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Colloids and Surfaces A: Physicochemical andEngineering Aspects
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n the layer structures in acid- and amine-substituted calixareneangmuir–Blodgett films
.M. McCartneya, N. Cowlama, F. Davisb,∗,1, T. Richardsona,2, A. Desertc,
. Gibaudc, C.J.M. Stirlingd
Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UKCranfield Health, Cranfield University, Cranfield MK43 0AL, UKLaboratoire P.E.C. Rayons-X, Faculté des Sciences, Université du Maine, 72085 Le Mans Cedex 9, FranceDepartment of Chemistry, University of Sheffield, Sheffield S3 7HF, UK
i g h l i g h t s
X-ray reflectometry measurementsmade on LB films of calixarenes.Total film thickness and the bilayerspacing were obtained from reflectiv-ity profiles.The “pinched loop” molecular confor-mation found for calix[8]arenes.The classical bowl and chain configu-ration found for calix[4]arenes.
g r a p h i c a l a b s t r a c t
a r t i c l e i n f o
rticle history:eceived 16 March 2013eceived in revised form 30 May 2013ccepted 5 June 2013vailable online xxx
eywords:-ray reflectometry
a b s t r a c t
X-ray reflectometry measurements have been made on Langmuir–Blodgett (LB) films of an amine-substituted tertiary octyl calix[8]arene and of alternating layers of acid and amine-substituted tertiaryoctyl calix[8]arene as well on LB films of alternating layers acid/amine-substituted tertiary butylcalix[4]arenes. The total film thickness and the bilayer spacing were obtained for each sample fromits reflectivity profile R(Q) vs. Q. Simulations of the reflectivity profiles were made with a programmebased on Parratt’s recursive relations, using models of their multilayer structures. The “pinched loop”molecular conformation which, rather surprisingly, describes the layer structure in an acid substituted
alixareneB filminched loop
tert-octyl calix[8]arene, also describes the structures in the amine-substituted and the alternate layeracid/amine-substituted calix[8]arenes, with the exception that the amine head-groups are larger andtheir interpenetration is prevented. In contrast, the smaller calix bowl in the acid/amine-substitutedcalix[4]arene molecules defines their conformation more rigidly. The layer structure of the alternatelayer acid/amine-substituted calix[4]arene sample is therefore well described by the repetition of theclassical bowl and chain configuration.
The general characteristics of the calix[4]resorcinarenes andthe calixarenes [1] have been described in our previous paperson the layer structures in a calix[4]resorcinarene film [2,3] and
in a Langmuir–Blodgett (LB) film of an acid-substituted tertiaryoctyl calix[8]arene [4]. These compounds are of interest in host-guest chemistry and also as pyroelectric materials [5,6] for usein thermal detectors and thermal imaging devices. LB films can
e fabricated by the repetition of non-centrosymmetric molecularilayers, which usually have a low permittivity and a low dielectric
oss tangent. The classical fatty acid/fatty amine systems have ofteneen studied in the past, but functionalised calixarenes can also be
ncorporated into alternate layer LB films. They exhibit a high den-ity of acid/amine pairs and a high melting point for the completedlm. Pyroelectric coefficients of up to 15 �Cm−2 K−1 with figures oferit of FD = 75 �Cm−2 K−1 have been reported for calixarene based
lms [7], which are amongst the highest values of any LB films.Calixarenes have been widely studied and shown to bind a broad
ange of inorganic and organic guests, however many of the studiesn the specific complexation properties of calixarenes have onlyaken place in solution. For such materials to be of use in sen-ors there must be a transduction step which converts the bindingvent into a measurable signal. This will usually require the cal-xarene to be immobilised onto a solid surface such as an electrode,
piezoelectric quartz crystal or an optical chip. Therefore an under-tanding of the nature and structure of such layers in necessary.
