscopy The fractions of released isomersas a function of time (release profiles) at 40∘C are shown inFigures 5(a)ndash5(c) for m-HCn o-HCn and p-HCn respec-tively The release profiles at other temperatures show thesame general trend and are presented in Figure S2 in thesupporting information Although the chemical structures of
the isomers are similar and the release medium is thesame the release profiles and hence release properties aresignificantly different (Figure 5) The equilibrium amountsreleased at 40∘C together with calculated dipole momentsand selected atomic charges are shown in Table 2 As indi-cated in Table 2 the equilibrium amounts released from thenanohybrids were significantly different ZC-o-HCn released100 of the intercalated ions while ZC-m-HCn releasedabout 40 and ZC-p-HCn released about 22 While ZC-o-HCn showed complete release at all temperatures the extentof reaction for the release ofm-HCn and p-HCn showed tem-perature dependence (increasing with temperature as shownin Figure S2) The extents of reactions at 40∘C are confirmedby XRD data in Figure S3 in supporting information
The differences in the extent of reaction may indicatedifferences in the affinity of the HDS for the isomers anddifferences in the thermodynamic equilibrium constants ofthe systemsThe extent of anion exchange depends on severalfactors which include the solvation enthalpy of the isomers(in the bulk liquid and in the gallery) the binding enthalpy(between themetal hydroxide layers and the isomers) and theinter-intramolecular interactions of the anions within thelayers [4 28 31]Themajor contributing factor is expected tobe the electrostatic interaction between the positively chargedmetal hydroxide layers and the negatively charged anionswith the carboxylate group of the isomers being in closecontact with the layers Since the isomers have the samecharge and comparable size differences in dipole moments(an indicator of the charge distribution) could have moreinfluence on the thermodynamics of the release processThe magnitude of dipole moments has been shown to becorrelatedwith selectivity in intercalation reactions of layereddouble hydroxides [4 28 31] The affinity of the metalhydroxide layer is higher for the isomer with the highestdipole moment [13] therefore this isomer is expected to beretained within the layers more than the other isomers
The equilibrium amounts of anion released from thenanohybrids follow the following order o-HCn gt m-HCngt p-HCn which is opposite to that of the magnitude ofdipole moments as shown in Table 2 The order of isomerselectivity in intercalation reactions is as expected thereverse of the order of equilibrium amount release observedhere Since p-HCn has the highest dipole moments the metalhydroxide layers are expected to have the greatest affinityfor p-HCn as compared to the other isomers [13] this couldexplain the order observed here Although factors whichinfluence the extent of release are expected to be complexdue to the processes involved in anion exchange reactions[32] the results obtained here infer that the magnitudeof dipole moments plays a significant role in determining
Advances in Physical Chemistry 7
0 4 8 12 16
0
10
20
30
40
Frac
tion
rele
ased
()
Time (hours)
(a)
0 5 10 150
20
40
60
80
Frac
tion
rele
ased
()
Time (hours)
(b)
0 1 2 3 4 5
0
4
8
12
16
20
Frac
tion
rele
ased
()
Time (hours)
(c)
Figure 5 Release profiles for (a) ZC-m-HCn (b) ZC-o-HCn and (c) ZC-p-HCn at 40∘C
the equilibrium constant and hence the amount released atequilibrium The complex nature of the exchange reactionsis highlighted when our previous results for the release ofcinnamate anion are considered [21] Cinnamate anion hasa dipole moment of 146 D which is lower than that of m-HCn but there was only 31 Cn released at equilibriumThis indicates a complex interplay of the involved factors asit has been shown that highly hydrophobic anions are easyto intercalate into the layers Due to the absence of hydroxylgroup on the benzene ring of Cn anion this isomer is morehydrophobic than the others and is therefore not readilydeintercalated into the polar aqueous solvent environment
From Figure 5 it can qualitatively be observed that therelease rates and the release profiles of the isomers aresignificantly different While the profiles for o-HCn releaseare sigmoidal those for the meta- and paraisomers aredeceleratory Reaction rates can be quantified by fittingexperimental data to reaction models which best describe(reproduce) the experimental data in the so-called model
fitting procedureThis procedure is applicable to singlemech-anism reactions in which the mechanism does not change asthe reaction progresses [33] We have shown previously thatisoconversional methods can be extended to anion exchangereactions in layered metal hydroxide as a strategy to identifywhen using model based approaches are appropriate [21] Aconstant 119864
119886as a function of extent of reaction indicates that
the reaction may effectively be characterized with a singlemechanism and in these cases model based procedures areapplicable [33]The integral isoconversional method (2) wasapplied to the kinetic data obtained here
ln 119905 = ln119892 (120572)
119860+119864119886
119877119879 (2)
In (2) 119864119886is the activation energy 120572 is the extent of
reaction 119860 is the preexponential factor 119905 is the time 119879 thetemperature 119877 is the molar gas constant and 119892(120572) is theintegral reactionmodelThe isoconversional analysis data forall the nanohybrids is shown in Figure 6 in which the 119864
119886
8 Advances in Physical Chemistry
02 04 06 08 1065
70
75
80
85ZC-o-HCn
Extent of reaction (120572)
Ea
(kJm
ol)
(a)
03 04 05 06 07 08 09
30
40
50
60ZC-p-HCn
Extent of reaction (120572)
Ea
(kJm
ol)
(b)
03 04 05 06 07 08 09 10
50
60
70
80ZC-m-HCn
Extent of reaction (120572)
Ea
(kJ m
olminus1)
(c)
Figure 6 Variation of effective 119864119886with 120572 for ZC-m-HCn ZC-o-HCn and ZC-p-HCn
values remains constant within experimental error over theentire conversion range for ZC-m-HCn and ZC-p-HCn
The 119864119886values for ZC-o-HCn are within 10 although
the changes at higher conversion appear significant comparedwith experimental uncertainty The linear decrease of the 119864
119886
at higher conversion above 120572 = 05 has been attributed toweakening of the electrostatic attraction between the anionsand the layers as the anions are progressively exchanged fromlayer to layer [34] The constant 119864
119886at low conversion may
be due to some solid state transformations occurring whichmight be responsible for the induction period observedin the PXRD analysis (Figure S3) Although the inductionperiod is short the process might still be governing theenergetics of the reaction even during the exchange periodresulting in the constant energy being observed up to extentof reaction of 05 Since 119864
119886values for exchange reactions at
the anion binding sites have been shown to range from 30to 70 kJmolminus1 depending on the nature and size of the guestanions [31 32 35] the reactionmay be controlled by chemical
reaction at the exchange site It is also important to note thatdiffusion of anions in a dense organicmatrix can have high119864
119886
values sometimes being more than 100 kJmolminus1 [36] as suchdiffusion limited process cannot be ruled out if the anions areclosely packed in the gallery
The 119864119886values for the release of m-HCn and p-HCn
are in the range of both reaction controlled process anddiffusion controlled processes [21] The fact that the 119864
119886does
not significantly change over the entire reaction may indicatethat the reaction is diffusion controlled The differences inthe mechanism between o-HCn release and the other twosystems may be due to different strength of interactions withthe layersThe presence of H-bonding between the layers andthe o-HCn anions may bring the anions closer to the layersresulting in an increased electrostatic attraction resulting inthe process being controlled by reaction at the exchangesite Since there was no significant variation of the 119864
119886with
conversion for m-HCn and p-HCn release and the energydifference for o-HCn release is too low to reflect a change in
Advances in Physical Chemistry 9
mechanism model based approach was used to obtain rateconstants for the exchange reactions The Avrami-Erofersquoevnucleation-growth model [37 38] was used here since ithas the advantage that it can be used to describe reactionsoccurring via different mechanisms making comparison ofthe processes possible since the order of reactions will be thesame In the model the extent of reaction (120572) depends on therate constant 119896 and a coefficient 119899 as shown in the followingequation
119886 (119905) = 1 minus 119890[minus(119896119905)
119899
] (3)
The coefficient 119899 (Avrami exponent) potentially containsinformation about the mechanism of the reaction [39] TheAvrami-Erofersquoev model has been applied successfully to solu-tion phase anion exchange reactions in layeredmaterials andother fluid phase reactions [2 10 40 41] we have also shownthat values of parameters obtained from analysis of bothsolid state transformations and solution phase processesemploying the Avrami model are comparable indicating thatthere is correlation between solid state transformation andfluid phase processes [21] The Avrami exponent is usuallyobtained by taking double logarithm of the Avrami-Erofersquoevequation obtaining (4) which was popularized by Sharp andHancock in 1972 [42] Consider
A plot of ln[minus ln(1 minus 120572)] as a function of ln 119905 (doublelogarithmic plot) gives a linear plot in which the value of 119899 isobtained from the slope and the value of 119896 is evaluated fromthe intercept
The extent of reaction versus time plots obtained for theZC-o-HCnClminus exchange reactions at different temperaturesis shown in Figure 7(a) The Avrami-Erofersquoev model wasapplied and correctly described the obtained experimentaldatawithin the120572 range of 015ndash10The validity of theAvrami-Erofersquoevmodel can be confirmedby the corresponding doublelogarithmic plots in Figure 7(b) where straight lines wereobtained with 1198772 values ranging from 0996 to 0999 Thedouble logarithmic plots are almost parallel indicating thatthe reaction proceeds with the same mechanism over thetemperature range used here The obtained 119899 value was closeto 10 consistent with a nucleation controlled mechanismwhich agrees well with the activation energies obtained fromisoconversional methods and discussed before
The release profiles together with the double logarithmicplots for the release of m-HCn and p-HCn are shown in thesupporting information Figures S4 and S5 respectively Thekinetics of m-HCn release also follows the Avrami-Erofersquoevmodel the data provided a good fit within the range 015 lt120572 lt 085 the fit to limited range of 120572 has been observedin other intercalation reactions Anomalously low values ofthe Avrami exponent observed here (shown in Table 3) havebeen reported in polymer and alloy crystallization and donot have physical mechanistic meaning [43 44] The Avramiexponent values obtained for the release of p-HCn are closeto 05 which is consistent with diffusion controlled reactions[10] Diffusion control of such a high 119864
119886highlights two
0 2 4 6 8 10 12 1400
02
04
06
08
10
Time (hours)
Exte
nt o
f rea
ctio
n (120572
)
300∘C400∘C500∘C600∘C
(a)
4 6 8 10
0
1
minus1
minus2
ln (t minus t0)
lnminusln(1
minus120572)
30∘C40∘C50∘C60∘C
(b)
Figure 7 Extent of reaction as a function of time (a) and doublelogarithmic plots (b) for the exchange reaction of Clminus anion andZC-o-HCn at various temperatures 60∘C (◻) 50∘C (Q) 40∘C (998771)and 30∘C (e)
issues (a) the interlayer space is densely packed and thediffusional resistance within the interlayer space approachesthat in polymers in which119864
119886values can be above 100 kJmolminus1
depending on the diffusing molecule [36] and (b) Kineticambiguity is common in model based approaches in thiscase the mechanistic interpretation from the model is not inline with the kinetic parameters obtained
FromTable 3 it can be concluded that the rate of release ofm-HCn from the nanohybrids is much faster compared with
10 Advances in Physical Chemistry
Table 3 A summary of kinetic parameters obtained at different temperatures
Nanohybrid Temperature (∘C) 119899 119896 sminus1 (lowast10minus3)
ZC-o-HCn
600 082 plusmn 002 17 plusmn 03
500 090 plusmn 002 07 plusmn 01
400 088 plusmn 001 029 plusmn 005
300 102 plusmn 001 010 plusmn 001
ZC-m-HCn
600 024 plusmn 001 31 plusmn 9
500 024 plusmn 001 20 plusmn 7
400 021 plusmn 002 7 plusmn 2
300 0225 plusmn 0007 35 plusmn 08
ZC-p-HCn
600 041 plusmn 001 29 plusmn 05
500 045 plusmn 002 16 plusmn 04
400 046 plusmn 001 10 plusmn 02
300 039 plusmn 001 05 plusmn 01
the other isomers As an example the rate constants at 40∘Cwere (7 plusmn 2) times 10minus3 sminus1 for m-HCn (10 plusmn 02) times 10minus3 sminus1 forp-HCn and (029 plusmn 005) times 10minus3 sminus1 for o-HCn The rate ofrelease follows the following order
119898ndashHCn gt 119901-HCn gt 119900-HCn
The release profile form-HCn at 40∘C (Figure 5(a)) showsthat there is a burst and rapid release of the intercalated anionin the first 10 minutes followed by a more sustained releasewith equilibrium being attained after 3 hours This burstrelease is observed for all the temperatures used here (FigureS2a in supporting information) and has been observed beforein anion exchange reactions in LDHs [10] The burst releaseobserved here is due to anion exchange as confirmed byPXRD results in Figure S3 The chloride phase representedby an asterisk in Figure S3 was significant at 30 seconds (theearliest we could sample) of ZC-m-HCn being in contact withthe chloride solution The initial slow and linear release inZC-o-HCn indicated in Figure 5(b) and confirmed by XRDprofiles in Figure S3 may be due to the increased stabilityof the HDS structure due to extensive hydrogen bonding asindicated by FTIR data in Figure 3 and the close arrangementof the anions implied by XRDprofiles in Figure 2(a)This firststage of release has a strong temperature dependence beingmore pronounced at low temperature and almost absent athigher temperatures (Figure S2b in supporting information)
Factors which may influence the rates of release includeinteractions between the metal hydroxide layers and theanions anion-anion interactions within the layers and in theexchange medium and solvent-anion interaction [10] It isexpected that these factors will have a complex effect on thereaction rates since they affect each isomer differently Thecomplex interplay of the abovementioned factors oftenmakeit difficult to come up with a simple relationship betweenthe rate of release and the aforementioned factors [10] It isimportant to note that these factors are expected to dependon the types and positions of chemical groups on the anionThepresence of hydrogen bondingwithin the interlayer spacereduces the Gibbs free energy of formation of the nanohy-brids and stabilizes the system [45]The rate of release of ionsthen depends on the differences in the energy of nanohybrid
and the barrier to reaction As indicated in Figure 3 there issignificant level of hydrogen bonding in ZC-o-HCn whichthen resulted in a close interaction with the layers Thehydrogen bond network in the nanohybrid and the closeinteraction of the anions with the layers stabilized the systemas compared to the other nanohybrids The nanohybrid inwhich these interactions are low is relatively less stable and isexpected to proceed faster to equilibrium this is consistentwith results obtained here The anions in the unstablelessstable nanohybrid prefer bulk solution (in which there isstabilization by solvation with water molecules) to galleryspace which may result in the burst and fast release observedin ZC-m-HCnThe modification of bioactive molecules withgroups capable of hydrogen bonding for example hydroxylgroups especially at positions which allow interactions withthe layers may enable tuning of the release behavior ofthese molecules from layered materials It is important tonote that the modification should be at positions whichdo not affect the bioactivity of the molecules Additionalexperiments including studies on other isomeric anions withgroups capable of forming hydrogen bonds and effect ofmetallayer composition will be helpful in further elucidating howthe position of substituents control the rate as well as theextent of reaction
4 Conclusions
Anion exchange reactions in hydroxy double salts are affectedby the structure of the intercalated anions and there is astrong interplay between thermodynamics and kinetics ofthe release reactions In the present study we have shownthat there is a significant effect of the position of substituentgroups on the rate and extent of release of isomers ofhydroxycinnamate While the release of the metaisomer wasinstantaneous and fast the orthoisomer release exhibiteda slower more sustained and complete release the orderof reaction rates was m-HCn gt p-HCn gt o-HCn withthe o-HCn nanohybrid showing an induction period Theposition of the hydroxyl groups resulted in differences inthe interlayer interactions of the anions with presence ofinterlayer hydrogen bonds affecting the rates of the reaction
Advances in Physical Chemistry 11
The formation of hydrogen bonds between the layers and theintercalated orthoisomer resulted in greater stabilization ofthe nanohybrid and resulted in the observed induction periodand slow release The extent of reaction was correlated withthe magnitude of dipole moments These results highlightthe potential of tuning the release behavior of intercalatedbioactive compounds which bring control into both theamount released per given time and the period for totalrelease
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
This work was supported by the National Science Foundation(CHE-0809751)
References
[1] S Yamanaka T Sako andM Hattori ldquoAnion exchange in basiccopper acetaterdquo Chemistry Letters pp 1869ndash1872 1989
[2] E Kandare and J M Hossenlopp ldquoHydroxy double salt anionexchange kinetics effects of precursor structure and anionsizetrdquo Journal of Physical Chemistry B vol 109 no 17 pp 8469ndash8475 2005
[3] P K Dutta and M Puri ldquoAnion exchange in lithium aluminatehydroxidesrdquo Journal of Physical Chemistry vol 93 no 1 pp376ndash381 1989
[4] B M Choudary V S Jaya B R Reddy M L Kantam M MRao and S S Madhavendra ldquoSynthesis characterization ionexchange and catalytic properties of nanobinary and ternarymetal oxyhydroxidesrdquo Chemistry of Materials vol 17 no 10pp 2740ndash2743 2005
[5] A M Fogg J S Dunn S-G Shyu D R Cary and D OrsquoHareldquoSelective ion-exchange intercalation of isomeric dicarboxylateanions into the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Chemistry of Materials vol 10 no 1 pp 351ndash355 1998[6] T Ikeda H Amoh and T Yasunaga ldquoStereoselective exchange
kinetics of L- and D-histidines for Cl- in the interlayerof a hydrotalcite-like compound by the chemical relaxationmethodrdquo Journal of the American Chemical Society vol 106 no20 pp 5772ndash5775 1984
[7] M Meyn K Beneke and G Lagaly ldquoAnion-exchange reactionsof hydroxy double saltsrdquo Inorganic Chemistry vol 32 no 7 pp1209ndash1215 1993
[8] V Ambrogi G Fardella G Grandolini L Perioli and M CTiralti ldquoIntercalation compounds of hydrotalcite-like anionicclays with anti-inflammatory agents II uptake of diclofenac fora controlled release formulationrdquo AAPS PharmSciTech vol 3no 3 p E26 2002
[9] M Z Bin Hussein Z Zainal A H Yahaya and D WV Foo ldquoControlled release of a plant growth regulator 120572-naphthaleneacetate from the lamella of Zn-Al-layered doublehydroxide nanocompositerdquo Journal of Controlled Release vol82 no 2-3 pp 417ndash427 2002
[10] A I Khan A Ragavan B Fong et al ldquoRecent developments inthe use of layered double hydroxides as host materials for the
storage and triggered release of functional anionsrdquo Industrialand Engineering Chemistry Research vol 48 no 23 pp 10196ndash10205 2009
[11] M Silion D Hritcu I M Jaba et al ldquoIn vitro and invivo behavior of ketoprofen intercalated into layered doublehydroxidesrdquo Journal of Materials Science vol 21 no 11 pp3009ndash3018 2010
[12] J-H Yang Y-S Han M Park T Park S-J Hwang andJ-H Choy ldquoNew inorganic-based drug delivery system ofindole-3-acetic acid-layered metal hydroxide nanohybrids withcontrolled release raterdquo Chemistry of Materials vol 19 no 10pp 2679ndash2685 2007
[13] L Lei R P Vijayan and D OrsquoHare ldquoPreferential anionexchange intercalation of pyridinecarboxylate and toluate iso-mers in the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Journal of Materials Chemistry vol 11 no 12 pp 3276ndash32802001
[14] B Lotsch F Millange R I Walton and D OrsquoHare ldquoSepara-tion of nucleoside monophosphates using preferential anionexchange intercalation in layered double hydroxidesrdquo Solid StateSciences vol 3 no 8 pp 883ndash886 2001
[15] H Tagaya N Sasaki H Morioka and J Kadokawa ldquoPrepara-tion of new inorganicmdashorganic layered compounds hydroxydouble salts and preferential intercalation of organic carboxylicacids into themrdquoMolecular Crystals and Liquid Crystals Scienceand Technology A vol 341 pp 413ndash418 2000
[16] P P Kumar A G Kalinichev and R J Kirkpatrick ldquoHydra-tion swelling interlayer structure and hydrogen bondingin organolayered double hydroxides insights from moleculardynamics simulation of citrate-intercalated hydrotalciterdquo Jour-nal of Physical Chemistry B vol 110 no 9 pp 3841ndash3844 2006
[17] S V Prasanna and P V Kamath ldquoAnion-exchange reactions oflayered double hydroxides interplay between coulombic andh-bonding interactionsrdquo Industrial and Engineering ChemistryResearch vol 48 no 13 pp 6315ndash6320 2009
[18] Z Gu A CThomas Z P Xu J H Campbell and G Q Lu ldquoInvitro sustained release of LMWH from MgAl-layered doublehydroxide nanohybridsrdquo Chemistry of Materials vol 20 no 11pp 3715ndash3722 2008
[19] J T Rajamathi S Britto and M Rajamathi ldquoSynthesis andanion exchange reactions of a layered Copper-Zincrdquo Journal ofChemical Sciences vol 117 no 6 pp 629ndash633 2005
[20] M Frisch G Trucks H Schlegel et al ldquoGaussian 98 RevisionA114rdquo 2002
[21] S Majoni and J M Hossenlopp ldquoAnion exchange kinetics ofnanodimensional layered metal hydroxides use of isoconver-sional analysisrdquo Journal of Physical Chemistry A vol 114 no 49pp 12858ndash12869 2010
[22] C Chouillet J-M Krafft C Louis and H Lauron-PernotldquoCharacterization of zinc hydroxynitrates by diffuse reflectanceinfrared spectroscopymdashstructural modifications during ther-mal treatmentrdquo Spectrochimica Acta A vol 60 no 3 pp 505ndash511 2004
[23] W Fujita and K Awaga ldquoMagnetic properties of Cu2(OH)
3(al-
kanecarboxylate) compounds drastic mod-ification withextension of the Alkyl chainrdquo Inorganic Chemistry vol 35 no7 pp 1915ndash1917 1996
[24] R Swisłocka M Kowczyk-Sadowy M Kalinowska and WLewandowski ldquoSpectroscopic (FT-IR FT-Raman1H and13CNMR) and theoretical studies of p-coumaric acid and alkalimetal p-coumaratesrdquo Spectroscopy vol 27 no 1 pp 35ndash48 2012
12 Advances in Physical Chemistry
[25] T Biswick W Jones A Pacuła E Serwicka and J PodobinskildquoThe role of anhydrous zinc nitrate in the thermal decomposi-tion of the zinc hydroxy nitrates Zn
5(OH)
8(NO3)2sdot2H2O and
ZnOHNO3sdotH2Ordquo Journal of Solid State Chemistry vol 180 no
4 pp 1171ndash1179 2007[26] S-H Park and E L Cheol ldquoSynthesis characterization and
magnetic properties of a novel disulfonate-pillared copperhydroxide Cu
2(OH)
3(DS4)
12 DS4 = 14-butanedisulfonaterdquo
Bulletin of the Korean Chemical Society vol 27 no 10 pp 1587ndash1592 2006
[27] P Rabu M Drillon and C Hornick ldquoStructural and spectro-scopic study of organic-inorganic transitionmetal based layeredmagnetsrdquo Analusis vol 28 no 2 pp 103ndash108 2000
[28] L Lei A Khan and D OrsquoHare ldquoSelective anion-exchangeintercalation of isomeric benzoate anions into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo Journal of Solid State
Chemistry vol 178 no 12 pp 3648ndash3654 2005[29] W Krause H-J Bernhardt R S W Braithwaite U Kolitsch
and R Pritchard ldquoKapellasite Cu3Zn(OH)
6Cl2 a new mineral
from Lavrion Greece and its crystal structurerdquo MineralogicalMagazine vol 70 no 3 pp 329ndash340 2006
[30] L C Andrews L R Bernstein C M Foris et al PowderDiffraction File Inorganic International Centre for DiffractionData Newtown Square Pennsylvania 2009
[31] A Ragavan A I Khan and D OrsquoHare ldquoIsomer selectiveion-exchange intercalation of nitrophenolates into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotxH
2Ordquo Journal of Materials
Chemistry vol 16 no 6 pp 602ndash608 2006[32] A Ragavan A Khan and D OrsquoHare ldquoSelective intercalation
of chlorophenoxyacetates into the layered double hydroxide[LiAl2(OH)
6]ClsdotxH
2Ordquo Journal of Materials Chemistry vol 16
no 42 pp 4155ndash4159 2006[33] S Vyazovkin and C A Wight ldquoModel-free and model-fitting
approaches to kinetic analysis of isothermal and nonisothermaldatardquoThermochimica Acta vol 340-341 pp 53ndash68 1999
[34] R E Johnsen F Krumeich and P Norby ldquoStructural andmicrostructural changes during anion exchange ofCoAl layereddouble hydroxides an in situ X-ray powder diffraction studyrdquoJournal of Applied Crystallography vol 43 no 3 pp 434ndash4472010
[35] G R Williams T G Dunbar A J Beer A M Fogg and DOrsquoHare ldquoIntercalation chemistry of the novel layered doublehydroxides [MAl
4(OH)
12](NO
3)2sdotyH2O (M=Zn Cu Ni and
Co) 1 new organic intercalates and reaction mechanismsrdquoJournal of Materials Chemistry vol 16 no 13 pp 1222ndash12302006
[36] T E M Ten Hulscher and G Cornelissen ldquoEffect of tempera-ture on sorption equilibrium and sorption kinetics of organicmicropollutantsmdasha reviewrdquo Chemosphere vol 32 no 4 pp609ndash626 1996
[37] M Avrami ldquoKinetics of phase change I general theoryrdquo TheJournal of Chemical Physics vol 7 no 12 pp 1103ndash1112 1939
[38] M Avrami ldquoKinetics of phase change II Transformation-timerelations for random distribution of nucleirdquo The Journal ofChemical Physics vol 8 no 2 pp 212ndash224 1940
[39] R J Francis S OrsquoBrien A M Fogg et al ldquoTime-resolved in-situ energy and angular dispersive X-ray diffraction studies ofthe formation of themicroporous gallophosphateULM-5 underhydrothermal conditionsrdquo Journal of the American ChemicalSociety vol 121 no 5 pp 1002ndash1015 1999
[40] Z Miladinovic J Zakrzewska B Kovacevic and G BacicldquoMonitoring of crystallization processes during synthesis of
zeolite A by in situ 27Al NMR spectroscopyrdquo Materials Chem-istry and Physics vol 104 no 2-3 pp 384ndash389 2007
[41] R Serna-Guerrero and A Sayari ldquoModeling adsorption of CO2
on amine-functionalized mesoporous silica 2 kinetics andbreakthrough curvesrdquo Chemical Engineering Journal vol 161no 1-2 pp 182ndash190 2010
[42] J D Hancock and J H Sharp ldquoMethod of comparing solid-state kinetic data and its application to the decomposition ofkaolinite brucite and BaCO
3rdquo Journal of the American Ceramic
Society vol 55 no 2 pp 74ndash77 1972[43] F Auriemma C De Rosa and R Triolo ldquoSlow crystallization
kinetics of poly(vinyl alcohol) in confined environment duringcryotropic gelation of aqueous solutionsrdquo Macromolecules vol39 no 26 pp 9429ndash9434 2006
[44] R V Ramanujan and Y R Zhang ldquoQuantitative transmissionelectronmicroscopy analysis of the nanocrystallization kineticsof softmagnetic alloysrdquoPhysical ReviewB vol 74 no 22 ArticleID 224408 2006
[45] A G Kalinichev P Padma Kumar and R James KirkpatrickldquoMolecular dynamics computer simulations of the effects ofhydrogen bonding on the properties of layered double hydrox-ides intercalated with organic acidsrdquo Philosophical Magazinevol 90 no 17-18 pp 2475ndash2488 2010
Figure 4 PXRD profile of a representative ZC-Cl (kapellasite)obtained from exchange reactions (ZC-o-HCnClminus reaction) Millerhkl indices for selected Clminus reflections are given PDF no 56ndash71
anion being tilted closer to the layers which may explain thesmaller gallery height (as compared to that inZC-m-HCn andZC-p-HCn) observed from PXRD analysis In ZC-p-HCnOH groups may be oriented in a way so as to participate inintermolecular H-bonding between anions
32 Kinetic Analysis The release of n-HCn from nanohy-brids was achieved by ion exchange using Clminus as exchangeanions After specified times supernatants were collected andanalyzed for released anions and the residues were analyzedfor solid state transformations using PXRD Analysis of thesolid samples recovered from the reaction mixtures aftereach contact time indicated that the same chloride phasewas obtained for all the nanohybrids A representative PXRDprofile for the zinc copper chloride (ZC-Cl) obtained fromexchange reactions is shown in Figure 4 The chloride phasehas a 119889-spacing of 574 plusmn 001 A which is in close agreementwith the literature value of 573 A [29] and has been indexedas kapellasite Cu
3Zn(OH)
6Cl2(PDF no 56ndash71) [28 30] The
decrease of the interlayer space as the isomers are released isconsistent with large anions being replaced by smaller ions
The concentrations of released anions were evaluatedusing UV-Vis spectroscopy The fractions of released isomersas a function of time (release profiles) at 40∘C are shown inFigures 5(a)ndash5(c) for m-HCn o-HCn and p-HCn respec-tively The release profiles at other temperatures show thesame general trend and are presented in Figure S2 in thesupporting information Although the chemical structures of
the isomers are similar and the release medium is thesame the release profiles and hence release properties aresignificantly different (Figure 5) The equilibrium amountsreleased at 40∘C together with calculated dipole momentsand selected atomic charges are shown in Table 2 As indi-cated in Table 2 the equilibrium amounts released from thenanohybrids were significantly different ZC-o-HCn released100 of the intercalated ions while ZC-m-HCn releasedabout 40 and ZC-p-HCn released about 22 While ZC-o-HCn showed complete release at all temperatures the extentof reaction for the release ofm-HCn and p-HCn showed tem-perature dependence (increasing with temperature as shownin Figure S2) The extents of reactions at 40∘C are confirmedby XRD data in Figure S3 in supporting information
The differences in the extent of reaction may indicatedifferences in the affinity of the HDS for the isomers anddifferences in the thermodynamic equilibrium constants ofthe systemsThe extent of anion exchange depends on severalfactors which include the solvation enthalpy of the isomers(in the bulk liquid and in the gallery) the binding enthalpy(between themetal hydroxide layers and the isomers) and theinter-intramolecular interactions of the anions within thelayers [4 28 31]Themajor contributing factor is expected tobe the electrostatic interaction between the positively chargedmetal hydroxide layers and the negatively charged anionswith the carboxylate group of the isomers being in closecontact with the layers Since the isomers have the samecharge and comparable size differences in dipole moments(an indicator of the charge distribution) could have moreinfluence on the thermodynamics of the release processThe magnitude of dipole moments has been shown to becorrelatedwith selectivity in intercalation reactions of layereddouble hydroxides [4 28 31] The affinity of the metalhydroxide layer is higher for the isomer with the highestdipole moment [13] therefore this isomer is expected to beretained within the layers more than the other isomers
The equilibrium amounts of anion released from thenanohybrids follow the following order o-HCn gt m-HCngt p-HCn which is opposite to that of the magnitude ofdipole moments as shown in Table 2 The order of isomerselectivity in intercalation reactions is as expected thereverse of the order of equilibrium amount release observedhere Since p-HCn has the highest dipole moments the metalhydroxide layers are expected to have the greatest affinityfor p-HCn as compared to the other isomers [13] this couldexplain the order observed here Although factors whichinfluence the extent of release are expected to be complexdue to the processes involved in anion exchange reactions[32] the results obtained here infer that the magnitudeof dipole moments plays a significant role in determining
Advances in Physical Chemistry 7
0 4 8 12 16
0
10
20
30
40
Frac
tion
rele
ased
()
Time (hours)
(a)
0 5 10 150
20
40
60
80
Frac
tion
rele
ased
()
Time (hours)
(b)
0 1 2 3 4 5
0
4
8
12
16
20
Frac
tion
rele
ased
()
Time (hours)
(c)
Figure 5 Release profiles for (a) ZC-m-HCn (b) ZC-o-HCn and (c) ZC-p-HCn at 40∘C
the equilibrium constant and hence the amount released atequilibrium The complex nature of the exchange reactionsis highlighted when our previous results for the release ofcinnamate anion are considered [21] Cinnamate anion hasa dipole moment of 146 D which is lower than that of m-HCn but there was only 31 Cn released at equilibriumThis indicates a complex interplay of the involved factors asit has been shown that highly hydrophobic anions are easyto intercalate into the layers Due to the absence of hydroxylgroup on the benzene ring of Cn anion this isomer is morehydrophobic than the others and is therefore not readilydeintercalated into the polar aqueous solvent environment
From Figure 5 it can qualitatively be observed that therelease rates and the release profiles of the isomers aresignificantly different While the profiles for o-HCn releaseare sigmoidal those for the meta- and paraisomers aredeceleratory Reaction rates can be quantified by fittingexperimental data to reaction models which best describe(reproduce) the experimental data in the so-called model
fitting procedureThis procedure is applicable to singlemech-anism reactions in which the mechanism does not change asthe reaction progresses [33] We have shown previously thatisoconversional methods can be extended to anion exchangereactions in layered metal hydroxide as a strategy to identifywhen using model based approaches are appropriate [21] Aconstant 119864
119886as a function of extent of reaction indicates that
the reaction may effectively be characterized with a singlemechanism and in these cases model based procedures areapplicable [33]The integral isoconversional method (2) wasapplied to the kinetic data obtained here
ln 119905 = ln119892 (120572)
119860+119864119886
119877119879 (2)
In (2) 119864119886is the activation energy 120572 is the extent of
reaction 119860 is the preexponential factor 119905 is the time 119879 thetemperature 119877 is the molar gas constant and 119892(120572) is theintegral reactionmodelThe isoconversional analysis data forall the nanohybrids is shown in Figure 6 in which the 119864
119886
8 Advances in Physical Chemistry
02 04 06 08 1065
70
75
80
85ZC-o-HCn
Extent of reaction (120572)
Ea
(kJm
ol)
(a)
03 04 05 06 07 08 09
30
40
50
60ZC-p-HCn
Extent of reaction (120572)
Ea
(kJm
ol)
(b)
03 04 05 06 07 08 09 10
50
60
70
80ZC-m-HCn
Extent of reaction (120572)
Ea
(kJ m
olminus1)
(c)
Figure 6 Variation of effective 119864119886with 120572 for ZC-m-HCn ZC-o-HCn and ZC-p-HCn
values remains constant within experimental error over theentire conversion range for ZC-m-HCn and ZC-p-HCn
The 119864119886values for ZC-o-HCn are within 10 although
the changes at higher conversion appear significant comparedwith experimental uncertainty The linear decrease of the 119864
119886
at higher conversion above 120572 = 05 has been attributed toweakening of the electrostatic attraction between the anionsand the layers as the anions are progressively exchanged fromlayer to layer [34] The constant 119864
119886at low conversion may
be due to some solid state transformations occurring whichmight be responsible for the induction period observedin the PXRD analysis (Figure S3) Although the inductionperiod is short the process might still be governing theenergetics of the reaction even during the exchange periodresulting in the constant energy being observed up to extentof reaction of 05 Since 119864
119886values for exchange reactions at
the anion binding sites have been shown to range from 30to 70 kJmolminus1 depending on the nature and size of the guestanions [31 32 35] the reactionmay be controlled by chemical
reaction at the exchange site It is also important to note thatdiffusion of anions in a dense organicmatrix can have high119864
119886
values sometimes being more than 100 kJmolminus1 [36] as suchdiffusion limited process cannot be ruled out if the anions areclosely packed in the gallery
The 119864119886values for the release of m-HCn and p-HCn
are in the range of both reaction controlled process anddiffusion controlled processes [21] The fact that the 119864
119886does
not significantly change over the entire reaction may indicatethat the reaction is diffusion controlled The differences inthe mechanism between o-HCn release and the other twosystems may be due to different strength of interactions withthe layersThe presence of H-bonding between the layers andthe o-HCn anions may bring the anions closer to the layersresulting in an increased electrostatic attraction resulting inthe process being controlled by reaction at the exchangesite Since there was no significant variation of the 119864
119886with
conversion for m-HCn and p-HCn release and the energydifference for o-HCn release is too low to reflect a change in
Advances in Physical Chemistry 9
mechanism model based approach was used to obtain rateconstants for the exchange reactions The Avrami-Erofersquoevnucleation-growth model [37 38] was used here since ithas the advantage that it can be used to describe reactionsoccurring via different mechanisms making comparison ofthe processes possible since the order of reactions will be thesame In the model the extent of reaction (120572) depends on therate constant 119896 and a coefficient 119899 as shown in the followingequation
119886 (119905) = 1 minus 119890[minus(119896119905)
119899
] (3)
The coefficient 119899 (Avrami exponent) potentially containsinformation about the mechanism of the reaction [39] TheAvrami-Erofersquoev model has been applied successfully to solu-tion phase anion exchange reactions in layeredmaterials andother fluid phase reactions [2 10 40 41] we have also shownthat values of parameters obtained from analysis of bothsolid state transformations and solution phase processesemploying the Avrami model are comparable indicating thatthere is correlation between solid state transformation andfluid phase processes [21] The Avrami exponent is usuallyobtained by taking double logarithm of the Avrami-Erofersquoevequation obtaining (4) which was popularized by Sharp andHancock in 1972 [42] Consider