There has been a number of works reported where the interac-ion of various guests with calixarene monolayers and multilayersas been studied. For example, monolayers of a calix[4]arene and achiff base modified calix[4]arene on various subphases were stud-ed and showed the Schiff base calixarene demonstrated a muchtronger interaction with dissolved Li+ and Cu2+ ions [8]. Otherorkers studied the interaction of a calix[7]arene monolayer with
arious metal ions in the subphase and showed that incorpora-ion of certain ions led to a more uniform LB film structure [9]. LBlms of p-allyl calix[4]arene could be deposited onto glassy carbonlectrodes which then had a much higher sensitivity for mercury
ig. 1. The general schemes of the (a) calix[8]arenes and (b) the calix[4]arenes are given
sotherms of the calixarenes.
hysicochem. Eng. Aspects 436 (2013) 41– 48
ions than unmodified carbon [10]. Thiacalixarene LB films have alsobeen shown to greatly improve sensitivity for mercury at glassy car-bon electrodes [11] using absorptive stripping voltammetry and inother work demonstrated simultaneous determination of lead andcadmium [12] as well as developing a selective electrochemicalsensor for silver ions using LB films of calixarenes [13]. Incorpo-ration of sodium ions in the subphase has also been shown to affectthe properties of calixarene monolayers and also the gas perme-ability of LB bilayers deposited on a permeable substrate [14].
Calixarene LB films have also been shown to interactwith organic guests, for example LB films of phosphorylatedcalix[4]arenes could be deposited on piezoelectric quartz crystalsand shown to reversibly form complexes with volatile compoundssuch as acetone and chloroform [15]. LB films of the similarcalix[4]resorcinarene have also been shown to detect organicvapours using quartz crystal microbalance [16] or surface plasmonresonance [17].
We previously reported [4] the construction of LB films of an acidsubstituted calix[8]arene and determination of its layer structure.Our findings led us to conclude that the calix[8]arene existed inthe LB film in the unusual “pinched loop” state rather than a coneor pleated loop conformation. Despite the rather unusual molec-ular conformation of the “pinched loop” state, it alone describedthe layer structure in an acid substituted tert-octyl calix[8]arenesample [4]. We were keen therefore, to establish whether it could
be applied to other substituted calixarenes as well as investigatingstructures made from calix[4]arenes.
The general schemes of calix[8]arene and of calix[4]arene sam-ples measured in the present work are shown in Fig. 1, and they
and the R- and X- substitutions of the examples studied are specified in Table 1; (c)
C.M. McCartney et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 41– 48 43
Table 1The calixarene samples used in the present measurements by X-ray reflectometryare specified.
Molecule Alias Chemical structure
t-octyl calix[8]arene–aminesubstituted
to8am R = -C(CH3)2CH2(CH3)3
X = -O(CH2)3NH2
t-octyl calix[8]arene–acidco-deposited with t-octylcalix[8]arene–amine
to8ac/am R = -C(CH3)2CH2(CH3)3
X = -OCH2COOH (acid)X = -O(CH2)3NH2 (amine)
t-butyl calix[4]arene–acid,co-deposited with t-butylcalix[4]arene–amine
tb4ac/am R = -C(CH3)3
X = -OCH2COOH (acid)X = -O(CH2)3NH2 (amine)
Table 2Parameters derived from measurements of the �–A isotherms and the CPK modelsof the four molecules are given.
Sample Solid phasepressure (m Nm−1)
Molecular area (from�–A isotherm) (Å2)
Molecular area (fromCPK model) (Å2)
to8ac 25 313 310to8am 25 380 310
coatraboea
2
mhtArpet
rwmCaCavdappilXw(th4
tb4ac 20 183 130tb4am 25 173 128
ame from a series designed to optimise the pyroelectric responsef the resultant LB films [7]. Substitution of either carboxyl- ormino- groups around the lower rim of the phenolic basket in both-octyl and t-butyl calix[n]arenes changes the total height of theesulting molecule and hence the number of acid/amine pairs in
given thickness of film. In addition, equivalent substitutions inoth calix[8]arene and of calix[4]arene lead to different densitiesf acid/amine pair within the layer planes, on account of the differ-nt sizes of the phenolic baskets. Details of the three calixarenesre given in Table 1, together with their aliases.