A plot of ln[minus ln(1 minus 120572)] as a function of ln 119905 (doublelogarithmic plot) gives a linear plot in which the value of 119899 isobtained from the slope and the value of 119896 is evaluated fromthe intercept
The extent of reaction versus time plots obtained for theZC-o-HCnClminus exchange reactions at different temperaturesis shown in Figure 7(a) The Avrami-Erofersquoev model wasapplied and correctly described the obtained experimentaldatawithin the120572 range of 015ndash10The validity of theAvrami-Erofersquoevmodel can be confirmedby the corresponding doublelogarithmic plots in Figure 7(b) where straight lines wereobtained with 1198772 values ranging from 0996 to 0999 Thedouble logarithmic plots are almost parallel indicating thatthe reaction proceeds with the same mechanism over thetemperature range used here The obtained 119899 value was closeto 10 consistent with a nucleation controlled mechanismwhich agrees well with the activation energies obtained fromisoconversional methods and discussed before
The release profiles together with the double logarithmicplots for the release of m-HCn and p-HCn are shown in thesupporting information Figures S4 and S5 respectively Thekinetics of m-HCn release also follows the Avrami-Erofersquoevmodel the data provided a good fit within the range 015 lt120572 lt 085 the fit to limited range of 120572 has been observedin other intercalation reactions Anomalously low values ofthe Avrami exponent observed here (shown in Table 3) havebeen reported in polymer and alloy crystallization and donot have physical mechanistic meaning [43 44] The Avramiexponent values obtained for the release of p-HCn are closeto 05 which is consistent with diffusion controlled reactions[10] Diffusion control of such a high 119864
119886highlights two
0 2 4 6 8 10 12 1400
02
04
06
08
10
Time (hours)
Exte
nt o
f rea
ctio
n (120572
)
300∘C400∘C500∘C600∘C
(a)
4 6 8 10
0
1
minus1
minus2
ln (t minus t0)
lnminusln(1
minus120572)
30∘C40∘C50∘C60∘C
(b)
Figure 7 Extent of reaction as a function of time (a) and doublelogarithmic plots (b) for the exchange reaction of Clminus anion andZC-o-HCn at various temperatures 60∘C (◻) 50∘C (Q) 40∘C (998771)and 30∘C (e)
issues (a) the interlayer space is densely packed and thediffusional resistance within the interlayer space approachesthat in polymers in which119864
119886values can be above 100 kJmolminus1
depending on the diffusing molecule [36] and (b) Kineticambiguity is common in model based approaches in thiscase the mechanistic interpretation from the model is not inline with the kinetic parameters obtained
FromTable 3 it can be concluded that the rate of release ofm-HCn from the nanohybrids is much faster compared with
10 Advances in Physical Chemistry
Table 3 A summary of kinetic parameters obtained at different temperatures
Nanohybrid Temperature (∘C) 119899 119896 sminus1 (lowast10minus3)
ZC-o-HCn
600 082 plusmn 002 17 plusmn 03
500 090 plusmn 002 07 plusmn 01
400 088 plusmn 001 029 plusmn 005
300 102 plusmn 001 010 plusmn 001
ZC-m-HCn
600 024 plusmn 001 31 plusmn 9
500 024 plusmn 001 20 plusmn 7
400 021 plusmn 002 7 plusmn 2
300 0225 plusmn 0007 35 plusmn 08
ZC-p-HCn
600 041 plusmn 001 29 plusmn 05
500 045 plusmn 002 16 plusmn 04
400 046 plusmn 001 10 plusmn 02
300 039 plusmn 001 05 plusmn 01
the other isomers As an example the rate constants at 40∘Cwere (7 plusmn 2) times 10minus3 sminus1 for m-HCn (10 plusmn 02) times 10minus3 sminus1 forp-HCn and (029 plusmn 005) times 10minus3 sminus1 for o-HCn The rate ofrelease follows the following order
119898ndashHCn gt 119901-HCn gt 119900-HCn
The release profile form-HCn at 40∘C (Figure 5(a)) showsthat there is a burst and rapid release of the intercalated anionin the first 10 minutes followed by a more sustained releasewith equilibrium being attained after 3 hours This burstrelease is observed for all the temperatures used here (FigureS2a in supporting information) and has been observed beforein anion exchange reactions in LDHs [10] The burst releaseobserved here is due to anion exchange as confirmed byPXRD results in Figure S3 The chloride phase representedby an asterisk in Figure S3 was significant at 30 seconds (theearliest we could sample) of ZC-m-HCn being in contact withthe chloride solution The initial slow and linear release inZC-o-HCn indicated in Figure 5(b) and confirmed by XRDprofiles in Figure S3 may be due to the increased stabilityof the HDS structure due to extensive hydrogen bonding asindicated by FTIR data in Figure 3 and the close arrangementof the anions implied by XRDprofiles in Figure 2(a)This firststage of release has a strong temperature dependence beingmore pronounced at low temperature and almost absent athigher temperatures (Figure S2b in supporting information)
Factors which may influence the rates of release includeinteractions between the metal hydroxide layers and theanions anion-anion interactions within the layers and in theexchange medium and solvent-anion interaction [10] It isexpected that these factors will have a complex effect on thereaction rates since they affect each isomer differently Thecomplex interplay of the abovementioned factors oftenmakeit difficult to come up with a simple relationship betweenthe rate of release and the aforementioned factors [10] It isimportant to note that these factors are expected to dependon the types and positions of chemical groups on the anionThepresence of hydrogen bondingwithin the interlayer spacereduces the Gibbs free energy of formation of the nanohy-brids and stabilizes the system [45]The rate of release of ionsthen depends on the differences in the energy of nanohybrid
and the barrier to reaction As indicated in Figure 3 there issignificant level of hydrogen bonding in ZC-o-HCn whichthen resulted in a close interaction with the layers Thehydrogen bond network in the nanohybrid and the closeinteraction of the anions with the layers stabilized the systemas compared to the other nanohybrids The nanohybrid inwhich these interactions are low is relatively less stable and isexpected to proceed faster to equilibrium this is consistentwith results obtained here The anions in the unstablelessstable nanohybrid prefer bulk solution (in which there isstabilization by solvation with water molecules) to galleryspace which may result in the burst and fast release observedin ZC-m-HCnThe modification of bioactive molecules withgroups capable of hydrogen bonding for example hydroxylgroups especially at positions which allow interactions withthe layers may enable tuning of the release behavior ofthese molecules from layered materials It is important tonote that the modification should be at positions whichdo not affect the bioactivity of the molecules Additionalexperiments including studies on other isomeric anions withgroups capable of forming hydrogen bonds and effect ofmetallayer composition will be helpful in further elucidating howthe position of substituents control the rate as well as theextent of reaction
4 Conclusions
Anion exchange reactions in hydroxy double salts are affectedby the structure of the intercalated anions and there is astrong interplay between thermodynamics and kinetics ofthe release reactions In the present study we have shownthat there is a significant effect of the position of substituentgroups on the rate and extent of release of isomers ofhydroxycinnamate While the release of the metaisomer wasinstantaneous and fast the orthoisomer release exhibiteda slower more sustained and complete release the orderof reaction rates was m-HCn gt p-HCn gt o-HCn withthe o-HCn nanohybrid showing an induction period Theposition of the hydroxyl groups resulted in differences inthe interlayer interactions of the anions with presence ofinterlayer hydrogen bonds affecting the rates of the reaction
Advances in Physical Chemistry 11
The formation of hydrogen bonds between the layers and theintercalated orthoisomer resulted in greater stabilization ofthe nanohybrid and resulted in the observed induction periodand slow release The extent of reaction was correlated withthe magnitude of dipole moments These results highlightthe potential of tuning the release behavior of intercalatedbioactive compounds which bring control into both theamount released per given time and the period for totalrelease
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
This work was supported by the National Science Foundation(CHE-0809751)
References
[1] S Yamanaka T Sako andM Hattori ldquoAnion exchange in basiccopper acetaterdquo Chemistry Letters pp 1869ndash1872 1989
[2] E Kandare and J M Hossenlopp ldquoHydroxy double salt anionexchange kinetics effects of precursor structure and anionsizetrdquo Journal of Physical Chemistry B vol 109 no 17 pp 8469ndash8475 2005
[3] P K Dutta and M Puri ldquoAnion exchange in lithium aluminatehydroxidesrdquo Journal of Physical Chemistry vol 93 no 1 pp376ndash381 1989
[4] B M Choudary V S Jaya B R Reddy M L Kantam M MRao and S S Madhavendra ldquoSynthesis characterization ionexchange and catalytic properties of nanobinary and ternarymetal oxyhydroxidesrdquo Chemistry of Materials vol 17 no 10pp 2740ndash2743 2005
[5] A M Fogg J S Dunn S-G Shyu D R Cary and D OrsquoHareldquoSelective ion-exchange intercalation of isomeric dicarboxylateanions into the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Chemistry of Materials vol 10 no 1 pp 351ndash355 1998[6] T Ikeda H Amoh and T Yasunaga ldquoStereoselective exchange
kinetics of L- and D-histidines for Cl- in the interlayerof a hydrotalcite-like compound by the chemical relaxationmethodrdquo Journal of the American Chemical Society vol 106 no20 pp 5772ndash5775 1984
[7] M Meyn K Beneke and G Lagaly ldquoAnion-exchange reactionsof hydroxy double saltsrdquo Inorganic Chemistry vol 32 no 7 pp1209ndash1215 1993
[8] V Ambrogi G Fardella G Grandolini L Perioli and M CTiralti ldquoIntercalation compounds of hydrotalcite-like anionicclays with anti-inflammatory agents II uptake of diclofenac fora controlled release formulationrdquo AAPS PharmSciTech vol 3no 3 p E26 2002
[9] M Z Bin Hussein Z Zainal A H Yahaya and D WV Foo ldquoControlled release of a plant growth regulator 120572-naphthaleneacetate from the lamella of Zn-Al-layered doublehydroxide nanocompositerdquo Journal of Controlled Release vol82 no 2-3 pp 417ndash427 2002
[10] A I Khan A Ragavan B Fong et al ldquoRecent developments inthe use of layered double hydroxides as host materials for the
storage and triggered release of functional anionsrdquo Industrialand Engineering Chemistry Research vol 48 no 23 pp 10196ndash10205 2009
[11] M Silion D Hritcu I M Jaba et al ldquoIn vitro and invivo behavior of ketoprofen intercalated into layered doublehydroxidesrdquo Journal of Materials Science vol 21 no 11 pp3009ndash3018 2010
[12] J-H Yang Y-S Han M Park T Park S-J Hwang andJ-H Choy ldquoNew inorganic-based drug delivery system ofindole-3-acetic acid-layered metal hydroxide nanohybrids withcontrolled release raterdquo Chemistry of Materials vol 19 no 10pp 2679ndash2685 2007
[13] L Lei R P Vijayan and D OrsquoHare ldquoPreferential anionexchange intercalation of pyridinecarboxylate and toluate iso-mers in the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Journal of Materials Chemistry vol 11 no 12 pp 3276ndash32802001
[14] B Lotsch F Millange R I Walton and D OrsquoHare ldquoSepara-tion of nucleoside monophosphates using preferential anionexchange intercalation in layered double hydroxidesrdquo Solid StateSciences vol 3 no 8 pp 883ndash886 2001
[15] H Tagaya N Sasaki H Morioka and J Kadokawa ldquoPrepara-tion of new inorganicmdashorganic layered compounds hydroxydouble salts and preferential intercalation of organic carboxylicacids into themrdquoMolecular Crystals and Liquid Crystals Scienceand Technology A vol 341 pp 413ndash418 2000
[16] P P Kumar A G Kalinichev and R J Kirkpatrick ldquoHydra-tion swelling interlayer structure and hydrogen bondingin organolayered double hydroxides insights from moleculardynamics simulation of citrate-intercalated hydrotalciterdquo Jour-nal of Physical Chemistry B vol 110 no 9 pp 3841ndash3844 2006
[17] S V Prasanna and P V Kamath ldquoAnion-exchange reactions oflayered double hydroxides interplay between coulombic andh-bonding interactionsrdquo Industrial and Engineering ChemistryResearch vol 48 no 13 pp 6315ndash6320 2009
[18] Z Gu A CThomas Z P Xu J H Campbell and G Q Lu ldquoInvitro sustained release of LMWH from MgAl-layered doublehydroxide nanohybridsrdquo Chemistry of Materials vol 20 no 11pp 3715ndash3722 2008
[19] J T Rajamathi S Britto and M Rajamathi ldquoSynthesis andanion exchange reactions of a layered Copper-Zincrdquo Journal ofChemical Sciences vol 117 no 6 pp 629ndash633 2005
[20] M Frisch G Trucks H Schlegel et al ldquoGaussian 98 RevisionA114rdquo 2002
[21] S Majoni and J M Hossenlopp ldquoAnion exchange kinetics ofnanodimensional layered metal hydroxides use of isoconver-sional analysisrdquo Journal of Physical Chemistry A vol 114 no 49pp 12858ndash12869 2010
[22] C Chouillet J-M Krafft C Louis and H Lauron-PernotldquoCharacterization of zinc hydroxynitrates by diffuse reflectanceinfrared spectroscopymdashstructural modifications during ther-mal treatmentrdquo Spectrochimica Acta A vol 60 no 3 pp 505ndash511 2004
[23] W Fujita and K Awaga ldquoMagnetic properties of Cu2(OH)
3(al-
kanecarboxylate) compounds drastic mod-ification withextension of the Alkyl chainrdquo Inorganic Chemistry vol 35 no7 pp 1915ndash1917 1996
[24] R Swisłocka M Kowczyk-Sadowy M Kalinowska and WLewandowski ldquoSpectroscopic (FT-IR FT-Raman1H and13CNMR) and theoretical studies of p-coumaric acid and alkalimetal p-coumaratesrdquo Spectroscopy vol 27 no 1 pp 35ndash48 2012
12 Advances in Physical Chemistry
[25] T Biswick W Jones A Pacuła E Serwicka and J PodobinskildquoThe role of anhydrous zinc nitrate in the thermal decomposi-tion of the zinc hydroxy nitrates Zn
5(OH)
8(NO3)2sdot2H2O and
ZnOHNO3sdotH2Ordquo Journal of Solid State Chemistry vol 180 no
4 pp 1171ndash1179 2007[26] S-H Park and E L Cheol ldquoSynthesis characterization and
magnetic properties of a novel disulfonate-pillared copperhydroxide Cu
2(OH)
3(DS4)
12 DS4 = 14-butanedisulfonaterdquo
Bulletin of the Korean Chemical Society vol 27 no 10 pp 1587ndash1592 2006
[27] P Rabu M Drillon and C Hornick ldquoStructural and spectro-scopic study of organic-inorganic transitionmetal based layeredmagnetsrdquo Analusis vol 28 no 2 pp 103ndash108 2000
[28] L Lei A Khan and D OrsquoHare ldquoSelective anion-exchangeintercalation of isomeric benzoate anions into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo Journal of Solid State
Chemistry vol 178 no 12 pp 3648ndash3654 2005[29] W Krause H-J Bernhardt R S W Braithwaite U Kolitsch
and R Pritchard ldquoKapellasite Cu3Zn(OH)
6Cl2 a new mineral
from Lavrion Greece and its crystal structurerdquo MineralogicalMagazine vol 70 no 3 pp 329ndash340 2006
[30] L C Andrews L R Bernstein C M Foris et al PowderDiffraction File Inorganic International Centre for DiffractionData Newtown Square Pennsylvania 2009
[31] A Ragavan A I Khan and D OrsquoHare ldquoIsomer selectiveion-exchange intercalation of nitrophenolates into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotxH
2Ordquo Journal of Materials
Chemistry vol 16 no 6 pp 602ndash608 2006[32] A Ragavan A Khan and D OrsquoHare ldquoSelective intercalation
of chlorophenoxyacetates into the layered double hydroxide[LiAl2(OH)
6]ClsdotxH
2Ordquo Journal of Materials Chemistry vol 16
no 42 pp 4155ndash4159 2006[33] S Vyazovkin and C A Wight ldquoModel-free and model-fitting
approaches to kinetic analysis of isothermal and nonisothermaldatardquoThermochimica Acta vol 340-341 pp 53ndash68 1999
[34] R E Johnsen F Krumeich and P Norby ldquoStructural andmicrostructural changes during anion exchange ofCoAl layereddouble hydroxides an in situ X-ray powder diffraction studyrdquoJournal of Applied Crystallography vol 43 no 3 pp 434ndash4472010
[35] G R Williams T G Dunbar A J Beer A M Fogg and DOrsquoHare ldquoIntercalation chemistry of the novel layered doublehydroxides [MAl
4(OH)
12](NO
3)2sdotyH2O (M=Zn Cu Ni and
Co) 1 new organic intercalates and reaction mechanismsrdquoJournal of Materials Chemistry vol 16 no 13 pp 1222ndash12302006
[36] T E M Ten Hulscher and G Cornelissen ldquoEffect of tempera-ture on sorption equilibrium and sorption kinetics of organicmicropollutantsmdasha reviewrdquo Chemosphere vol 32 no 4 pp609ndash626 1996
[37] M Avrami ldquoKinetics of phase change I general theoryrdquo TheJournal of Chemical Physics vol 7 no 12 pp 1103ndash1112 1939
[38] M Avrami ldquoKinetics of phase change II Transformation-timerelations for random distribution of nucleirdquo The Journal ofChemical Physics vol 8 no 2 pp 212ndash224 1940
[39] R J Francis S OrsquoBrien A M Fogg et al ldquoTime-resolved in-situ energy and angular dispersive X-ray diffraction studies ofthe formation of themicroporous gallophosphateULM-5 underhydrothermal conditionsrdquo Journal of the American ChemicalSociety vol 121 no 5 pp 1002ndash1015 1999
[40] Z Miladinovic J Zakrzewska B Kovacevic and G BacicldquoMonitoring of crystallization processes during synthesis of
zeolite A by in situ 27Al NMR spectroscopyrdquo Materials Chem-istry and Physics vol 104 no 2-3 pp 384ndash389 2007
[41] R Serna-Guerrero and A Sayari ldquoModeling adsorption of CO2
on amine-functionalized mesoporous silica 2 kinetics andbreakthrough curvesrdquo Chemical Engineering Journal vol 161no 1-2 pp 182ndash190 2010
[42] J D Hancock and J H Sharp ldquoMethod of comparing solid-state kinetic data and its application to the decomposition ofkaolinite brucite and BaCO
3rdquo Journal of the American Ceramic
Society vol 55 no 2 pp 74ndash77 1972[43] F Auriemma C De Rosa and R Triolo ldquoSlow crystallization
kinetics of poly(vinyl alcohol) in confined environment duringcryotropic gelation of aqueous solutionsrdquo Macromolecules vol39 no 26 pp 9429ndash9434 2006
[44] R V Ramanujan and Y R Zhang ldquoQuantitative transmissionelectronmicroscopy analysis of the nanocrystallization kineticsof softmagnetic alloysrdquoPhysical ReviewB vol 74 no 22 ArticleID 224408 2006
[45] A G Kalinichev P Padma Kumar and R James KirkpatrickldquoMolecular dynamics computer simulations of the effects ofhydrogen bonding on the properties of layered double hydrox-ides intercalated with organic acidsrdquo Philosophical Magazinevol 90 no 17-18 pp 2475ndash2488 2010
Figure 5 Release profiles for (a) ZC-m-HCn (b) ZC-o-HCn and (c) ZC-p-HCn at 40∘C
the equilibrium constant and hence the amount released atequilibrium The complex nature of the exchange reactionsis highlighted when our previous results for the release ofcinnamate anion are considered [21] Cinnamate anion hasa dipole moment of 146 D which is lower than that of m-HCn but there was only 31 Cn released at equilibriumThis indicates a complex interplay of the involved factors asit has been shown that highly hydrophobic anions are easyto intercalate into the layers Due to the absence of hydroxylgroup on the benzene ring of Cn anion this isomer is morehydrophobic than the others and is therefore not readilydeintercalated into the polar aqueous solvent environment
From Figure 5 it can qualitatively be observed that therelease rates and the release profiles of the isomers aresignificantly different While the profiles for o-HCn releaseare sigmoidal those for the meta- and paraisomers aredeceleratory Reaction rates can be quantified by fittingexperimental data to reaction models which best describe(reproduce) the experimental data in the so-called model
fitting procedureThis procedure is applicable to singlemech-anism reactions in which the mechanism does not change asthe reaction progresses [33] We have shown previously thatisoconversional methods can be extended to anion exchangereactions in layered metal hydroxide as a strategy to identifywhen using model based approaches are appropriate [21] Aconstant 119864
119886as a function of extent of reaction indicates that
the reaction may effectively be characterized with a singlemechanism and in these cases model based procedures areapplicable [33]The integral isoconversional method (2) wasapplied to the kinetic data obtained here
ln 119905 = ln119892 (120572)
119860+119864119886
119877119879 (2)
In (2) 119864119886is the activation energy 120572 is the extent of
reaction 119860 is the preexponential factor 119905 is the time 119879 thetemperature 119877 is the molar gas constant and 119892(120572) is theintegral reactionmodelThe isoconversional analysis data forall the nanohybrids is shown in Figure 6 in which the 119864
119886
8 Advances in Physical Chemistry
02 04 06 08 1065
70
75
80
85ZC-o-HCn
Extent of reaction (120572)
Ea
(kJm
ol)
(a)
03 04 05 06 07 08 09
30
40
50
60ZC-p-HCn
Extent of reaction (120572)
Ea
(kJm
ol)
(b)
03 04 05 06 07 08 09 10
50
60
70
80ZC-m-HCn
Extent of reaction (120572)
Ea
(kJ m
olminus1)
(c)
Figure 6 Variation of effective 119864119886with 120572 for ZC-m-HCn ZC-o-HCn and ZC-p-HCn
values remains constant within experimental error over theentire conversion range for ZC-m-HCn and ZC-p-HCn
The 119864119886values for ZC-o-HCn are within 10 although
the changes at higher conversion appear significant comparedwith experimental uncertainty The linear decrease of the 119864
119886
at higher conversion above 120572 = 05 has been attributed toweakening of the electrostatic attraction between the anionsand the layers as the anions are progressively exchanged fromlayer to layer [34] The constant 119864
119886at low conversion may
be due to some solid state transformations occurring whichmight be responsible for the induction period observedin the PXRD analysis (Figure S3) Although the inductionperiod is short the process might still be governing theenergetics of the reaction even during the exchange periodresulting in the constant energy being observed up to extentof reaction of 05 Since 119864
119886values for exchange reactions at
the anion binding sites have been shown to range from 30to 70 kJmolminus1 depending on the nature and size of the guestanions [31 32 35] the reactionmay be controlled by chemical
reaction at the exchange site It is also important to note thatdiffusion of anions in a dense organicmatrix can have high119864
119886
values sometimes being more than 100 kJmolminus1 [36] as suchdiffusion limited process cannot be ruled out if the anions areclosely packed in the gallery
The 119864119886values for the release of m-HCn and p-HCn
are in the range of both reaction controlled process anddiffusion controlled processes [21] The fact that the 119864
119886does
not significantly change over the entire reaction may indicatethat the reaction is diffusion controlled The differences inthe mechanism between o-HCn release and the other twosystems may be due to different strength of interactions withthe layersThe presence of H-bonding between the layers andthe o-HCn anions may bring the anions closer to the layersresulting in an increased electrostatic attraction resulting inthe process being controlled by reaction at the exchangesite Since there was no significant variation of the 119864
119886with
conversion for m-HCn and p-HCn release and the energydifference for o-HCn release is too low to reflect a change in
Advances in Physical Chemistry 9
mechanism model based approach was used to obtain rateconstants for the exchange reactions The Avrami-Erofersquoevnucleation-growth model [37 38] was used here since ithas the advantage that it can be used to describe reactionsoccurring via different mechanisms making comparison ofthe processes possible since the order of reactions will be thesame In the model the extent of reaction (120572) depends on therate constant 119896 and a coefficient 119899 as shown in the followingequation
119886 (119905) = 1 minus 119890[minus(119896119905)
119899
] (3)
The coefficient 119899 (Avrami exponent) potentially containsinformation about the mechanism of the reaction [39] TheAvrami-Erofersquoev model has been applied successfully to solu-tion phase anion exchange reactions in layeredmaterials andother fluid phase reactions [2 10 40 41] we have also shownthat values of parameters obtained from analysis of bothsolid state transformations and solution phase processesemploying the Avrami model are comparable indicating thatthere is correlation between solid state transformation andfluid phase processes [21] The Avrami exponent is usuallyobtained by taking double logarithm of the Avrami-Erofersquoevequation obtaining (4) which was popularized by Sharp andHancock in 1972 [42] Consider
A plot of ln[minus ln(1 minus 120572)] as a function of ln 119905 (doublelogarithmic plot) gives a linear plot in which the value of 119899 isobtained from the slope and the value of 119896 is evaluated fromthe intercept
The extent of reaction versus time plots obtained for theZC-o-HCnClminus exchange reactions at different temperaturesis shown in Figure 7(a) The Avrami-Erofersquoev model wasapplied and correctly described the obtained experimentaldatawithin the120572 range of 015ndash10The validity of theAvrami-Erofersquoevmodel can be confirmedby the corresponding doublelogarithmic plots in Figure 7(b) where straight lines wereobtained with 1198772 values ranging from 0996 to 0999 Thedouble logarithmic plots are almost parallel indicating thatthe reaction proceeds with the same mechanism over thetemperature range used here The obtained 119899 value was closeto 10 consistent with a nucleation controlled mechanismwhich agrees well with the activation energies obtained fromisoconversional methods and discussed before
The release profiles together with the double logarithmicplots for the release of m-HCn and p-HCn are shown in thesupporting information Figures S4 and S5 respectively Thekinetics of m-HCn release also follows the Avrami-Erofersquoevmodel the data provided a good fit within the range 015 lt120572 lt 085 the fit to limited range of 120572 has been observedin other intercalation reactions Anomalously low values ofthe Avrami exponent observed here (shown in Table 3) havebeen reported in polymer and alloy crystallization and donot have physical mechanistic meaning [43 44] The Avramiexponent values obtained for the release of p-HCn are closeto 05 which is consistent with diffusion controlled reactions[10] Diffusion control of such a high 119864
119886highlights two
0 2 4 6 8 10 12 1400
02
04
06
08
10
Time (hours)
Exte
nt o
f rea
ctio
n (120572
)
300∘C400∘C500∘C600∘C
(a)
4 6 8 10
0
1
minus1
minus2
ln (t minus t0)
lnminusln(1
minus120572)
30∘C40∘C50∘C60∘C
(b)
Figure 7 Extent of reaction as a function of time (a) and doublelogarithmic plots (b) for the exchange reaction of Clminus anion andZC-o-HCn at various temperatures 60∘C (◻) 50∘C (Q) 40∘C (998771)and 30∘C (e)
issues (a) the interlayer space is densely packed and thediffusional resistance within the interlayer space approachesthat in polymers in which119864
119886values can be above 100 kJmolminus1
depending on the diffusing molecule [36] and (b) Kineticambiguity is common in model based approaches in thiscase the mechanistic interpretation from the model is not inline with the kinetic parameters obtained
FromTable 3 it can be concluded that the rate of release ofm-HCn from the nanohybrids is much faster compared with
10 Advances in Physical Chemistry
Table 3 A summary of kinetic parameters obtained at different temperatures
Nanohybrid Temperature (∘C) 119899 119896 sminus1 (lowast10minus3)
ZC-o-HCn
600 082 plusmn 002 17 plusmn 03
500 090 plusmn 002 07 plusmn 01
400 088 plusmn 001 029 plusmn 005
300 102 plusmn 001 010 plusmn 001
ZC-m-HCn
600 024 plusmn 001 31 plusmn 9
500 024 plusmn 001 20 plusmn 7
400 021 plusmn 002 7 plusmn 2
300 0225 plusmn 0007 35 plusmn 08
ZC-p-HCn
600 041 plusmn 001 29 plusmn 05
500 045 plusmn 002 16 plusmn 04
400 046 plusmn 001 10 plusmn 02
300 039 plusmn 001 05 plusmn 01
the other isomers As an example the rate constants at 40∘Cwere (7 plusmn 2) times 10minus3 sminus1 for m-HCn (10 plusmn 02) times 10minus3 sminus1 forp-HCn and (029 plusmn 005) times 10minus3 sminus1 for o-HCn The rate ofrelease follows the following order
119898ndashHCn gt 119901-HCn gt 119900-HCn
The release profile form-HCn at 40∘C (Figure 5(a)) showsthat there is a burst and rapid release of the intercalated anionin the first 10 minutes followed by a more sustained releasewith equilibrium being attained after 3 hours This burstrelease is observed for all the temperatures used here (FigureS2a in supporting information) and has been observed beforein anion exchange reactions in LDHs [10] The burst releaseobserved here is due to anion exchange as confirmed byPXRD results in Figure S3 The chloride phase representedby an asterisk in Figure S3 was significant at 30 seconds (theearliest we could sample) of ZC-m-HCn being in contact withthe chloride solution The initial slow and linear release inZC-o-HCn indicated in Figure 5(b) and confirmed by XRDprofiles in Figure S3 may be due to the increased stabilityof the HDS structure due to extensive hydrogen bonding asindicated by FTIR data in Figure 3 and the close arrangementof the anions implied by XRDprofiles in Figure 2(a)This firststage of release has a strong temperature dependence beingmore pronounced at low temperature and almost absent athigher temperatures (Figure S2b in supporting information)
Factors which may influence the rates of release includeinteractions between the metal hydroxide layers and theanions anion-anion interactions within the layers and in theexchange medium and solvent-anion interaction [10] It isexpected that these factors will have a complex effect on thereaction rates since they affect each isomer differently Thecomplex interplay of the abovementioned factors oftenmakeit difficult to come up with a simple relationship betweenthe rate of release and the aforementioned factors [10] It isimportant to note that these factors are expected to dependon the types and positions of chemical groups on the anionThepresence of hydrogen bondingwithin the interlayer spacereduces the Gibbs free energy of formation of the nanohy-brids and stabilizes the system [45]The rate of release of ionsthen depends on the differences in the energy of nanohybrid
and the barrier to reaction As indicated in Figure 3 there issignificant level of hydrogen bonding in ZC-o-HCn whichthen resulted in a close interaction with the layers Thehydrogen bond network in the nanohybrid and the closeinteraction of the anions with the layers stabilized the systemas compared to the other nanohybrids The nanohybrid inwhich these interactions are low is relatively less stable and isexpected to proceed faster to equilibrium this is consistentwith results obtained here The anions in the unstablelessstable nanohybrid prefer bulk solution (in which there isstabilization by solvation with water molecules) to galleryspace which may result in the burst and fast release observedin ZC-m-HCnThe modification of bioactive molecules withgroups capable of hydrogen bonding for example hydroxylgroups especially at positions which allow interactions withthe layers may enable tuning of the release behavior ofthese molecules from layered materials It is important tonote that the modification should be at positions whichdo not affect the bioactivity of the molecules Additionalexperiments including studies on other isomeric anions withgroups capable of forming hydrogen bonds and effect ofmetallayer composition will be helpful in further elucidating howthe position of substituents control the rate as well as theextent of reaction
4 Conclusions
Anion exchange reactions in hydroxy double salts are affectedby the structure of the intercalated anions and there is astrong interplay between thermodynamics and kinetics ofthe release reactions In the present study we have shownthat there is a significant effect of the position of substituentgroups on the rate and extent of release of isomers ofhydroxycinnamate While the release of the metaisomer wasinstantaneous and fast the orthoisomer release exhibiteda slower more sustained and complete release the orderof reaction rates was m-HCn gt p-HCn gt o-HCn withthe o-HCn nanohybrid showing an induction period Theposition of the hydroxyl groups resulted in differences inthe interlayer interactions of the anions with presence ofinterlayer hydrogen bonds affecting the rates of the reaction
Advances in Physical Chemistry 11
The formation of hydrogen bonds between the layers and theintercalated orthoisomer resulted in greater stabilization ofthe nanohybrid and resulted in the observed induction periodand slow release The extent of reaction was correlated withthe magnitude of dipole moments These results highlightthe potential of tuning the release behavior of intercalatedbioactive compounds which bring control into both theamount released per given time and the period for totalrelease
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
This work was supported by the National Science Foundation(CHE-0809751)
References
[1] S Yamanaka T Sako andM Hattori ldquoAnion exchange in basiccopper acetaterdquo Chemistry Letters pp 1869ndash1872 1989
[2] E Kandare and J M Hossenlopp ldquoHydroxy double salt anionexchange kinetics effects of precursor structure and anionsizetrdquo Journal of Physical Chemistry B vol 109 no 17 pp 8469ndash8475 2005
[3] P K Dutta and M Puri ldquoAnion exchange in lithium aluminatehydroxidesrdquo Journal of Physical Chemistry vol 93 no 1 pp376ndash381 1989
[4] B M Choudary V S Jaya B R Reddy M L Kantam M MRao and S S Madhavendra ldquoSynthesis characterization ionexchange and catalytic properties of nanobinary and ternarymetal oxyhydroxidesrdquo Chemistry of Materials vol 17 no 10pp 2740ndash2743 2005
[5] A M Fogg J S Dunn S-G Shyu D R Cary and D OrsquoHareldquoSelective ion-exchange intercalation of isomeric dicarboxylateanions into the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Chemistry of Materials vol 10 no 1 pp 351ndash355 1998[6] T Ikeda H Amoh and T Yasunaga ldquoStereoselective exchange
kinetics of L- and D-histidines for Cl- in the interlayerof a hydrotalcite-like compound by the chemical relaxationmethodrdquo Journal of the American Chemical Society vol 106 no20 pp 5772ndash5775 1984
[7] M Meyn K Beneke and G Lagaly ldquoAnion-exchange reactionsof hydroxy double saltsrdquo Inorganic Chemistry vol 32 no 7 pp1209ndash1215 1993
[8] V Ambrogi G Fardella G Grandolini L Perioli and M CTiralti ldquoIntercalation compounds of hydrotalcite-like anionicclays with anti-inflammatory agents II uptake of diclofenac fora controlled release formulationrdquo AAPS PharmSciTech vol 3no 3 p E26 2002
[9] M Z Bin Hussein Z Zainal A H Yahaya and D WV Foo ldquoControlled release of a plant growth regulator 120572-naphthaleneacetate from the lamella of Zn-Al-layered doublehydroxide nanocompositerdquo Journal of Controlled Release vol82 no 2-3 pp 417ndash427 2002
[10] A I Khan A Ragavan B Fong et al ldquoRecent developments inthe use of layered double hydroxides as host materials for the
storage and triggered release of functional anionsrdquo Industrialand Engineering Chemistry Research vol 48 no 23 pp 10196ndash10205 2009
[11] M Silion D Hritcu I M Jaba et al ldquoIn vitro and invivo behavior of ketoprofen intercalated into layered doublehydroxidesrdquo Journal of Materials Science vol 21 no 11 pp3009ndash3018 2010
[12] J-H Yang Y-S Han M Park T Park S-J Hwang andJ-H Choy ldquoNew inorganic-based drug delivery system ofindole-3-acetic acid-layered metal hydroxide nanohybrids withcontrolled release raterdquo Chemistry of Materials vol 19 no 10pp 2679ndash2685 2007
[13] L Lei R P Vijayan and D OrsquoHare ldquoPreferential anionexchange intercalation of pyridinecarboxylate and toluate iso-mers in the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Journal of Materials Chemistry vol 11 no 12 pp 3276ndash32802001
[14] B Lotsch F Millange R I Walton and D OrsquoHare ldquoSepara-tion of nucleoside monophosphates using preferential anionexchange intercalation in layered double hydroxidesrdquo Solid StateSciences vol 3 no 8 pp 883ndash886 2001
[15] H Tagaya N Sasaki H Morioka and J Kadokawa ldquoPrepara-tion of new inorganicmdashorganic layered compounds hydroxydouble salts and preferential intercalation of organic carboxylicacids into themrdquoMolecular Crystals and Liquid Crystals Scienceand Technology A vol 341 pp 413ndash418 2000
[16] P P Kumar A G Kalinichev and R J Kirkpatrick ldquoHydra-tion swelling interlayer structure and hydrogen bondingin organolayered double hydroxides insights from moleculardynamics simulation of citrate-intercalated hydrotalciterdquo Jour-nal of Physical Chemistry B vol 110 no 9 pp 3841ndash3844 2006
[17] S V Prasanna and P V Kamath ldquoAnion-exchange reactions oflayered double hydroxides interplay between coulombic andh-bonding interactionsrdquo Industrial and Engineering ChemistryResearch vol 48 no 13 pp 6315ndash6320 2009
[18] Z Gu A CThomas Z P Xu J H Campbell and G Q Lu ldquoInvitro sustained release of LMWH from MgAl-layered doublehydroxide nanohybridsrdquo Chemistry of Materials vol 20 no 11pp 3715ndash3722 2008
[19] J T Rajamathi S Britto and M Rajamathi ldquoSynthesis andanion exchange reactions of a layered Copper-Zincrdquo Journal ofChemical Sciences vol 117 no 6 pp 629ndash633 2005
[20] M Frisch G Trucks H Schlegel et al ldquoGaussian 98 RevisionA114rdquo 2002
[21] S Majoni and J M Hossenlopp ldquoAnion exchange kinetics ofnanodimensional layered metal hydroxides use of isoconver-sional analysisrdquo Journal of Physical Chemistry A vol 114 no 49pp 12858ndash12869 2010
[22] C Chouillet J-M Krafft C Louis and H Lauron-PernotldquoCharacterization of zinc hydroxynitrates by diffuse reflectanceinfrared spectroscopymdashstructural modifications during ther-mal treatmentrdquo Spectrochimica Acta A vol 60 no 3 pp 505ndash511 2004
[23] W Fujita and K Awaga ldquoMagnetic properties of Cu2(OH)
3(al-
kanecarboxylate) compounds drastic mod-ification withextension of the Alkyl chainrdquo Inorganic Chemistry vol 35 no7 pp 1915ndash1917 1996
[24] R Swisłocka M Kowczyk-Sadowy M Kalinowska and WLewandowski ldquoSpectroscopic (FT-IR FT-Raman1H and13CNMR) and theoretical studies of p-coumaric acid and alkalimetal p-coumaratesrdquo Spectroscopy vol 27 no 1 pp 35ndash48 2012
12 Advances in Physical Chemistry
[25] T Biswick W Jones A Pacuła E Serwicka and J PodobinskildquoThe role of anhydrous zinc nitrate in the thermal decomposi-tion of the zinc hydroxy nitrates Zn