. Sample preparation and experimental method
The synthesis of the calixarene samples has been described inore detail elsewhere [18]. Basically t-octyl phenol and formalde-
yde were condensed under alkaline conditions to afford the parent-octyl calix[8]arene; t-butyl calix[4]arene was purchased fromcros Organics. The phenol groups of the calixarenes were theneacted with either 2-bromoethyl acetate or N-(3-bromopropyl)hthalimide using potassium carbonate as a base, the resultantsters or phthalimides were then hydrolysed with KOH to affordhe acids or amines.
The LB films (which had additional cadmium ions incorpo-ated to aid the visibility of their layers in the X-ray experiments),ere fabricated using a single layer, constant perimeter, Lang-uir trough under computer control, containing a 5 × 10−4 mol L−1
dCl2 solution in water (ELGA UHP 18 M� cm) subphase withdded ammonia (to increase the pH from 5.45 to 7.5) at 20 ◦C.alixarene solutions (0.5 mg mL−1 in chloroform) were spread andllowed to settle for 30 min to facilitate the evaporation of the sol-ent and the uptake of the cadmium ions. The isotherms of theifferent materials were recorded by compressing the monolayert a rate of about 1% s−1. The deposition pressures and the areaer molecule from the �–A isotherms are given in Table 2. Areaser molecule were calculated by extrapolating the slope of the
sotherm, at the deposition pressure, back to zero pressure. Mono-ayers were deposited onto the substrates at a speed of 5 mm min−1.-ray photoelectron spectroscopy (XPS) measurements were madeith a VG Clam 2 spectrometer with an Mg K� X-ray source
100 W, pass energy of 100 eV). Unlike the to8ac sample in [4], thehree new samples were deposited onto silicon substrates renderedydrophilic by cleaning with an alkaline etch (2% NaOH, 2% H2O2,0 ◦C, 5 min). X-ray reflectometry measurements were made with
Fig. 2. The X-ray reflectivity profile R(Q) vs. Q for the to8am, to8ac/am and tb4ac/amLB film samples are shown as obtained with Cu K� radiation � = 1.514 A.
a horizontal Huber diffractometer (mounted on Philips PW 1130generator, Cu K�, � = 1.542 A) with separate � and 2� motions and asystem of collimators optimised for reflectometry at small angles.Analysis of the R(Q) vs. Q reflectivity profiles was carried out aspreviously described [4].
The to8am sample was 4 bilayers thick constructed so that theamine X-groups were in contact with the substrate. The to8ac/amsample had 13 layers, deposited in an alternate layer trough withone layer of to8ac acid headgroups in contact with the substratefollowed by six bilayers of alternate amine and acid-substitutedcompound. This same layer sequence was used for the tb4ac/amsample. The XPS measurements showed that the to8am, to8ac/amand tb4ac/am contained 0.5 cadmium ions per molecule which wasmuch lower than in to8ac [4]. In the to8am this is probably due toweaker interactions between the amine groups and the cadmiumions, while in the case of the alternating films, the amine groupsbecome protonated and compete with the cadmium for binding tothe acid groups.
The X-ray reflectometry profiles R(Q) vs. Q were measured toQmax ≈ 0.55 A−1 for the to8am and to8ac/am and Qmax ≈ 0.60 A−1
for the tb4ac/am sample and are shown in Fig. 2. They exhibitthe same overall features, which are clearest in the profile ofthe tb4ac/am sample, and include first and second order Braggpeaks – although the second peak for both to8am and to8ac/amis broad and indistinct. Several orders of well-developed interfer-ence fringes can be seen – again less so for the to8ac/am sample.They begin at Qm = 2 = 0.039 A−1 for tb4ac/am and can still be iden-tified beyond the second Bragg peak at Q ≈ 0.68 A−1. The number(n − 1) of fringes present between the two Bragg peaks in each pro-file relates to the n layers present in each sample The R(Q) vs. Qprofiles for the three new samples all reach a limiting value ofR(Q) ≈ 1.5 × 10−7, but there are subtle differences between themwhich are partly hidden by the logarithmic R(Q) scale. The “peak-to-background ratio” of the first Bragg peak is (8:1) for to8am; (3:1)for to8ac/am and rises to (16:1) for tb4ac/am. It will be explainedbelow how these values illustrate the different degrees of contrastbetween the constituent (sub) layers of the samples.
The parameters obtained from these R(Q) vs. Q profiles are givenin Table 3, together with the corresponding values obtained sub-sequently with the PARRATT 32 programme. The bilayer spacing dwas obtained for each sample from the positions of the Bragg peaksand with only two data points are obviously subject to large errors.The interference fringes with m = 1–4 were measured for to8ac;
m = 1–3, 6–8 for to8am; m = 2–5, 8–12 for to8ac/am and m = 2–5,8–12, 15–18 for tb4ac/am which allowed an accurate determina-tion of the total thickness D of the samples to be made.
44 C.M. McCartney et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 41– 48
Table 3The bilayer thickness d and the total film thickness D of the LB film samples, obtained directly from their reflectivity profiles are given, together with the values from theleast squares fits to their profiles using the PARRATT 32 programme.
Fig. 3. Schematic views of the to8ac (a) and to8am (b) molecules in the “pinched loop” conformation are given, together with the tb4ac (c) and tb4am (d) molecules in the“bowl and chain” conformation. The dimensions were derived from the CPK models of the two conformations and are accurate to ± 0.3 A.
Fig. 4. Schematic diagrams of layer structures in to8am created by the packing of neighbouring molecules in the “pinched loop” conformation are shown. The larger aminehead-groups prevent the interpenetration of the X-groups which occurs in to8ac. Two variations of the structure are given, which cannot be distinguished in the reflectometrymeasurements.
C.M. McCartney et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 41– 48 45
F givent
ipwidtept
3
3
to
FtTp
the spacing between the X-groups on neighbouring moleculesis sufficiently large (15.7 A) to allow the insertion of a third X-group from a molecule which is inverted and in a plane below thetwo molecules shown. This can occur even when the size of the
ig. 5. A schematic diagram of the structure of the to8ac/am alternate layer film ishe layer structure shown in Fig. 4 are permitted, although only one is shown.
Finally, CPK models were made of each of the moleculesnvolved. They provided molecular dimensions for direct com-arison with the data from the X-ray reflectometry experiments,hich were also checked against the molecular areas from the �–A
sotherms. They helped to clarify how the molecular conformationsetermined the layer structures in the to8am and to8ac/am andb4ac/am samples. Unfortunately, diagrams based on the CPK mod-ls do not reproduce well, when reduced to the dimensions used inublications, (especially as inserts to figures) so schematic views ofhe molecules will be given in Figs. 3–5 and 7.
. Layer structures in the calixarene LB films
.1. Molecular conformation in calix[8]arenes
Fig. 3 shows the schematics of the to8ac and to8am molecules inhe “pinched loop” conformation, together with their dimensionsbtained directly from the CPK models. Fig. 3 helps to explain how
ig. 6. The least squares fits (continuous line) to the four experimental X-ray reflec-ivity profiles (– –) of the to8am, to8ac/am and tb4ac/am LB film samples are shown.hey are based on the layer structures depicted in Figs. 4, 5 and 7 and the structuralarameters obtained from these fits are given in Tables 4 and 5.
. The interpenetration of the X-groups is again prevented and the two variations of
the layers may form in an LB film sample. When two molecules arebrought together, their intermolecular distance is determined bythe partial interdigitation of the lateral R-groups and has a valueof ≈25 A (from CPK models). The combined width of the lowerX-groups is 9.3 A, so with an intermolecular distance of ≈25 A,
Fig. 7. A schematic diagram of the layer structure in the tb4ac/am alternate layerfilm is shown. It is based on the repetition of a simple “bowl and chain” configuration,with each molecule in the form of a short truncated cone.
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6 C.M. McCartney et al. / Colloids and Surface
-groups is increased to allow for the incorporated cadmium atoms.he interpenetration of the head-groups creates a high density ofadmium atoms in their layer plane which was observed in to8acsee Fig. 7a of [4]) Further layers can then be stacked on top ofhis bilayer, with a partial interdigitation of their four “vertical” R-roups and this “nesting” of the layers gives a structure with theorrect bilayer spacing.