5(OH)
8(NO3)2sdot2H2O and
ZnOHNO3sdotH2Ordquo Journal of Solid State Chemistry vol 180 no
4 pp 1171ndash1179 2007[26] S-H Park and E L Cheol ldquoSynthesis characterization and
magnetic properties of a novel disulfonate-pillared copperhydroxide Cu
2(OH)
3(DS4)
12 DS4 = 14-butanedisulfonaterdquo
Bulletin of the Korean Chemical Society vol 27 no 10 pp 1587ndash1592 2006
[27] P Rabu M Drillon and C Hornick ldquoStructural and spectro-scopic study of organic-inorganic transitionmetal based layeredmagnetsrdquo Analusis vol 28 no 2 pp 103ndash108 2000
[28] L Lei A Khan and D OrsquoHare ldquoSelective anion-exchangeintercalation of isomeric benzoate anions into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo Journal of Solid State
Chemistry vol 178 no 12 pp 3648ndash3654 2005[29] W Krause H-J Bernhardt R S W Braithwaite U Kolitsch
and R Pritchard ldquoKapellasite Cu3Zn(OH)
6Cl2 a new mineral
from Lavrion Greece and its crystal structurerdquo MineralogicalMagazine vol 70 no 3 pp 329ndash340 2006
[30] L C Andrews L R Bernstein C M Foris et al PowderDiffraction File Inorganic International Centre for DiffractionData Newtown Square Pennsylvania 2009
[31] A Ragavan A I Khan and D OrsquoHare ldquoIsomer selectiveion-exchange intercalation of nitrophenolates into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotxH
2Ordquo Journal of Materials
Chemistry vol 16 no 6 pp 602ndash608 2006[32] A Ragavan A Khan and D OrsquoHare ldquoSelective intercalation
of chlorophenoxyacetates into the layered double hydroxide[LiAl2(OH)
6]ClsdotxH
2Ordquo Journal of Materials Chemistry vol 16
no 42 pp 4155ndash4159 2006[33] S Vyazovkin and C A Wight ldquoModel-free and model-fitting
approaches to kinetic analysis of isothermal and nonisothermaldatardquoThermochimica Acta vol 340-341 pp 53ndash68 1999
[34] R E Johnsen F Krumeich and P Norby ldquoStructural andmicrostructural changes during anion exchange ofCoAl layereddouble hydroxides an in situ X-ray powder diffraction studyrdquoJournal of Applied Crystallography vol 43 no 3 pp 434ndash4472010
[35] G R Williams T G Dunbar A J Beer A M Fogg and DOrsquoHare ldquoIntercalation chemistry of the novel layered doublehydroxides [MAl
4(OH)
12](NO
3)2sdotyH2O (M=Zn Cu Ni and
Co) 1 new organic intercalates and reaction mechanismsrdquoJournal of Materials Chemistry vol 16 no 13 pp 1222ndash12302006
[36] T E M Ten Hulscher and G Cornelissen ldquoEffect of tempera-ture on sorption equilibrium and sorption kinetics of organicmicropollutantsmdasha reviewrdquo Chemosphere vol 32 no 4 pp609ndash626 1996
[37] M Avrami ldquoKinetics of phase change I general theoryrdquo TheJournal of Chemical Physics vol 7 no 12 pp 1103ndash1112 1939
[38] M Avrami ldquoKinetics of phase change II Transformation-timerelations for random distribution of nucleirdquo The Journal ofChemical Physics vol 8 no 2 pp 212ndash224 1940
[39] R J Francis S OrsquoBrien A M Fogg et al ldquoTime-resolved in-situ energy and angular dispersive X-ray diffraction studies ofthe formation of themicroporous gallophosphateULM-5 underhydrothermal conditionsrdquo Journal of the American ChemicalSociety vol 121 no 5 pp 1002ndash1015 1999
[40] Z Miladinovic J Zakrzewska B Kovacevic and G BacicldquoMonitoring of crystallization processes during synthesis of
zeolite A by in situ 27Al NMR spectroscopyrdquo Materials Chem-istry and Physics vol 104 no 2-3 pp 384ndash389 2007
[41] R Serna-Guerrero and A Sayari ldquoModeling adsorption of CO2
on amine-functionalized mesoporous silica 2 kinetics andbreakthrough curvesrdquo Chemical Engineering Journal vol 161no 1-2 pp 182ndash190 2010
[42] J D Hancock and J H Sharp ldquoMethod of comparing solid-state kinetic data and its application to the decomposition ofkaolinite brucite and BaCO
3rdquo Journal of the American Ceramic
Society vol 55 no 2 pp 74ndash77 1972[43] F Auriemma C De Rosa and R Triolo ldquoSlow crystallization
kinetics of poly(vinyl alcohol) in confined environment duringcryotropic gelation of aqueous solutionsrdquo Macromolecules vol39 no 26 pp 9429ndash9434 2006
[44] R V Ramanujan and Y R Zhang ldquoQuantitative transmissionelectronmicroscopy analysis of the nanocrystallization kineticsof softmagnetic alloysrdquoPhysical ReviewB vol 74 no 22 ArticleID 224408 2006
[45] A G Kalinichev P Padma Kumar and R James KirkpatrickldquoMolecular dynamics computer simulations of the effects ofhydrogen bonding on the properties of layered double hydrox-ides intercalated with organic acidsrdquo Philosophical Magazinevol 90 no 17-18 pp 2475ndash2488 2010
Figure 6 Variation of effective 119864119886with 120572 for ZC-m-HCn ZC-o-HCn and ZC-p-HCn
values remains constant within experimental error over theentire conversion range for ZC-m-HCn and ZC-p-HCn
The 119864119886values for ZC-o-HCn are within 10 although
the changes at higher conversion appear significant comparedwith experimental uncertainty The linear decrease of the 119864
119886
at higher conversion above 120572 = 05 has been attributed toweakening of the electrostatic attraction between the anionsand the layers as the anions are progressively exchanged fromlayer to layer [34] The constant 119864
119886at low conversion may
be due to some solid state transformations occurring whichmight be responsible for the induction period observedin the PXRD analysis (Figure S3) Although the inductionperiod is short the process might still be governing theenergetics of the reaction even during the exchange periodresulting in the constant energy being observed up to extentof reaction of 05 Since 119864
119886values for exchange reactions at
the anion binding sites have been shown to range from 30to 70 kJmolminus1 depending on the nature and size of the guestanions [31 32 35] the reactionmay be controlled by chemical
reaction at the exchange site It is also important to note thatdiffusion of anions in a dense organicmatrix can have high119864
119886
values sometimes being more than 100 kJmolminus1 [36] as suchdiffusion limited process cannot be ruled out if the anions areclosely packed in the gallery
The 119864119886values for the release of m-HCn and p-HCn
are in the range of both reaction controlled process anddiffusion controlled processes [21] The fact that the 119864
119886does
not significantly change over the entire reaction may indicatethat the reaction is diffusion controlled The differences inthe mechanism between o-HCn release and the other twosystems may be due to different strength of interactions withthe layersThe presence of H-bonding between the layers andthe o-HCn anions may bring the anions closer to the layersresulting in an increased electrostatic attraction resulting inthe process being controlled by reaction at the exchangesite Since there was no significant variation of the 119864
119886with
conversion for m-HCn and p-HCn release and the energydifference for o-HCn release is too low to reflect a change in
Advances in Physical Chemistry 9
mechanism model based approach was used to obtain rateconstants for the exchange reactions The Avrami-Erofersquoevnucleation-growth model [37 38] was used here since ithas the advantage that it can be used to describe reactionsoccurring via different mechanisms making comparison ofthe processes possible since the order of reactions will be thesame In the model the extent of reaction (120572) depends on therate constant 119896 and a coefficient 119899 as shown in the followingequation
119886 (119905) = 1 minus 119890[minus(119896119905)
119899
] (3)
The coefficient 119899 (Avrami exponent) potentially containsinformation about the mechanism of the reaction [39] TheAvrami-Erofersquoev model has been applied successfully to solu-tion phase anion exchange reactions in layeredmaterials andother fluid phase reactions [2 10 40 41] we have also shownthat values of parameters obtained from analysis of bothsolid state transformations and solution phase processesemploying the Avrami model are comparable indicating thatthere is correlation between solid state transformation andfluid phase processes [21] The Avrami exponent is usuallyobtained by taking double logarithm of the Avrami-Erofersquoevequation obtaining (4) which was popularized by Sharp andHancock in 1972 [42] Consider
A plot of ln[minus ln(1 minus 120572)] as a function of ln 119905 (doublelogarithmic plot) gives a linear plot in which the value of 119899 isobtained from the slope and the value of 119896 is evaluated fromthe intercept
The extent of reaction versus time plots obtained for theZC-o-HCnClminus exchange reactions at different temperaturesis shown in Figure 7(a) The Avrami-Erofersquoev model wasapplied and correctly described the obtained experimentaldatawithin the120572 range of 015ndash10The validity of theAvrami-Erofersquoevmodel can be confirmedby the corresponding doublelogarithmic plots in Figure 7(b) where straight lines wereobtained with 1198772 values ranging from 0996 to 0999 Thedouble logarithmic plots are almost parallel indicating thatthe reaction proceeds with the same mechanism over thetemperature range used here The obtained 119899 value was closeto 10 consistent with a nucleation controlled mechanismwhich agrees well with the activation energies obtained fromisoconversional methods and discussed before
The release profiles together with the double logarithmicplots for the release of m-HCn and p-HCn are shown in thesupporting information Figures S4 and S5 respectively Thekinetics of m-HCn release also follows the Avrami-Erofersquoevmodel the data provided a good fit within the range 015 lt120572 lt 085 the fit to limited range of 120572 has been observedin other intercalation reactions Anomalously low values ofthe Avrami exponent observed here (shown in Table 3) havebeen reported in polymer and alloy crystallization and donot have physical mechanistic meaning [43 44] The Avramiexponent values obtained for the release of p-HCn are closeto 05 which is consistent with diffusion controlled reactions[10] Diffusion control of such a high 119864
119886highlights two
0 2 4 6 8 10 12 1400
02
04
06
08
10
Time (hours)
Exte
nt o
f rea
ctio
n (120572
)
300∘C400∘C500∘C600∘C
(a)
4 6 8 10
0
1
minus1
minus2
ln (t minus t0)
lnminusln(1
minus120572)
30∘C40∘C50∘C60∘C
(b)
Figure 7 Extent of reaction as a function of time (a) and doublelogarithmic plots (b) for the exchange reaction of Clminus anion andZC-o-HCn at various temperatures 60∘C (◻) 50∘C (Q) 40∘C (998771)and 30∘C (e)
issues (a) the interlayer space is densely packed and thediffusional resistance within the interlayer space approachesthat in polymers in which119864
119886values can be above 100 kJmolminus1
depending on the diffusing molecule [36] and (b) Kineticambiguity is common in model based approaches in thiscase the mechanistic interpretation from the model is not inline with the kinetic parameters obtained
FromTable 3 it can be concluded that the rate of release ofm-HCn from the nanohybrids is much faster compared with
10 Advances in Physical Chemistry
Table 3 A summary of kinetic parameters obtained at different temperatures
Nanohybrid Temperature (∘C) 119899 119896 sminus1 (lowast10minus3)
ZC-o-HCn
600 082 plusmn 002 17 plusmn 03
500 090 plusmn 002 07 plusmn 01
400 088 plusmn 001 029 plusmn 005
300 102 plusmn 001 010 plusmn 001
ZC-m-HCn
600 024 plusmn 001 31 plusmn 9
500 024 plusmn 001 20 plusmn 7
400 021 plusmn 002 7 plusmn 2
300 0225 plusmn 0007 35 plusmn 08
ZC-p-HCn
600 041 plusmn 001 29 plusmn 05
500 045 plusmn 002 16 plusmn 04
400 046 plusmn 001 10 plusmn 02
300 039 plusmn 001 05 plusmn 01
the other isomers As an example the rate constants at 40∘Cwere (7 plusmn 2) times 10minus3 sminus1 for m-HCn (10 plusmn 02) times 10minus3 sminus1 forp-HCn and (029 plusmn 005) times 10minus3 sminus1 for o-HCn The rate ofrelease follows the following order
119898ndashHCn gt 119901-HCn gt 119900-HCn
The release profile form-HCn at 40∘C (Figure 5(a)) showsthat there is a burst and rapid release of the intercalated anionin the first 10 minutes followed by a more sustained releasewith equilibrium being attained after 3 hours This burstrelease is observed for all the temperatures used here (FigureS2a in supporting information) and has been observed beforein anion exchange reactions in LDHs [10] The burst releaseobserved here is due to anion exchange as confirmed byPXRD results in Figure S3 The chloride phase representedby an asterisk in Figure S3 was significant at 30 seconds (theearliest we could sample) of ZC-m-HCn being in contact withthe chloride solution The initial slow and linear release inZC-o-HCn indicated in Figure 5(b) and confirmed by XRDprofiles in Figure S3 may be due to the increased stabilityof the HDS structure due to extensive hydrogen bonding asindicated by FTIR data in Figure 3 and the close arrangementof the anions implied by XRDprofiles in Figure 2(a)This firststage of release has a strong temperature dependence beingmore pronounced at low temperature and almost absent athigher temperatures (Figure S2b in supporting information)
Factors which may influence the rates of release includeinteractions between the metal hydroxide layers and theanions anion-anion interactions within the layers and in theexchange medium and solvent-anion interaction [10] It isexpected that these factors will have a complex effect on thereaction rates since they affect each isomer differently Thecomplex interplay of the abovementioned factors oftenmakeit difficult to come up with a simple relationship betweenthe rate of release and the aforementioned factors [10] It isimportant to note that these factors are expected to dependon the types and positions of chemical groups on the anionThepresence of hydrogen bondingwithin the interlayer spacereduces the Gibbs free energy of formation of the nanohy-brids and stabilizes the system [45]The rate of release of ionsthen depends on the differences in the energy of nanohybrid
and the barrier to reaction As indicated in Figure 3 there issignificant level of hydrogen bonding in ZC-o-HCn whichthen resulted in a close interaction with the layers Thehydrogen bond network in the nanohybrid and the closeinteraction of the anions with the layers stabilized the systemas compared to the other nanohybrids The nanohybrid inwhich these interactions are low is relatively less stable and isexpected to proceed faster to equilibrium this is consistentwith results obtained here The anions in the unstablelessstable nanohybrid prefer bulk solution (in which there isstabilization by solvation with water molecules) to galleryspace which may result in the burst and fast release observedin ZC-m-HCnThe modification of bioactive molecules withgroups capable of hydrogen bonding for example hydroxylgroups especially at positions which allow interactions withthe layers may enable tuning of the release behavior ofthese molecules from layered materials It is important tonote that the modification should be at positions whichdo not affect the bioactivity of the molecules Additionalexperiments including studies on other isomeric anions withgroups capable of forming hydrogen bonds and effect ofmetallayer composition will be helpful in further elucidating howthe position of substituents control the rate as well as theextent of reaction
4 Conclusions
Anion exchange reactions in hydroxy double salts are affectedby the structure of the intercalated anions and there is astrong interplay between thermodynamics and kinetics ofthe release reactions In the present study we have shownthat there is a significant effect of the position of substituentgroups on the rate and extent of release of isomers ofhydroxycinnamate While the release of the metaisomer wasinstantaneous and fast the orthoisomer release exhibiteda slower more sustained and complete release the orderof reaction rates was m-HCn gt p-HCn gt o-HCn withthe o-HCn nanohybrid showing an induction period Theposition of the hydroxyl groups resulted in differences inthe interlayer interactions of the anions with presence ofinterlayer hydrogen bonds affecting the rates of the reaction
Advances in Physical Chemistry 11
The formation of hydrogen bonds between the layers and theintercalated orthoisomer resulted in greater stabilization ofthe nanohybrid and resulted in the observed induction periodand slow release The extent of reaction was correlated withthe magnitude of dipole moments These results highlightthe potential of tuning the release behavior of intercalatedbioactive compounds which bring control into both theamount released per given time and the period for totalrelease
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
This work was supported by the National Science Foundation(CHE-0809751)
References
[1] S Yamanaka T Sako andM Hattori ldquoAnion exchange in basiccopper acetaterdquo Chemistry Letters pp 1869ndash1872 1989
[2] E Kandare and J M Hossenlopp ldquoHydroxy double salt anionexchange kinetics effects of precursor structure and anionsizetrdquo Journal of Physical Chemistry B vol 109 no 17 pp 8469ndash8475 2005
[3] P K Dutta and M Puri ldquoAnion exchange in lithium aluminatehydroxidesrdquo Journal of Physical Chemistry vol 93 no 1 pp376ndash381 1989
[4] B M Choudary V S Jaya B R Reddy M L Kantam M MRao and S S Madhavendra ldquoSynthesis characterization ionexchange and catalytic properties of nanobinary and ternarymetal oxyhydroxidesrdquo Chemistry of Materials vol 17 no 10pp 2740ndash2743 2005
[5] A M Fogg J S Dunn S-G Shyu D R Cary and D OrsquoHareldquoSelective ion-exchange intercalation of isomeric dicarboxylateanions into the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Chemistry of Materials vol 10 no 1 pp 351ndash355 1998[6] T Ikeda H Amoh and T Yasunaga ldquoStereoselective exchange
kinetics of L- and D-histidines for Cl- in the interlayerof a hydrotalcite-like compound by the chemical relaxationmethodrdquo Journal of the American Chemical Society vol 106 no20 pp 5772ndash5775 1984
[7] M Meyn K Beneke and G Lagaly ldquoAnion-exchange reactionsof hydroxy double saltsrdquo Inorganic Chemistry vol 32 no 7 pp1209ndash1215 1993
[8] V Ambrogi G Fardella G Grandolini L Perioli and M CTiralti ldquoIntercalation compounds of hydrotalcite-like anionicclays with anti-inflammatory agents II uptake of diclofenac fora controlled release formulationrdquo AAPS PharmSciTech vol 3no 3 p E26 2002
[9] M Z Bin Hussein Z Zainal A H Yahaya and D WV Foo ldquoControlled release of a plant growth regulator 120572-naphthaleneacetate from the lamella of Zn-Al-layered doublehydroxide nanocompositerdquo Journal of Controlled Release vol82 no 2-3 pp 417ndash427 2002
[10] A I Khan A Ragavan B Fong et al ldquoRecent developments inthe use of layered double hydroxides as host materials for the
storage and triggered release of functional anionsrdquo Industrialand Engineering Chemistry Research vol 48 no 23 pp 10196ndash10205 2009
[11] M Silion D Hritcu I M Jaba et al ldquoIn vitro and invivo behavior of ketoprofen intercalated into layered doublehydroxidesrdquo Journal of Materials Science vol 21 no 11 pp3009ndash3018 2010
[12] J-H Yang Y-S Han M Park T Park S-J Hwang andJ-H Choy ldquoNew inorganic-based drug delivery system ofindole-3-acetic acid-layered metal hydroxide nanohybrids withcontrolled release raterdquo Chemistry of Materials vol 19 no 10pp 2679ndash2685 2007