The intermolecular distance for the to8am has the same value25 A as to8ac, but the amine head-groups are larger than their
arboxyl analogues. There is a 9% increase in their width to 10.1 A,o that the space between the neighbouring X-groups is reducedo 10.6 A. The complete interpenetration of the head-groups is lessikely to occur, especially when the size of the substituted cadmiumtoms is taken into account. The increase in the bilayer spacingrom 25 A in to8ac [4] to ≈31 A in to8am also suggests that com-lete interpenetration of the X-groups does not occur. The aminoead-groups are certainly longer than the carboxyl groups, but thislone does not account for the increase in the bilayer spacing, asill be shown in the structural refinement below. On the otherand, there is no reason to suggest that the interdigitation of the-groups is different in to8ac and to8am. The proposed layer struc-ure in the to8am film is shown in Fig. 4, which can be comparedirectly with Fig. 7a of [4]. Note that in to8am, the first layer has theydrophilic X-groups in contact with the standard substrate, whileo8ac had the hydrophobic R-groups in contact with the treatedubstrate. Two variations of the proposed layer structure in theo8am film are shown in Fig. 4 – one where the molecules are stag-ered in neighbouring planes and one where the molecules stackirectly head-to-head. The sub-layers in each of these will have theame values of electronic density, so the reflectivity measurementsannot detect the difference between them. The model with thetaggered molecules is perhaps preferred as it may provide moreexibility in the molecular packing.
There is a smaller increase in the bilayer spacing from 25 A ino8ac to ≈27 A in the to8ac/am sample (Table 3). This is almostccounted for by the extra length of a single amine head-group perilayer. The distances between the neighbouring X-groups (15.7 Aor to8ac [4] and 14.9 A for to8am from CPK models) are also suf-cient to allow some interpenetration of the X-groups to occur.owever the detailed analysis of the reflectivity profile of the
o8ac/am sample, which will be presented below, suggest thathis probably does not occur. A consideration of the pyroelectric
echanism also supports this, because the proton transfer woulde hindered if the carboxyl and amine groups interpenetratedompletely and the pyroelectric response of the system would beeduced rather than enhanced [7]. The proposed layer structure ofhe to8ac/am sample is shown in Fig. 5, with the X-groups in therst layer again in contact with the substrate and the R-groups (ofhe 13th layer) forming the upper surface of the film.
.2. Calculation of the reflectivity profiles for the calix[8]arenes
The essential first step in simulating a reflectivity profile, is toivide the sample into a series of layers normal to the surface [see]. These are usually sub-layers to the actual molecular layers. Theon-penetrating structure of to8am in Fig. 4 can be described by
similar sequence of five sub-layers to to8ac [4] – of which onlyhree are distinct,
hydrophobic R-groups; hydrophobic R-groups and phenolic rings; hydrophilic X-groups and cadmium ions; = 2 hydrophobic R-groups and phenolic rings;
= 1 hydrophobic R-groups;
In Fig. 4 for example, sub-layer 3 lies in the middle of the ×3region, with layers 2–1 above and 4–5 below. The structure of
hysicochem. Eng. Aspects 436 (2013) 41– 48
to8ac/am requires a sixth sub-layer to describe the different elec-tronic densities of the alternating acid/amine head groups, whichare shown in the middle of the ×5 {region in Fig. 5. Obviously, thefirst and last molecular layers of these films are not interdigitatedand their sub-layers will have half the complement of atoms ofthose in the bulk. It turned out to be more convenient to take anasymmetric set of four sub-layers for to8am (namely 2, 3, 4 and 5)instead of the sequence 1–5 above, to form the repeat unit in thecalculation and to deal with the layers remaining at the top andbottom film individually. Thus to8am had two bottom sub-layersand three top sub-layers and to8ac/am had two bottom sub-layersand one top sub-layer, respectively. The electronic scattering lengthdensity profiles �(z) vs. z were derived for each sample and usedto calculate the R(Q) vs. Q reflectivity profiles with the PARRATT 32programme [4]. The calculated parameters were adjusted to obtainthe best fit between the simulated and the experimental reflectivityprofiles and in each case a sufficient level of agreement was reachedto allow a least squares refinement to be made.