[13] L Lei R P Vijayan and D OrsquoHare ldquoPreferential anionexchange intercalation of pyridinecarboxylate and toluate iso-mers in the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Journal of Materials Chemistry vol 11 no 12 pp 3276ndash32802001
[14] B Lotsch F Millange R I Walton and D OrsquoHare ldquoSepara-tion of nucleoside monophosphates using preferential anionexchange intercalation in layered double hydroxidesrdquo Solid StateSciences vol 3 no 8 pp 883ndash886 2001
[15] H Tagaya N Sasaki H Morioka and J Kadokawa ldquoPrepara-tion of new inorganicmdashorganic layered compounds hydroxydouble salts and preferential intercalation of organic carboxylicacids into themrdquoMolecular Crystals and Liquid Crystals Scienceand Technology A vol 341 pp 413ndash418 2000
[16] P P Kumar A G Kalinichev and R J Kirkpatrick ldquoHydra-tion swelling interlayer structure and hydrogen bondingin organolayered double hydroxides insights from moleculardynamics simulation of citrate-intercalated hydrotalciterdquo Jour-nal of Physical Chemistry B vol 110 no 9 pp 3841ndash3844 2006
[17] S V Prasanna and P V Kamath ldquoAnion-exchange reactions oflayered double hydroxides interplay between coulombic andh-bonding interactionsrdquo Industrial and Engineering ChemistryResearch vol 48 no 13 pp 6315ndash6320 2009
[18] Z Gu A CThomas Z P Xu J H Campbell and G Q Lu ldquoInvitro sustained release of LMWH from MgAl-layered doublehydroxide nanohybridsrdquo Chemistry of Materials vol 20 no 11pp 3715ndash3722 2008
[19] J T Rajamathi S Britto and M Rajamathi ldquoSynthesis andanion exchange reactions of a layered Copper-Zincrdquo Journal ofChemical Sciences vol 117 no 6 pp 629ndash633 2005
[20] M Frisch G Trucks H Schlegel et al ldquoGaussian 98 RevisionA114rdquo 2002
[21] S Majoni and J M Hossenlopp ldquoAnion exchange kinetics ofnanodimensional layered metal hydroxides use of isoconver-sional analysisrdquo Journal of Physical Chemistry A vol 114 no 49pp 12858ndash12869 2010
[22] C Chouillet J-M Krafft C Louis and H Lauron-PernotldquoCharacterization of zinc hydroxynitrates by diffuse reflectanceinfrared spectroscopymdashstructural modifications during ther-mal treatmentrdquo Spectrochimica Acta A vol 60 no 3 pp 505ndash511 2004
[23] W Fujita and K Awaga ldquoMagnetic properties of Cu2(OH)
3(al-
kanecarboxylate) compounds drastic mod-ification withextension of the Alkyl chainrdquo Inorganic Chemistry vol 35 no7 pp 1915ndash1917 1996
[24] R Swisłocka M Kowczyk-Sadowy M Kalinowska and WLewandowski ldquoSpectroscopic (FT-IR FT-Raman1H and13CNMR) and theoretical studies of p-coumaric acid and alkalimetal p-coumaratesrdquo Spectroscopy vol 27 no 1 pp 35ndash48 2012
12 Advances in Physical Chemistry
[25] T Biswick W Jones A Pacuła E Serwicka and J PodobinskildquoThe role of anhydrous zinc nitrate in the thermal decomposi-tion of the zinc hydroxy nitrates Zn
5(OH)
8(NO3)2sdot2H2O and
ZnOHNO3sdotH2Ordquo Journal of Solid State Chemistry vol 180 no
4 pp 1171ndash1179 2007[26] S-H Park and E L Cheol ldquoSynthesis characterization and
magnetic properties of a novel disulfonate-pillared copperhydroxide Cu
2(OH)
3(DS4)
12 DS4 = 14-butanedisulfonaterdquo
Bulletin of the Korean Chemical Society vol 27 no 10 pp 1587ndash1592 2006
[27] P Rabu M Drillon and C Hornick ldquoStructural and spectro-scopic study of organic-inorganic transitionmetal based layeredmagnetsrdquo Analusis vol 28 no 2 pp 103ndash108 2000
[28] L Lei A Khan and D OrsquoHare ldquoSelective anion-exchangeintercalation of isomeric benzoate anions into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo Journal of Solid State
Chemistry vol 178 no 12 pp 3648ndash3654 2005[29] W Krause H-J Bernhardt R S W Braithwaite U Kolitsch
and R Pritchard ldquoKapellasite Cu3Zn(OH)
6Cl2 a new mineral
from Lavrion Greece and its crystal structurerdquo MineralogicalMagazine vol 70 no 3 pp 329ndash340 2006
[30] L C Andrews L R Bernstein C M Foris et al PowderDiffraction File Inorganic International Centre for DiffractionData Newtown Square Pennsylvania 2009
[31] A Ragavan A I Khan and D OrsquoHare ldquoIsomer selectiveion-exchange intercalation of nitrophenolates into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotxH
2Ordquo Journal of Materials
Chemistry vol 16 no 6 pp 602ndash608 2006[32] A Ragavan A Khan and D OrsquoHare ldquoSelective intercalation
of chlorophenoxyacetates into the layered double hydroxide[LiAl2(OH)
6]ClsdotxH
2Ordquo Journal of Materials Chemistry vol 16
no 42 pp 4155ndash4159 2006[33] S Vyazovkin and C A Wight ldquoModel-free and model-fitting
approaches to kinetic analysis of isothermal and nonisothermaldatardquoThermochimica Acta vol 340-341 pp 53ndash68 1999
[34] R E Johnsen F Krumeich and P Norby ldquoStructural andmicrostructural changes during anion exchange ofCoAl layereddouble hydroxides an in situ X-ray powder diffraction studyrdquoJournal of Applied Crystallography vol 43 no 3 pp 434ndash4472010
[35] G R Williams T G Dunbar A J Beer A M Fogg and DOrsquoHare ldquoIntercalation chemistry of the novel layered doublehydroxides [MAl
4(OH)
12](NO
3)2sdotyH2O (M=Zn Cu Ni and
Co) 1 new organic intercalates and reaction mechanismsrdquoJournal of Materials Chemistry vol 16 no 13 pp 1222ndash12302006
[36] T E M Ten Hulscher and G Cornelissen ldquoEffect of tempera-ture on sorption equilibrium and sorption kinetics of organicmicropollutantsmdasha reviewrdquo Chemosphere vol 32 no 4 pp609ndash626 1996
[37] M Avrami ldquoKinetics of phase change I general theoryrdquo TheJournal of Chemical Physics vol 7 no 12 pp 1103ndash1112 1939
[38] M Avrami ldquoKinetics of phase change II Transformation-timerelations for random distribution of nucleirdquo The Journal ofChemical Physics vol 8 no 2 pp 212ndash224 1940
[39] R J Francis S OrsquoBrien A M Fogg et al ldquoTime-resolved in-situ energy and angular dispersive X-ray diffraction studies ofthe formation of themicroporous gallophosphateULM-5 underhydrothermal conditionsrdquo Journal of the American ChemicalSociety vol 121 no 5 pp 1002ndash1015 1999
[40] Z Miladinovic J Zakrzewska B Kovacevic and G BacicldquoMonitoring of crystallization processes during synthesis of
zeolite A by in situ 27Al NMR spectroscopyrdquo Materials Chem-istry and Physics vol 104 no 2-3 pp 384ndash389 2007
[41] R Serna-Guerrero and A Sayari ldquoModeling adsorption of CO2
on amine-functionalized mesoporous silica 2 kinetics andbreakthrough curvesrdquo Chemical Engineering Journal vol 161no 1-2 pp 182ndash190 2010
[42] J D Hancock and J H Sharp ldquoMethod of comparing solid-state kinetic data and its application to the decomposition ofkaolinite brucite and BaCO
3rdquo Journal of the American Ceramic
Society vol 55 no 2 pp 74ndash77 1972[43] F Auriemma C De Rosa and R Triolo ldquoSlow crystallization
kinetics of poly(vinyl alcohol) in confined environment duringcryotropic gelation of aqueous solutionsrdquo Macromolecules vol39 no 26 pp 9429ndash9434 2006
[44] R V Ramanujan and Y R Zhang ldquoQuantitative transmissionelectronmicroscopy analysis of the nanocrystallization kineticsof softmagnetic alloysrdquoPhysical ReviewB vol 74 no 22 ArticleID 224408 2006
[45] A G Kalinichev P Padma Kumar and R James KirkpatrickldquoMolecular dynamics computer simulations of the effects ofhydrogen bonding on the properties of layered double hydrox-ides intercalated with organic acidsrdquo Philosophical Magazinevol 90 no 17-18 pp 2475ndash2488 2010
mechanism model based approach was used to obtain rateconstants for the exchange reactions The Avrami-Erofersquoevnucleation-growth model [37 38] was used here since ithas the advantage that it can be used to describe reactionsoccurring via different mechanisms making comparison ofthe processes possible since the order of reactions will be thesame In the model the extent of reaction (120572) depends on therate constant 119896 and a coefficient 119899 as shown in the followingequation
119886 (119905) = 1 minus 119890[minus(119896119905)
119899
] (3)
The coefficient 119899 (Avrami exponent) potentially containsinformation about the mechanism of the reaction [39] TheAvrami-Erofersquoev model has been applied successfully to solu-tion phase anion exchange reactions in layeredmaterials andother fluid phase reactions [2 10 40 41] we have also shownthat values of parameters obtained from analysis of bothsolid state transformations and solution phase processesemploying the Avrami model are comparable indicating thatthere is correlation between solid state transformation andfluid phase processes [21] The Avrami exponent is usuallyobtained by taking double logarithm of the Avrami-Erofersquoevequation obtaining (4) which was popularized by Sharp andHancock in 1972 [42] Consider
A plot of ln[minus ln(1 minus 120572)] as a function of ln 119905 (doublelogarithmic plot) gives a linear plot in which the value of 119899 isobtained from the slope and the value of 119896 is evaluated fromthe intercept
The extent of reaction versus time plots obtained for theZC-o-HCnClminus exchange reactions at different temperaturesis shown in Figure 7(a) The Avrami-Erofersquoev model wasapplied and correctly described the obtained experimentaldatawithin the120572 range of 015ndash10The validity of theAvrami-Erofersquoevmodel can be confirmedby the corresponding doublelogarithmic plots in Figure 7(b) where straight lines wereobtained with 1198772 values ranging from 0996 to 0999 Thedouble logarithmic plots are almost parallel indicating thatthe reaction proceeds with the same mechanism over thetemperature range used here The obtained 119899 value was closeto 10 consistent with a nucleation controlled mechanismwhich agrees well with the activation energies obtained fromisoconversional methods and discussed before
The release profiles together with the double logarithmicplots for the release of m-HCn and p-HCn are shown in thesupporting information Figures S4 and S5 respectively Thekinetics of m-HCn release also follows the Avrami-Erofersquoevmodel the data provided a good fit within the range 015 lt120572 lt 085 the fit to limited range of 120572 has been observedin other intercalation reactions Anomalously low values ofthe Avrami exponent observed here (shown in Table 3) havebeen reported in polymer and alloy crystallization and donot have physical mechanistic meaning [43 44] The Avramiexponent values obtained for the release of p-HCn are closeto 05 which is consistent with diffusion controlled reactions[10] Diffusion control of such a high 119864
119886highlights two
0 2 4 6 8 10 12 1400
02
04
06
08
10
Time (hours)
Exte
nt o
f rea
ctio
n (120572
)
300∘C400∘C500∘C600∘C
(a)
4 6 8 10
0
1
minus1
minus2
ln (t minus t0)
lnminusln(1
minus120572)
30∘C40∘C50∘C60∘C
(b)
Figure 7 Extent of reaction as a function of time (a) and doublelogarithmic plots (b) for the exchange reaction of Clminus anion andZC-o-HCn at various temperatures 60∘C (◻) 50∘C (Q) 40∘C (998771)and 30∘C (e)
issues (a) the interlayer space is densely packed and thediffusional resistance within the interlayer space approachesthat in polymers in which119864
119886values can be above 100 kJmolminus1
depending on the diffusing molecule [36] and (b) Kineticambiguity is common in model based approaches in thiscase the mechanistic interpretation from the model is not inline with the kinetic parameters obtained
FromTable 3 it can be concluded that the rate of release ofm-HCn from the nanohybrids is much faster compared with
10 Advances in Physical Chemistry
Table 3 A summary of kinetic parameters obtained at different temperatures
Nanohybrid Temperature (∘C) 119899 119896 sminus1 (lowast10minus3)
ZC-o-HCn
600 082 plusmn 002 17 plusmn 03
500 090 plusmn 002 07 plusmn 01
400 088 plusmn 001 029 plusmn 005
300 102 plusmn 001 010 plusmn 001
ZC-m-HCn
600 024 plusmn 001 31 plusmn 9
500 024 plusmn 001 20 plusmn 7
400 021 plusmn 002 7 plusmn 2
300 0225 plusmn 0007 35 plusmn 08
ZC-p-HCn
600 041 plusmn 001 29 plusmn 05
500 045 plusmn 002 16 plusmn 04
400 046 plusmn 001 10 plusmn 02
300 039 plusmn 001 05 plusmn 01
the other isomers As an example the rate constants at 40∘Cwere (7 plusmn 2) times 10minus3 sminus1 for m-HCn (10 plusmn 02) times 10minus3 sminus1 forp-HCn and (029 plusmn 005) times 10minus3 sminus1 for o-HCn The rate ofrelease follows the following order
119898ndashHCn gt 119901-HCn gt 119900-HCn
The release profile form-HCn at 40∘C (Figure 5(a)) showsthat there is a burst and rapid release of the intercalated anionin the first 10 minutes followed by a more sustained releasewith equilibrium being attained after 3 hours This burstrelease is observed for all the temperatures used here (FigureS2a in supporting information) and has been observed beforein anion exchange reactions in LDHs [10] The burst releaseobserved here is due to anion exchange as confirmed byPXRD results in Figure S3 The chloride phase representedby an asterisk in Figure S3 was significant at 30 seconds (theearliest we could sample) of ZC-m-HCn being in contact withthe chloride solution The initial slow and linear release inZC-o-HCn indicated in Figure 5(b) and confirmed by XRDprofiles in Figure S3 may be due to the increased stabilityof the HDS structure due to extensive hydrogen bonding asindicated by FTIR data in Figure 3 and the close arrangementof the anions implied by XRDprofiles in Figure 2(a)This firststage of release has a strong temperature dependence beingmore pronounced at low temperature and almost absent athigher temperatures (Figure S2b in supporting information)
Factors which may influence the rates of release includeinteractions between the metal hydroxide layers and theanions anion-anion interactions within the layers and in theexchange medium and solvent-anion interaction [10] It isexpected that these factors will have a complex effect on thereaction rates since they affect each isomer differently Thecomplex interplay of the abovementioned factors oftenmakeit difficult to come up with a simple relationship betweenthe rate of release and the aforementioned factors [10] It isimportant to note that these factors are expected to dependon the types and positions of chemical groups on the anionThepresence of hydrogen bondingwithin the interlayer spacereduces the Gibbs free energy of formation of the nanohy-brids and stabilizes the system [45]The rate of release of ionsthen depends on the differences in the energy of nanohybrid
and the barrier to reaction As indicated in Figure 3 there issignificant level of hydrogen bonding in ZC-o-HCn whichthen resulted in a close interaction with the layers Thehydrogen bond network in the nanohybrid and the closeinteraction of the anions with the layers stabilized the systemas compared to the other nanohybrids The nanohybrid inwhich these interactions are low is relatively less stable and isexpected to proceed faster to equilibrium this is consistentwith results obtained here The anions in the unstablelessstable nanohybrid prefer bulk solution (in which there isstabilization by solvation with water molecules) to galleryspace which may result in the burst and fast release observedin ZC-m-HCnThe modification of bioactive molecules withgroups capable of hydrogen bonding for example hydroxylgroups especially at positions which allow interactions withthe layers may enable tuning of the release behavior ofthese molecules from layered materials It is important tonote that the modification should be at positions whichdo not affect the bioactivity of the molecules Additionalexperiments including studies on other isomeric anions withgroups capable of forming hydrogen bonds and effect ofmetallayer composition will be helpful in further elucidating howthe position of substituents control the rate as well as theextent of reaction
4 Conclusions
Anion exchange reactions in hydroxy double salts are affectedby the structure of the intercalated anions and there is astrong interplay between thermodynamics and kinetics ofthe release reactions In the present study we have shownthat there is a significant effect of the position of substituentgroups on the rate and extent of release of isomers ofhydroxycinnamate While the release of the metaisomer wasinstantaneous and fast the orthoisomer release exhibiteda slower more sustained and complete release the orderof reaction rates was m-HCn gt p-HCn gt o-HCn withthe o-HCn nanohybrid showing an induction period Theposition of the hydroxyl groups resulted in differences inthe interlayer interactions of the anions with presence ofinterlayer hydrogen bonds affecting the rates of the reaction
Advances in Physical Chemistry 11
The formation of hydrogen bonds between the layers and theintercalated orthoisomer resulted in greater stabilization ofthe nanohybrid and resulted in the observed induction periodand slow release The extent of reaction was correlated withthe magnitude of dipole moments These results highlightthe potential of tuning the release behavior of intercalatedbioactive compounds which bring control into both theamount released per given time and the period for totalrelease
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
This work was supported by the National Science Foundation(CHE-0809751)
References
[1] S Yamanaka T Sako andM Hattori ldquoAnion exchange in basiccopper acetaterdquo Chemistry Letters pp 1869ndash1872 1989
[2] E Kandare and J M Hossenlopp ldquoHydroxy double salt anionexchange kinetics effects of precursor structure and anionsizetrdquo Journal of Physical Chemistry B vol 109 no 17 pp 8469ndash8475 2005
[3] P K Dutta and M Puri ldquoAnion exchange in lithium aluminatehydroxidesrdquo Journal of Physical Chemistry vol 93 no 1 pp376ndash381 1989
[4] B M Choudary V S Jaya B R Reddy M L Kantam M MRao and S S Madhavendra ldquoSynthesis characterization ionexchange and catalytic properties of nanobinary and ternarymetal oxyhydroxidesrdquo Chemistry of Materials vol 17 no 10pp 2740ndash2743 2005
[5] A M Fogg J S Dunn S-G Shyu D R Cary and D OrsquoHareldquoSelective ion-exchange intercalation of isomeric dicarboxylateanions into the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Chemistry of Materials vol 10 no 1 pp 351ndash355 1998[6] T Ikeda H Amoh and T Yasunaga ldquoStereoselective exchange
kinetics of L- and D-histidines for Cl- in the interlayerof a hydrotalcite-like compound by the chemical relaxationmethodrdquo Journal of the American Chemical Society vol 106 no20 pp 5772ndash5775 1984
[7] M Meyn K Beneke and G Lagaly ldquoAnion-exchange reactionsof hydroxy double saltsrdquo Inorganic Chemistry vol 32 no 7 pp1209ndash1215 1993
[8] V Ambrogi G Fardella G Grandolini L Perioli and M CTiralti ldquoIntercalation compounds of hydrotalcite-like anionicclays with anti-inflammatory agents II uptake of diclofenac fora controlled release formulationrdquo AAPS PharmSciTech vol 3no 3 p E26 2002
[9] M Z Bin Hussein Z Zainal A H Yahaya and D WV Foo ldquoControlled release of a plant growth regulator 120572-naphthaleneacetate from the lamella of Zn-Al-layered doublehydroxide nanocompositerdquo Journal of Controlled Release vol82 no 2-3 pp 417ndash427 2002
[10] A I Khan A Ragavan B Fong et al ldquoRecent developments inthe use of layered double hydroxides as host materials for the
storage and triggered release of functional anionsrdquo Industrialand Engineering Chemistry Research vol 48 no 23 pp 10196ndash10205 2009
[11] M Silion D Hritcu I M Jaba et al ldquoIn vitro and invivo behavior of ketoprofen intercalated into layered doublehydroxidesrdquo Journal of Materials Science vol 21 no 11 pp3009ndash3018 2010