The fits to the experimental reflectivity profiles are shown inFig. 6 and it is clear that there is excellent agreement between theexperimental data and the calculated profiles. Table 4 shows thereis a very close correspondence between the parameters obtainedfrom the least squares refinements of the R(Q) vs. Q profiles andthose measured directly from the CPK models (which are given inbrackets). In addition, the summation of the thickness of the sub-layers in the repeat units agrees in every case with the d valuesderived from the positions of the Bragg peaks. When these sum-mations are extended to include the separate sub-layers at the topand bottom of the film and the oxide layer, an equally good agree-ment is found with the values of the total film thickness D derivedfrom the positions of the interference fringes, as shown in Table 3.
The parameters given in Table 4, also explain the relative visi-bilities of the features in the different R(Q) vs. Q profiles. The lack ofinterpenetration of the headgroups and the lower concentration ofcadmium ions in to8am and to8ac/am leads to values of electronicscattering length density which are similar to those of the adjacentlayers, so the intensity of their Bragg peaks is significantly reducedin comparison with the one observed for to8ac [4]. The Table alsoshows that the values of the electronic density, layer thickness andlayer roughness of those separate sub-layers which were commonto the to8ac [4], to8am and to8ac/am samples are carried throughconsistently in the refinements. Finally, the values of r.m.s. inter-face roughness for the to8am sample actually reduce from theirstarting values of 1 A in the refinement, which is indicative of ahigh quality multilayer film. In to8ac/am the r.m.s. roughness of thelayers containing the hydrophobic R-groups and the R-groups plusthe phenolic rings both increase. The former is the upper layer ofthis sample and the roughness in this layer causes the interferencefringes to be less well developed in its reflectivity profile than inthose of the to8am and tb4ac/am. The overall similarity of the filmparameters for these fittings confirms the successful application ofthe pinched loop molecular conformation to the layer structures ofthe three related compounds.
3.3. Molecular conformation and layer structure in thecalix[4]arene LB film
The reasons for studying the alternate layer tb4ac/am sample insearch for an optimum pyroelectric response were explained in theIntroduction and its R(Q) vs. Q profile is shown in the lower part ofFig. 2.
The fact that the circumference of the phenolic basket in the
calix[4]arenes is only half that of the calix[8]arenes, means thatthey have a much greater steric stability and the movement ofits attached groups is considerably hindered. The calix[4]areneshave therefore a classical bowl-and-chain configuration. This is a
C.M. McCartney et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 436 (2013) 41– 48 47
Table 4The electronic scattering length density, thickness and interface roughness of the sub-layers which constitute the to8am and to8ac/am LB film samples are given and can becompared with Table 3 of [4]. They were derived from the least squares fits to the experimental reflectivity profiles shown in Fig. 6 and the corresponding values obtaineddirectly from the CPK models given in brackets.
Sample to8am to8ac/am
Layer number and constitution Electronic s.l. density(×10−6 A−2)
Table 5The electronic scattering length density, thickness and interface roughness of the sub-layers which constitute the tb4ac/am LB film sample are given. They were derivedfrom the least squares fits to the experimental reflectivity profile and the values calculated directly from the CPK models given in brackets.
Layer number and constitution Electronic s.l. density(×10−6 A−2)
ompact, cone-like, molecular conformation in both tb4ac andb4am, as shown schematically in Fig. 3, with the dimensionserived from their CPK models. Fig. 3 shows that the possibility ofransforming this molecule into an alternative conformation (suchs the pinched loop structure) is unlikely. The diameter (≈5.8 A) ofne R-group alone occupies half the molecular diameter (≈11.3 A),o the possibility of interdigitation is also reduced. Finally, theolecular diameters measured from the cone-like CPK models areithin 16% of the same values derived from the �–A isotherms and
he very formation of the Langmuir film infers a clear segregationetween the hydrophilic and hydrophobic parts of the molecule. Allf this suggest that the only practical model for the layer structuren tb4ac/am film is the one shown in Fig. 7. The X-groups of the firstcid- substituted layer are in contact with the substrate and therere six repetitions of the acid/amine bilayer, finishing with a singlecid-substituted layer, whose R-groups form the top surface of thelm.
This structure can be described by the repetition of five sub-ayers – of which four are distinct and can be identified as,
amine X-groups and cadmium ions; phenolic rings; a double layer of hydrophobic R-groups; = 2 phenolic rings; acid X-groups and cadmium ions.
Sub-layer 3 is obviously in the middle of the ×5 {region in Fig. 7.he addition of sub-layers 5, 4, and a sub-layer 3 with half thick-ess at the upper surface will complete the 13 layers of the film.he electronic scattering length density profile �(z) vs. z and the(Q) vs. Q reflectivity profile were obtained using the PARRATT 32rogramme. Only minor modifications were necessary to the inputata before the least squares refinement was achieved. The fit to thexperimental reflectivity profile is given at the bottom of Fig. 6 andt reproduces the wealth of detail in the reflectivity profile quiteuccessfully. The derived parameters are given in Tables 3 and 5
able 5 shows that although the acid and amine-substituted X-roups have relatively low concentrations of cadmium ions, theirlectronic scattering length densities are sufficiently different fromhose of the other sub-layers to produce the stronger Bragg peaks
7.2 (10.0) 0 (1.0)“infinite” 2.0 (1.0)
that are observed. In addition, the low interface roughness (0.6 A) ofthe upper layer containing the hydrophobic R-groups helps to cre-ate the well defined interference fringes. One minor feature of theparameters obtained is that the derived lengths for the tb4ac andtb4am molecules are slightly smaller (by 0.95 A and 0.55 A) thanthe values obtained from the CPK models. This may be accountedfor if the measurements had slightly overestimated the length ofthe amine head-group and/or if there was a small degree ≈10% ofinterdigitation between the neighbouring R-groups. In any case, thevalue of the bilayer spacing obtained from the refinement is in goodagreement with the value derived from the positions of the Braggpeaks (see Table 3).
4. Discussion and conclusions
X-ray reflectometry is an established method for studying thestructures and structural defects in LB films, especially those basedon fatty acids. The method has been extended in the present workto another class of multilayer structures. The layer structure ofan acid substituted, tert octyl calix[8]arene LB film was success-fully described using the somewhat unexpected “pinched loop”molecular conformation [4] and it has now been shown that thesame model can be used for amine- and acid/amine-substitutedfilms. Only minor modifications involving the interpenetration ofthe head-groups were required. In contrast, the greater steric sta-bility of the calix[4]arene molecule means that the layer structureof an alternate layer acid/amine-substituted calix[4]arene LB filmsample is accurately described by the classical bowl and chain con-figuration.
The values of the molecular areas measured from �–A isotherms,(to8ac = 313 A2 and to8am = 380 A2) given in Table 3 are compatiblewith the areas obtained from the CPK models and they show thatthe pinched loop conformation of the molecules must exist in thecompressed Langmuir film on the liquid subphase. In contrast, the
large diameters of the bowl and chain and the pleated loop molec-ular conformations (ϕ = 24 A and ϕ = 26.5 A respectively) lead tosubstantially greater molecular areas – between 471 A2 and 702 A2
[4]. The �–A isotherms have shown that the to8ac and to8am films
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8 C.M. McCartney et al. / Colloids and Surface
ave a fairly high compressibility for surface pressures between0–35 m Nm−1, so it is tempting to associated this with a grad-al transition from, say, the bowl-and-chain to the pinched looponformation of the molecules.
cknowledgements
The support of the University of Sheffield for this programme isratefully acknowledged. The X-ray reflectometry measurementsere made at the Laboratoire P.E.C. Rayons-X, Faculté des Sciences,niversité du Maine and the collaboration between Dr Cowlam androf Gibaud was supported by the British Council. Tim Richardsonied after a short illness while this manuscript was being producednd it is dedicated to his memory.
eferences
[1] C.D. Gutsche, Calixarenes Monographs in Supramolecular Chemistry, RoyalSociety of Chemistry, Cambridge, 1989.
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