[12] J-H Yang Y-S Han M Park T Park S-J Hwang andJ-H Choy ldquoNew inorganic-based drug delivery system ofindole-3-acetic acid-layered metal hydroxide nanohybrids withcontrolled release raterdquo Chemistry of Materials vol 19 no 10pp 2679ndash2685 2007
[13] L Lei R P Vijayan and D OrsquoHare ldquoPreferential anionexchange intercalation of pyridinecarboxylate and toluate iso-mers in the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Journal of Materials Chemistry vol 11 no 12 pp 3276ndash32802001
[14] B Lotsch F Millange R I Walton and D OrsquoHare ldquoSepara-tion of nucleoside monophosphates using preferential anionexchange intercalation in layered double hydroxidesrdquo Solid StateSciences vol 3 no 8 pp 883ndash886 2001
[15] H Tagaya N Sasaki H Morioka and J Kadokawa ldquoPrepara-tion of new inorganicmdashorganic layered compounds hydroxydouble salts and preferential intercalation of organic carboxylicacids into themrdquoMolecular Crystals and Liquid Crystals Scienceand Technology A vol 341 pp 413ndash418 2000
[16] P P Kumar A G Kalinichev and R J Kirkpatrick ldquoHydra-tion swelling interlayer structure and hydrogen bondingin organolayered double hydroxides insights from moleculardynamics simulation of citrate-intercalated hydrotalciterdquo Jour-nal of Physical Chemistry B vol 110 no 9 pp 3841ndash3844 2006
[17] S V Prasanna and P V Kamath ldquoAnion-exchange reactions oflayered double hydroxides interplay between coulombic andh-bonding interactionsrdquo Industrial and Engineering ChemistryResearch vol 48 no 13 pp 6315ndash6320 2009
[18] Z Gu A CThomas Z P Xu J H Campbell and G Q Lu ldquoInvitro sustained release of LMWH from MgAl-layered doublehydroxide nanohybridsrdquo Chemistry of Materials vol 20 no 11pp 3715ndash3722 2008
[19] J T Rajamathi S Britto and M Rajamathi ldquoSynthesis andanion exchange reactions of a layered Copper-Zincrdquo Journal ofChemical Sciences vol 117 no 6 pp 629ndash633 2005
[20] M Frisch G Trucks H Schlegel et al ldquoGaussian 98 RevisionA114rdquo 2002
[21] S Majoni and J M Hossenlopp ldquoAnion exchange kinetics ofnanodimensional layered metal hydroxides use of isoconver-sional analysisrdquo Journal of Physical Chemistry A vol 114 no 49pp 12858ndash12869 2010
[22] C Chouillet J-M Krafft C Louis and H Lauron-PernotldquoCharacterization of zinc hydroxynitrates by diffuse reflectanceinfrared spectroscopymdashstructural modifications during ther-mal treatmentrdquo Spectrochimica Acta A vol 60 no 3 pp 505ndash511 2004
[23] W Fujita and K Awaga ldquoMagnetic properties of Cu2(OH)
3(al-
kanecarboxylate) compounds drastic mod-ification withextension of the Alkyl chainrdquo Inorganic Chemistry vol 35 no7 pp 1915ndash1917 1996
[24] R Swisłocka M Kowczyk-Sadowy M Kalinowska and WLewandowski ldquoSpectroscopic (FT-IR FT-Raman1H and13CNMR) and theoretical studies of p-coumaric acid and alkalimetal p-coumaratesrdquo Spectroscopy vol 27 no 1 pp 35ndash48 2012
12 Advances in Physical Chemistry
[25] T Biswick W Jones A Pacuła E Serwicka and J PodobinskildquoThe role of anhydrous zinc nitrate in the thermal decomposi-tion of the zinc hydroxy nitrates Zn
5(OH)
8(NO3)2sdot2H2O and
ZnOHNO3sdotH2Ordquo Journal of Solid State Chemistry vol 180 no
4 pp 1171ndash1179 2007[26] S-H Park and E L Cheol ldquoSynthesis characterization and
magnetic properties of a novel disulfonate-pillared copperhydroxide Cu
2(OH)
3(DS4)
12 DS4 = 14-butanedisulfonaterdquo
Bulletin of the Korean Chemical Society vol 27 no 10 pp 1587ndash1592 2006
[27] P Rabu M Drillon and C Hornick ldquoStructural and spectro-scopic study of organic-inorganic transitionmetal based layeredmagnetsrdquo Analusis vol 28 no 2 pp 103ndash108 2000
[28] L Lei A Khan and D OrsquoHare ldquoSelective anion-exchangeintercalation of isomeric benzoate anions into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo Journal of Solid State
Chemistry vol 178 no 12 pp 3648ndash3654 2005[29] W Krause H-J Bernhardt R S W Braithwaite U Kolitsch
and R Pritchard ldquoKapellasite Cu3Zn(OH)
6Cl2 a new mineral
from Lavrion Greece and its crystal structurerdquo MineralogicalMagazine vol 70 no 3 pp 329ndash340 2006
[30] L C Andrews L R Bernstein C M Foris et al PowderDiffraction File Inorganic International Centre for DiffractionData Newtown Square Pennsylvania 2009
[31] A Ragavan A I Khan and D OrsquoHare ldquoIsomer selectiveion-exchange intercalation of nitrophenolates into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotxH
2Ordquo Journal of Materials
Chemistry vol 16 no 6 pp 602ndash608 2006[32] A Ragavan A Khan and D OrsquoHare ldquoSelective intercalation
of chlorophenoxyacetates into the layered double hydroxide[LiAl2(OH)
6]ClsdotxH
2Ordquo Journal of Materials Chemistry vol 16
no 42 pp 4155ndash4159 2006[33] S Vyazovkin and C A Wight ldquoModel-free and model-fitting
approaches to kinetic analysis of isothermal and nonisothermaldatardquoThermochimica Acta vol 340-341 pp 53ndash68 1999
[34] R E Johnsen F Krumeich and P Norby ldquoStructural andmicrostructural changes during anion exchange ofCoAl layereddouble hydroxides an in situ X-ray powder diffraction studyrdquoJournal of Applied Crystallography vol 43 no 3 pp 434ndash4472010
[35] G R Williams T G Dunbar A J Beer A M Fogg and DOrsquoHare ldquoIntercalation chemistry of the novel layered doublehydroxides [MAl
4(OH)
12](NO
3)2sdotyH2O (M=Zn Cu Ni and
Co) 1 new organic intercalates and reaction mechanismsrdquoJournal of Materials Chemistry vol 16 no 13 pp 1222ndash12302006
[36] T E M Ten Hulscher and G Cornelissen ldquoEffect of tempera-ture on sorption equilibrium and sorption kinetics of organicmicropollutantsmdasha reviewrdquo Chemosphere vol 32 no 4 pp609ndash626 1996
[37] M Avrami ldquoKinetics of phase change I general theoryrdquo TheJournal of Chemical Physics vol 7 no 12 pp 1103ndash1112 1939
[38] M Avrami ldquoKinetics of phase change II Transformation-timerelations for random distribution of nucleirdquo The Journal ofChemical Physics vol 8 no 2 pp 212ndash224 1940
[39] R J Francis S OrsquoBrien A M Fogg et al ldquoTime-resolved in-situ energy and angular dispersive X-ray diffraction studies ofthe formation of themicroporous gallophosphateULM-5 underhydrothermal conditionsrdquo Journal of the American ChemicalSociety vol 121 no 5 pp 1002ndash1015 1999
[40] Z Miladinovic J Zakrzewska B Kovacevic and G BacicldquoMonitoring of crystallization processes during synthesis of
zeolite A by in situ 27Al NMR spectroscopyrdquo Materials Chem-istry and Physics vol 104 no 2-3 pp 384ndash389 2007
[41] R Serna-Guerrero and A Sayari ldquoModeling adsorption of CO2
on amine-functionalized mesoporous silica 2 kinetics andbreakthrough curvesrdquo Chemical Engineering Journal vol 161no 1-2 pp 182ndash190 2010
[42] J D Hancock and J H Sharp ldquoMethod of comparing solid-state kinetic data and its application to the decomposition ofkaolinite brucite and BaCO
3rdquo Journal of the American Ceramic
Society vol 55 no 2 pp 74ndash77 1972[43] F Auriemma C De Rosa and R Triolo ldquoSlow crystallization
kinetics of poly(vinyl alcohol) in confined environment duringcryotropic gelation of aqueous solutionsrdquo Macromolecules vol39 no 26 pp 9429ndash9434 2006
[44] R V Ramanujan and Y R Zhang ldquoQuantitative transmissionelectronmicroscopy analysis of the nanocrystallization kineticsof softmagnetic alloysrdquoPhysical ReviewB vol 74 no 22 ArticleID 224408 2006
[45] A G Kalinichev P Padma Kumar and R James KirkpatrickldquoMolecular dynamics computer simulations of the effects ofhydrogen bonding on the properties of layered double hydrox-ides intercalated with organic acidsrdquo Philosophical Magazinevol 90 no 17-18 pp 2475ndash2488 2010
Table 3 A summary of kinetic parameters obtained at different temperatures
Nanohybrid Temperature (∘C) 119899 119896 sminus1 (lowast10minus3)
ZC-o-HCn
600 082 plusmn 002 17 plusmn 03
500 090 plusmn 002 07 plusmn 01
400 088 plusmn 001 029 plusmn 005
300 102 plusmn 001 010 plusmn 001
ZC-m-HCn
600 024 plusmn 001 31 plusmn 9
500 024 plusmn 001 20 plusmn 7
400 021 plusmn 002 7 plusmn 2
300 0225 plusmn 0007 35 plusmn 08
ZC-p-HCn
600 041 plusmn 001 29 plusmn 05
500 045 plusmn 002 16 plusmn 04
400 046 plusmn 001 10 plusmn 02
300 039 plusmn 001 05 plusmn 01
the other isomers As an example the rate constants at 40∘Cwere (7 plusmn 2) times 10minus3 sminus1 for m-HCn (10 plusmn 02) times 10minus3 sminus1 forp-HCn and (029 plusmn 005) times 10minus3 sminus1 for o-HCn The rate ofrelease follows the following order
119898ndashHCn gt 119901-HCn gt 119900-HCn
The release profile form-HCn at 40∘C (Figure 5(a)) showsthat there is a burst and rapid release of the intercalated anionin the first 10 minutes followed by a more sustained releasewith equilibrium being attained after 3 hours This burstrelease is observed for all the temperatures used here (FigureS2a in supporting information) and has been observed beforein anion exchange reactions in LDHs [10] The burst releaseobserved here is due to anion exchange as confirmed byPXRD results in Figure S3 The chloride phase representedby an asterisk in Figure S3 was significant at 30 seconds (theearliest we could sample) of ZC-m-HCn being in contact withthe chloride solution The initial slow and linear release inZC-o-HCn indicated in Figure 5(b) and confirmed by XRDprofiles in Figure S3 may be due to the increased stabilityof the HDS structure due to extensive hydrogen bonding asindicated by FTIR data in Figure 3 and the close arrangementof the anions implied by XRDprofiles in Figure 2(a)This firststage of release has a strong temperature dependence beingmore pronounced at low temperature and almost absent athigher temperatures (Figure S2b in supporting information)
Factors which may influence the rates of release includeinteractions between the metal hydroxide layers and theanions anion-anion interactions within the layers and in theexchange medium and solvent-anion interaction [10] It isexpected that these factors will have a complex effect on thereaction rates since they affect each isomer differently Thecomplex interplay of the abovementioned factors oftenmakeit difficult to come up with a simple relationship betweenthe rate of release and the aforementioned factors [10] It isimportant to note that these factors are expected to dependon the types and positions of chemical groups on the anionThepresence of hydrogen bondingwithin the interlayer spacereduces the Gibbs free energy of formation of the nanohy-brids and stabilizes the system [45]The rate of release of ionsthen depends on the differences in the energy of nanohybrid
and the barrier to reaction As indicated in Figure 3 there issignificant level of hydrogen bonding in ZC-o-HCn whichthen resulted in a close interaction with the layers Thehydrogen bond network in the nanohybrid and the closeinteraction of the anions with the layers stabilized the systemas compared to the other nanohybrids The nanohybrid inwhich these interactions are low is relatively less stable and isexpected to proceed faster to equilibrium this is consistentwith results obtained here The anions in the unstablelessstable nanohybrid prefer bulk solution (in which there isstabilization by solvation with water molecules) to galleryspace which may result in the burst and fast release observedin ZC-m-HCnThe modification of bioactive molecules withgroups capable of hydrogen bonding for example hydroxylgroups especially at positions which allow interactions withthe layers may enable tuning of the release behavior ofthese molecules from layered materials It is important tonote that the modification should be at positions whichdo not affect the bioactivity of the molecules Additionalexperiments including studies on other isomeric anions withgroups capable of forming hydrogen bonds and effect ofmetallayer composition will be helpful in further elucidating howthe position of substituents control the rate as well as theextent of reaction
4 Conclusions
Anion exchange reactions in hydroxy double salts are affectedby the structure of the intercalated anions and there is astrong interplay between thermodynamics and kinetics ofthe release reactions In the present study we have shownthat there is a significant effect of the position of substituentgroups on the rate and extent of release of isomers ofhydroxycinnamate While the release of the metaisomer wasinstantaneous and fast the orthoisomer release exhibiteda slower more sustained and complete release the orderof reaction rates was m-HCn gt p-HCn gt o-HCn withthe o-HCn nanohybrid showing an induction period Theposition of the hydroxyl groups resulted in differences inthe interlayer interactions of the anions with presence ofinterlayer hydrogen bonds affecting the rates of the reaction
Advances in Physical Chemistry 11
The formation of hydrogen bonds between the layers and theintercalated orthoisomer resulted in greater stabilization ofthe nanohybrid and resulted in the observed induction periodand slow release The extent of reaction was correlated withthe magnitude of dipole moments These results highlightthe potential of tuning the release behavior of intercalatedbioactive compounds which bring control into both theamount released per given time and the period for totalrelease
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
This work was supported by the National Science Foundation(CHE-0809751)
References
[1] S Yamanaka T Sako andM Hattori ldquoAnion exchange in basiccopper acetaterdquo Chemistry Letters pp 1869ndash1872 1989
[2] E Kandare and J M Hossenlopp ldquoHydroxy double salt anionexchange kinetics effects of precursor structure and anionsizetrdquo Journal of Physical Chemistry B vol 109 no 17 pp 8469ndash8475 2005
[3] P K Dutta and M Puri ldquoAnion exchange in lithium aluminatehydroxidesrdquo Journal of Physical Chemistry vol 93 no 1 pp376ndash381 1989
[4] B M Choudary V S Jaya B R Reddy M L Kantam M MRao and S S Madhavendra ldquoSynthesis characterization ionexchange and catalytic properties of nanobinary and ternarymetal oxyhydroxidesrdquo Chemistry of Materials vol 17 no 10pp 2740ndash2743 2005
[5] A M Fogg J S Dunn S-G Shyu D R Cary and D OrsquoHareldquoSelective ion-exchange intercalation of isomeric dicarboxylateanions into the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Chemistry of Materials vol 10 no 1 pp 351ndash355 1998[6] T Ikeda H Amoh and T Yasunaga ldquoStereoselective exchange
kinetics of L- and D-histidines for Cl- in the interlayerof a hydrotalcite-like compound by the chemical relaxationmethodrdquo Journal of the American Chemical Society vol 106 no20 pp 5772ndash5775 1984
[7] M Meyn K Beneke and G Lagaly ldquoAnion-exchange reactionsof hydroxy double saltsrdquo Inorganic Chemistry vol 32 no 7 pp1209ndash1215 1993
[8] V Ambrogi G Fardella G Grandolini L Perioli and M CTiralti ldquoIntercalation compounds of hydrotalcite-like anionicclays with anti-inflammatory agents II uptake of diclofenac fora controlled release formulationrdquo AAPS PharmSciTech vol 3no 3 p E26 2002
[9] M Z Bin Hussein Z Zainal A H Yahaya and D WV Foo ldquoControlled release of a plant growth regulator 120572-naphthaleneacetate from the lamella of Zn-Al-layered doublehydroxide nanocompositerdquo Journal of Controlled Release vol82 no 2-3 pp 417ndash427 2002
[10] A I Khan A Ragavan B Fong et al ldquoRecent developments inthe use of layered double hydroxides as host materials for the
storage and triggered release of functional anionsrdquo Industrialand Engineering Chemistry Research vol 48 no 23 pp 10196ndash10205 2009
[11] M Silion D Hritcu I M Jaba et al ldquoIn vitro and invivo behavior of ketoprofen intercalated into layered doublehydroxidesrdquo Journal of Materials Science vol 21 no 11 pp3009ndash3018 2010
[12] J-H Yang Y-S Han M Park T Park S-J Hwang andJ-H Choy ldquoNew inorganic-based drug delivery system ofindole-3-acetic acid-layered metal hydroxide nanohybrids withcontrolled release raterdquo Chemistry of Materials vol 19 no 10pp 2679ndash2685 2007
[13] L Lei R P Vijayan and D OrsquoHare ldquoPreferential anionexchange intercalation of pyridinecarboxylate and toluate iso-mers in the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Journal of Materials Chemistry vol 11 no 12 pp 3276ndash32802001
[14] B Lotsch F Millange R I Walton and D OrsquoHare ldquoSepara-tion of nucleoside monophosphates using preferential anionexchange intercalation in layered double hydroxidesrdquo Solid StateSciences vol 3 no 8 pp 883ndash886 2001
[15] H Tagaya N Sasaki H Morioka and J Kadokawa ldquoPrepara-tion of new inorganicmdashorganic layered compounds hydroxydouble salts and preferential intercalation of organic carboxylicacids into themrdquoMolecular Crystals and Liquid Crystals Scienceand Technology A vol 341 pp 413ndash418 2000
[16] P P Kumar A G Kalinichev and R J Kirkpatrick ldquoHydra-tion swelling interlayer structure and hydrogen bondingin organolayered double hydroxides insights from moleculardynamics simulation of citrate-intercalated hydrotalciterdquo Jour-nal of Physical Chemistry B vol 110 no 9 pp 3841ndash3844 2006
[17] S V Prasanna and P V Kamath ldquoAnion-exchange reactions oflayered double hydroxides interplay between coulombic andh-bonding interactionsrdquo Industrial and Engineering ChemistryResearch vol 48 no 13 pp 6315ndash6320 2009
[18] Z Gu A CThomas Z P Xu J H Campbell and G Q Lu ldquoInvitro sustained release of LMWH from MgAl-layered doublehydroxide nanohybridsrdquo Chemistry of Materials vol 20 no 11pp 3715ndash3722 2008
[19] J T Rajamathi S Britto and M Rajamathi ldquoSynthesis andanion exchange reactions of a layered Copper-Zincrdquo Journal ofChemical Sciences vol 117 no 6 pp 629ndash633 2005
[20] M Frisch G Trucks H Schlegel et al ldquoGaussian 98 RevisionA114rdquo 2002
[21] S Majoni and J M Hossenlopp ldquoAnion exchange kinetics ofnanodimensional layered metal hydroxides use of isoconver-sional analysisrdquo Journal of Physical Chemistry A vol 114 no 49pp 12858ndash12869 2010
[22] C Chouillet J-M Krafft C Louis and H Lauron-PernotldquoCharacterization of zinc hydroxynitrates by diffuse reflectanceinfrared spectroscopymdashstructural modifications during ther-mal treatmentrdquo Spectrochimica Acta A vol 60 no 3 pp 505ndash511 2004
[23] W Fujita and K Awaga ldquoMagnetic properties of Cu2(OH)
3(al-
kanecarboxylate) compounds drastic mod-ification withextension of the Alkyl chainrdquo Inorganic Chemistry vol 35 no7 pp 1915ndash1917 1996
[24] R Swisłocka M Kowczyk-Sadowy M Kalinowska and WLewandowski ldquoSpectroscopic (FT-IR FT-Raman1H and13CNMR) and theoretical studies of p-coumaric acid and alkalimetal p-coumaratesrdquo Spectroscopy vol 27 no 1 pp 35ndash48 2012
12 Advances in Physical Chemistry
[25] T Biswick W Jones A Pacuła E Serwicka and J PodobinskildquoThe role of anhydrous zinc nitrate in the thermal decomposi-tion of the zinc hydroxy nitrates Zn
5(OH)
8(NO3)2sdot2H2O and
ZnOHNO3sdotH2Ordquo Journal of Solid State Chemistry vol 180 no
4 pp 1171ndash1179 2007[26] S-H Park and E L Cheol ldquoSynthesis characterization and
magnetic properties of a novel disulfonate-pillared copperhydroxide Cu
2(OH)
3(DS4)
12 DS4 = 14-butanedisulfonaterdquo
Bulletin of the Korean Chemical Society vol 27 no 10 pp 1587ndash1592 2006
[27] P Rabu M Drillon and C Hornick ldquoStructural and spectro-scopic study of organic-inorganic transitionmetal based layeredmagnetsrdquo Analusis vol 28 no 2 pp 103ndash108 2000
[28] L Lei A Khan and D OrsquoHare ldquoSelective anion-exchangeintercalation of isomeric benzoate anions into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo Journal of Solid State
Chemistry vol 178 no 12 pp 3648ndash3654 2005[29] W Krause H-J Bernhardt R S W Braithwaite U Kolitsch
and R Pritchard ldquoKapellasite Cu3Zn(OH)
6Cl2 a new mineral
from Lavrion Greece and its crystal structurerdquo MineralogicalMagazine vol 70 no 3 pp 329ndash340 2006
[30] L C Andrews L R Bernstein C M Foris et al PowderDiffraction File Inorganic International Centre for DiffractionData Newtown Square Pennsylvania 2009
[31] A Ragavan A I Khan and D OrsquoHare ldquoIsomer selectiveion-exchange intercalation of nitrophenolates into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotxH
2Ordquo Journal of Materials
Chemistry vol 16 no 6 pp 602ndash608 2006[32] A Ragavan A Khan and D OrsquoHare ldquoSelective intercalation
of chlorophenoxyacetates into the layered double hydroxide[LiAl2(OH)
6]ClsdotxH
2Ordquo Journal of Materials Chemistry vol 16
no 42 pp 4155ndash4159 2006[33] S Vyazovkin and C A Wight ldquoModel-free and model-fitting
approaches to kinetic analysis of isothermal and nonisothermaldatardquoThermochimica Acta vol 340-341 pp 53ndash68 1999
[34] R E Johnsen F Krumeich and P Norby ldquoStructural andmicrostructural changes during anion exchange ofCoAl layereddouble hydroxides an in situ X-ray powder diffraction studyrdquoJournal of Applied Crystallography vol 43 no 3 pp 434ndash4472010
[35] G R Williams T G Dunbar A J Beer A M Fogg and DOrsquoHare ldquoIntercalation chemistry of the novel layered doublehydroxides [MAl
4(OH)
12](NO
3)2sdotyH2O (M=Zn Cu Ni and
Co) 1 new organic intercalates and reaction mechanismsrdquoJournal of Materials Chemistry vol 16 no 13 pp 1222ndash12302006
[36] T E M Ten Hulscher and G Cornelissen ldquoEffect of tempera-ture on sorption equilibrium and sorption kinetics of organicmicropollutantsmdasha reviewrdquo Chemosphere vol 32 no 4 pp609ndash626 1996
[37] M Avrami ldquoKinetics of phase change I general theoryrdquo TheJournal of Chemical Physics vol 7 no 12 pp 1103ndash1112 1939
[38] M Avrami ldquoKinetics of phase change II Transformation-timerelations for random distribution of nucleirdquo The Journal ofChemical Physics vol 8 no 2 pp 212ndash224 1940
[39] R J Francis S OrsquoBrien A M Fogg et al ldquoTime-resolved in-situ energy and angular dispersive X-ray diffraction studies ofthe formation of themicroporous gallophosphateULM-5 underhydrothermal conditionsrdquo Journal of the American ChemicalSociety vol 121 no 5 pp 1002ndash1015 1999
[40] Z Miladinovic J Zakrzewska B Kovacevic and G BacicldquoMonitoring of crystallization processes during synthesis of
zeolite A by in situ 27Al NMR spectroscopyrdquo Materials Chem-istry and Physics vol 104 no 2-3 pp 384ndash389 2007
[41] R Serna-Guerrero and A Sayari ldquoModeling adsorption of CO2
on amine-functionalized mesoporous silica 2 kinetics andbreakthrough curvesrdquo Chemical Engineering Journal vol 161no 1-2 pp 182ndash190 2010
[42] J D Hancock and J H Sharp ldquoMethod of comparing solid-state kinetic data and its application to the decomposition ofkaolinite brucite and BaCO
3rdquo Journal of the American Ceramic
Society vol 55 no 2 pp 74ndash77 1972[43] F Auriemma C De Rosa and R Triolo ldquoSlow crystallization
kinetics of poly(vinyl alcohol) in confined environment duringcryotropic gelation of aqueous solutionsrdquo Macromolecules vol39 no 26 pp 9429ndash9434 2006
[44] R V Ramanujan and Y R Zhang ldquoQuantitative transmissionelectronmicroscopy analysis of the nanocrystallization kineticsof softmagnetic alloysrdquoPhysical ReviewB vol 74 no 22 ArticleID 224408 2006
[45] A G Kalinichev P Padma Kumar and R James KirkpatrickldquoMolecular dynamics computer simulations of the effects ofhydrogen bonding on the properties of layered double hydrox-ides intercalated with organic acidsrdquo Philosophical Magazinevol 90 no 17-18 pp 2475ndash2488 2010
The formation of hydrogen bonds between the layers and theintercalated orthoisomer resulted in greater stabilization ofthe nanohybrid and resulted in the observed induction periodand slow release The extent of reaction was correlated withthe magnitude of dipole moments These results highlightthe potential of tuning the release behavior of intercalatedbioactive compounds which bring control into both theamount released per given time and the period for totalrelease
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper
Acknowledgment
This work was supported by the National Science Foundation(CHE-0809751)
References
[1] S Yamanaka T Sako andM Hattori ldquoAnion exchange in basiccopper acetaterdquo Chemistry Letters pp 1869ndash1872 1989
[2] E Kandare and J M Hossenlopp ldquoHydroxy double salt anionexchange kinetics effects of precursor structure and anionsizetrdquo Journal of Physical Chemistry B vol 109 no 17 pp 8469ndash8475 2005
[3] P K Dutta and M Puri ldquoAnion exchange in lithium aluminatehydroxidesrdquo Journal of Physical Chemistry vol 93 no 1 pp376ndash381 1989
[4] B M Choudary V S Jaya B R Reddy M L Kantam M MRao and S S Madhavendra ldquoSynthesis characterization ionexchange and catalytic properties of nanobinary and ternarymetal oxyhydroxidesrdquo Chemistry of Materials vol 17 no 10pp 2740ndash2743 2005
[5] A M Fogg J S Dunn S-G Shyu D R Cary and D OrsquoHareldquoSelective ion-exchange intercalation of isomeric dicarboxylateanions into the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Chemistry of Materials vol 10 no 1 pp 351ndash355 1998[6] T Ikeda H Amoh and T Yasunaga ldquoStereoselective exchange
kinetics of L- and D-histidines for Cl- in the interlayerof a hydrotalcite-like compound by the chemical relaxationmethodrdquo Journal of the American Chemical Society vol 106 no20 pp 5772ndash5775 1984
[7] M Meyn K Beneke and G Lagaly ldquoAnion-exchange reactionsof hydroxy double saltsrdquo Inorganic Chemistry vol 32 no 7 pp1209ndash1215 1993
[8] V Ambrogi G Fardella G Grandolini L Perioli and M CTiralti ldquoIntercalation compounds of hydrotalcite-like anionicclays with anti-inflammatory agents II uptake of diclofenac fora controlled release formulationrdquo AAPS PharmSciTech vol 3no 3 p E26 2002
[9] M Z Bin Hussein Z Zainal A H Yahaya and D WV Foo ldquoControlled release of a plant growth regulator 120572-naphthaleneacetate from the lamella of Zn-Al-layered doublehydroxide nanocompositerdquo Journal of Controlled Release vol82 no 2-3 pp 417ndash427 2002
[10] A I Khan A Ragavan B Fong et al ldquoRecent developments inthe use of layered double hydroxides as host materials for the
storage and triggered release of functional anionsrdquo Industrialand Engineering Chemistry Research vol 48 no 23 pp 10196ndash10205 2009
[11] M Silion D Hritcu I M Jaba et al ldquoIn vitro and invivo behavior of ketoprofen intercalated into layered doublehydroxidesrdquo Journal of Materials Science vol 21 no 11 pp3009ndash3018 2010
[12] J-H Yang Y-S Han M Park T Park S-J Hwang andJ-H Choy ldquoNew inorganic-based drug delivery system ofindole-3-acetic acid-layered metal hydroxide nanohybrids withcontrolled release raterdquo Chemistry of Materials vol 19 no 10pp 2679ndash2685 2007
[13] L Lei R P Vijayan and D OrsquoHare ldquoPreferential anionexchange intercalation of pyridinecarboxylate and toluate iso-mers in the layered double hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo
Journal of Materials Chemistry vol 11 no 12 pp 3276ndash32802001
[14] B Lotsch F Millange R I Walton and D OrsquoHare ldquoSepara-tion of nucleoside monophosphates using preferential anionexchange intercalation in layered double hydroxidesrdquo Solid StateSciences vol 3 no 8 pp 883ndash886 2001
[15] H Tagaya N Sasaki H Morioka and J Kadokawa ldquoPrepara-tion of new inorganicmdashorganic layered compounds hydroxydouble salts and preferential intercalation of organic carboxylicacids into themrdquoMolecular Crystals and Liquid Crystals Scienceand Technology A vol 341 pp 413ndash418 2000
[16] P P Kumar A G Kalinichev and R J Kirkpatrick ldquoHydra-tion swelling interlayer structure and hydrogen bondingin organolayered double hydroxides insights from moleculardynamics simulation of citrate-intercalated hydrotalciterdquo Jour-nal of Physical Chemistry B vol 110 no 9 pp 3841ndash3844 2006
[17] S V Prasanna and P V Kamath ldquoAnion-exchange reactions oflayered double hydroxides interplay between coulombic andh-bonding interactionsrdquo Industrial and Engineering ChemistryResearch vol 48 no 13 pp 6315ndash6320 2009
[18] Z Gu A CThomas Z P Xu J H Campbell and G Q Lu ldquoInvitro sustained release of LMWH from MgAl-layered doublehydroxide nanohybridsrdquo Chemistry of Materials vol 20 no 11pp 3715ndash3722 2008
[19] J T Rajamathi S Britto and M Rajamathi ldquoSynthesis andanion exchange reactions of a layered Copper-Zincrdquo Journal ofChemical Sciences vol 117 no 6 pp 629ndash633 2005
[20] M Frisch G Trucks H Schlegel et al ldquoGaussian 98 RevisionA114rdquo 2002
[21] S Majoni and J M Hossenlopp ldquoAnion exchange kinetics ofnanodimensional layered metal hydroxides use of isoconver-sional analysisrdquo Journal of Physical Chemistry A vol 114 no 49pp 12858ndash12869 2010
[22] C Chouillet J-M Krafft C Louis and H Lauron-PernotldquoCharacterization of zinc hydroxynitrates by diffuse reflectanceinfrared spectroscopymdashstructural modifications during ther-mal treatmentrdquo Spectrochimica Acta A vol 60 no 3 pp 505ndash511 2004
[23] W Fujita and K Awaga ldquoMagnetic properties of Cu2(OH)
3(al-
kanecarboxylate) compounds drastic mod-ification withextension of the Alkyl chainrdquo Inorganic Chemistry vol 35 no7 pp 1915ndash1917 1996
[24] R Swisłocka M Kowczyk-Sadowy M Kalinowska and WLewandowski ldquoSpectroscopic (FT-IR FT-Raman1H and13CNMR) and theoretical studies of p-coumaric acid and alkalimetal p-coumaratesrdquo Spectroscopy vol 27 no 1 pp 35ndash48 2012
12 Advances in Physical Chemistry
[25] T Biswick W Jones A Pacuła E Serwicka and J PodobinskildquoThe role of anhydrous zinc nitrate in the thermal decomposi-tion of the zinc hydroxy nitrates Zn
5(OH)
8(NO3)2sdot2H2O and
ZnOHNO3sdotH2Ordquo Journal of Solid State Chemistry vol 180 no
4 pp 1171ndash1179 2007[26] S-H Park and E L Cheol ldquoSynthesis characterization and
magnetic properties of a novel disulfonate-pillared copperhydroxide Cu
2(OH)
3(DS4)
12 DS4 = 14-butanedisulfonaterdquo
Bulletin of the Korean Chemical Society vol 27 no 10 pp 1587ndash1592 2006
[27] P Rabu M Drillon and C Hornick ldquoStructural and spectro-scopic study of organic-inorganic transitionmetal based layeredmagnetsrdquo Analusis vol 28 no 2 pp 103ndash108 2000
[28] L Lei A Khan and D OrsquoHare ldquoSelective anion-exchangeintercalation of isomeric benzoate anions into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo Journal of Solid State
Chemistry vol 178 no 12 pp 3648ndash3654 2005[29] W Krause H-J Bernhardt R S W Braithwaite U Kolitsch
and R Pritchard ldquoKapellasite Cu3Zn(OH)
6Cl2 a new mineral
from Lavrion Greece and its crystal structurerdquo MineralogicalMagazine vol 70 no 3 pp 329ndash340 2006
[30] L C Andrews L R Bernstein C M Foris et al PowderDiffraction File Inorganic International Centre for DiffractionData Newtown Square Pennsylvania 2009
[31] A Ragavan A I Khan and D OrsquoHare ldquoIsomer selectiveion-exchange intercalation of nitrophenolates into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotxH
2Ordquo Journal of Materials
Chemistry vol 16 no 6 pp 602ndash608 2006[32] A Ragavan A Khan and D OrsquoHare ldquoSelective intercalation
of chlorophenoxyacetates into the layered double hydroxide[LiAl2(OH)
6]ClsdotxH
2Ordquo Journal of Materials Chemistry vol 16
no 42 pp 4155ndash4159 2006[33] S Vyazovkin and C A Wight ldquoModel-free and model-fitting
approaches to kinetic analysis of isothermal and nonisothermaldatardquoThermochimica Acta vol 340-341 pp 53ndash68 1999
[34] R E Johnsen F Krumeich and P Norby ldquoStructural andmicrostructural changes during anion exchange ofCoAl layereddouble hydroxides an in situ X-ray powder diffraction studyrdquoJournal of Applied Crystallography vol 43 no 3 pp 434ndash4472010
[35] G R Williams T G Dunbar A J Beer A M Fogg and DOrsquoHare ldquoIntercalation chemistry of the novel layered doublehydroxides [MAl
4(OH)
12](NO
3)2sdotyH2O (M=Zn Cu Ni and
Co) 1 new organic intercalates and reaction mechanismsrdquoJournal of Materials Chemistry vol 16 no 13 pp 1222ndash12302006
[36] T E M Ten Hulscher and G Cornelissen ldquoEffect of tempera-ture on sorption equilibrium and sorption kinetics of organicmicropollutantsmdasha reviewrdquo Chemosphere vol 32 no 4 pp609ndash626 1996
[37] M Avrami ldquoKinetics of phase change I general theoryrdquo TheJournal of Chemical Physics vol 7 no 12 pp 1103ndash1112 1939
[38] M Avrami ldquoKinetics of phase change II Transformation-timerelations for random distribution of nucleirdquo The Journal ofChemical Physics vol 8 no 2 pp 212ndash224 1940
[39] R J Francis S OrsquoBrien A M Fogg et al ldquoTime-resolved in-situ energy and angular dispersive X-ray diffraction studies ofthe formation of themicroporous gallophosphateULM-5 underhydrothermal conditionsrdquo Journal of the American ChemicalSociety vol 121 no 5 pp 1002ndash1015 1999
[40] Z Miladinovic J Zakrzewska B Kovacevic and G BacicldquoMonitoring of crystallization processes during synthesis of
zeolite A by in situ 27Al NMR spectroscopyrdquo Materials Chem-istry and Physics vol 104 no 2-3 pp 384ndash389 2007
[41] R Serna-Guerrero and A Sayari ldquoModeling adsorption of CO2
on amine-functionalized mesoporous silica 2 kinetics andbreakthrough curvesrdquo Chemical Engineering Journal vol 161no 1-2 pp 182ndash190 2010
[42] J D Hancock and J H Sharp ldquoMethod of comparing solid-state kinetic data and its application to the decomposition ofkaolinite brucite and BaCO
3rdquo Journal of the American Ceramic
Society vol 55 no 2 pp 74ndash77 1972[43] F Auriemma C De Rosa and R Triolo ldquoSlow crystallization
kinetics of poly(vinyl alcohol) in confined environment duringcryotropic gelation of aqueous solutionsrdquo Macromolecules vol39 no 26 pp 9429ndash9434 2006
[44] R V Ramanujan and Y R Zhang ldquoQuantitative transmissionelectronmicroscopy analysis of the nanocrystallization kineticsof softmagnetic alloysrdquoPhysical ReviewB vol 74 no 22 ArticleID 224408 2006
[45] A G Kalinichev P Padma Kumar and R James KirkpatrickldquoMolecular dynamics computer simulations of the effects ofhydrogen bonding on the properties of layered double hydrox-ides intercalated with organic acidsrdquo Philosophical Magazinevol 90 no 17-18 pp 2475ndash2488 2010
[25] T Biswick W Jones A Pacuła E Serwicka and J PodobinskildquoThe role of anhydrous zinc nitrate in the thermal decomposi-tion of the zinc hydroxy nitrates Zn
5(OH)
8(NO3)2sdot2H2O and
ZnOHNO3sdotH2Ordquo Journal of Solid State Chemistry vol 180 no
4 pp 1171ndash1179 2007[26] S-H Park and E L Cheol ldquoSynthesis characterization and
magnetic properties of a novel disulfonate-pillared copperhydroxide Cu
2(OH)
3(DS4)
12 DS4 = 14-butanedisulfonaterdquo
Bulletin of the Korean Chemical Society vol 27 no 10 pp 1587ndash1592 2006
[27] P Rabu M Drillon and C Hornick ldquoStructural and spectro-scopic study of organic-inorganic transitionmetal based layeredmagnetsrdquo Analusis vol 28 no 2 pp 103ndash108 2000
[28] L Lei A Khan and D OrsquoHare ldquoSelective anion-exchangeintercalation of isomeric benzoate anions into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotH
2Ordquo Journal of Solid State
Chemistry vol 178 no 12 pp 3648ndash3654 2005[29] W Krause H-J Bernhardt R S W Braithwaite U Kolitsch
and R Pritchard ldquoKapellasite Cu3Zn(OH)
6Cl2 a new mineral
from Lavrion Greece and its crystal structurerdquo MineralogicalMagazine vol 70 no 3 pp 329ndash340 2006
[30] L C Andrews L R Bernstein C M Foris et al PowderDiffraction File Inorganic International Centre for DiffractionData Newtown Square Pennsylvania 2009
[31] A Ragavan A I Khan and D OrsquoHare ldquoIsomer selectiveion-exchange intercalation of nitrophenolates into the layereddouble hydroxide [LiAl
2(OH)
6]ClsdotxH
2Ordquo Journal of Materials
Chemistry vol 16 no 6 pp 602ndash608 2006[32] A Ragavan A Khan and D OrsquoHare ldquoSelective intercalation
of chlorophenoxyacetates into the layered double hydroxide[LiAl2(OH)
6]ClsdotxH
2Ordquo Journal of Materials Chemistry vol 16
no 42 pp 4155ndash4159 2006[33] S Vyazovkin and C A Wight ldquoModel-free and model-fitting
approaches to kinetic analysis of isothermal and nonisothermaldatardquoThermochimica Acta vol 340-341 pp 53ndash68 1999
[34] R E Johnsen F Krumeich and P Norby ldquoStructural andmicrostructural changes during anion exchange ofCoAl layereddouble hydroxides an in situ X-ray powder diffraction studyrdquoJournal of Applied Crystallography vol 43 no 3 pp 434ndash4472010
[35] G R Williams T G Dunbar A J Beer A M Fogg and DOrsquoHare ldquoIntercalation chemistry of the novel layered doublehydroxides [MAl
4(OH)
12](NO
3)2sdotyH2O (M=Zn Cu Ni and
Co) 1 new organic intercalates and reaction mechanismsrdquoJournal of Materials Chemistry vol 16 no 13 pp 1222ndash12302006
[36] T E M Ten Hulscher and G Cornelissen ldquoEffect of tempera-ture on sorption equilibrium and sorption kinetics of organicmicropollutantsmdasha reviewrdquo Chemosphere vol 32 no 4 pp609ndash626 1996
[37] M Avrami ldquoKinetics of phase change I general theoryrdquo TheJournal of Chemical Physics vol 7 no 12 pp 1103ndash1112 1939
[38] M Avrami ldquoKinetics of phase change II Transformation-timerelations for random distribution of nucleirdquo The Journal ofChemical Physics vol 8 no 2 pp 212ndash224 1940
[39] R J Francis S OrsquoBrien A M Fogg et al ldquoTime-resolved in-situ energy and angular dispersive X-ray diffraction studies ofthe formation of themicroporous gallophosphateULM-5 underhydrothermal conditionsrdquo Journal of the American ChemicalSociety vol 121 no 5 pp 1002ndash1015 1999
[40] Z Miladinovic J Zakrzewska B Kovacevic and G BacicldquoMonitoring of crystallization processes during synthesis of
zeolite A by in situ 27Al NMR spectroscopyrdquo Materials Chem-istry and Physics vol 104 no 2-3 pp 384ndash389 2007
[41] R Serna-Guerrero and A Sayari ldquoModeling adsorption of CO2
on amine-functionalized mesoporous silica 2 kinetics andbreakthrough curvesrdquo Chemical Engineering Journal vol 161no 1-2 pp 182ndash190 2010
[42] J D Hancock and J H Sharp ldquoMethod of comparing solid-state kinetic data and its application to the decomposition ofkaolinite brucite and BaCO
3rdquo Journal of the American Ceramic
Society vol 55 no 2 pp 74ndash77 1972[43] F Auriemma C De Rosa and R Triolo ldquoSlow crystallization
kinetics of poly(vinyl alcohol) in confined environment duringcryotropic gelation of aqueous solutionsrdquo Macromolecules vol39 no 26 pp 9429ndash9434 2006
[44] R V Ramanujan and Y R Zhang ldquoQuantitative transmissionelectronmicroscopy analysis of the nanocrystallization kineticsof softmagnetic alloysrdquoPhysical ReviewB vol 74 no 22 ArticleID 224408 2006
[45] A G Kalinichev P Padma Kumar and R James KirkpatrickldquoMolecular dynamics computer simulations of the effects ofhydrogen bonding on the properties of layered double hydrox-ides intercalated with organic acidsrdquo Philosophical Magazinevol 90 no 17-18 pp 2475ndash2488 2010
