Chemical Engineering Department, Hacettepe University, Beytepe 06800, Ankara, Turkey
Received February 5, 2009. Revised Manuscript Received April 20, 2009
The hydrophobic core of the multicompartment micelles consists of incompatible and clearly separated distinctsubdomains which make them different from the classical micelles. Owing to these properties multicompartmentmicelles have a great potential to be used as solubilization agents and carriers for a wide variety of applications where it isimportant to prevent the uncontrolled interaction of the solubilizates before reaching the target and to convey them tothe specified point simultaneously. Here we show that effective compartmentalization inside the micelle and highsolubilization capacity for the two immiscible water-insoluble materials in cases of both simultaneous and separatesolubilization can be achieved by newly designed ion-pair hybrid surfactant CH3(CH2)11(OCH2CH2)23N
+(C2H5)3-SO3
-(CF2)7CF3 (C12E23N+SO3
-F8) through the agency of favorable molecular design. Molecular structure is tailoredby the approach of using a balance of forces to obtain compartmentalization, which is without precedent. This newmolecule also has the properties of quite low critical micelle concentration and an extensive surfactant concentrationrange for solubilization which are additional important advantageous features.
Introduction
It is of great importance to transport several compounds to thesame place at a desired proportion simultaneously for a widevariety of processes. As a result, the studies in the field ofmulticompartment micelles have been accelerated recently.1-5
Multicompartment micelles are aggregates of surfactants, whichare composed of a hydrophilic shell and a multidomain hydro-phobic core with each domain having different properties.6
Existence of different subdomains in a micellar core makes itpossible to cosolubilize and transport several different andimmiscible materials in different subdomains selectively, prevent-ing any undesired interactions before reaching the target. Becauseof this property, multicompartment micelles have high potentialto be used especially in controlled drug delivery, cosmetics,imaging technology, selective entrapment and release of dyes,pesticides, gene therapy agents, etc. Until now, in all attempts tomake multicompartment micelles hydrocarbon-fluorocarbonhybrid systems were used. Fluorocarbons have unique propertiessuch as extraordinary thermal, chemical and biological inertness,low surface tension, high hydrophobicity, high fluidity, highrigidity, and high gas dissolving capacities caused by their strongintramolecular and weak intermolecular interactions.7 Thesecharacteristics of fluorocarbons increase the interest for utiliza-tion of them in recent researches especially in biomedicine.8,9
For the scope of multicompartment micelles, high tendency
to self-aggregate and lipophobicity along with hydrophobicityare the key features of fluorocarbons. So, hydrocarbon-fluor-ocarbon hybrid systems are considered as appropriate structuresfor preparing multicompartment micelles.
In this study, the novel hydrocarbon-fluorocarbon ion-pair hybrid surfactant CH3(CH2)11(OCH2CH2)23N
+(C2H5)3-SO3
-(CF2)7CF3 (C12E23N+SO3
-F8), which is thought to be ableto form multicompartment micelles with high solubilizationcapacity, is designed and the aggregation and the solubiliza-tion properties of this new molecule have been investigated withthe hope of introducing a new approach to multicompartmentmicelles. To our knowledge, this is the first time in literatureof utilization of ion-pair amphiphiles in order to obtain multi-compartment micelles although they have been used and studiedfor a long while. We aimed to reduce some problems facedin previous studies related to multicompartment micelles suchas low content of hydrophobic groups in the aggregate, absenceof common interface between two distinct hydrophobiccores, low solubilization capacity, and overlapping of hydro-phobic subdomains which is critical for cosolubilizationby making the appropriate molecular design. This new moleculehas gratifying results such as quite low CMC (critical micelleconcentration) and superior surface activity, high solubiliza-tion capacities for hydrocarbon-based and fluorocarbon-based probes both separately and simultaneously, which leadsto the deduction of achieving efficient compartmentalizationinside the micellar core.
Materials and Methods
Perfluoro-1-octanesulfonyl fluoride, triethylamine, 1H,1H,2H,2H-tridecafluor-1-octyliodid, orange OT, diethyl ether (max.0.2% H2O), ethanol, and hexane were purchased from Sigma(Germany). Brij 35 and DTAB were purchased from Across(Belgium). These chemicals were used without further purification.F8C2SO3
-Na+ (CF3(CF2)7(CH2)2SO3-Na+) was synthesized in
our laboratory as described elsewhere.10 The ion-pair hybrid
*To whom correspondence should be addressed. Fax:+90 312 2992124.E-mail:[email protected].(1) St€ahler, K.; Selb, J.; Candau, F. Langmuir 2005, 15, 7565–7576.(2) Kubowicz, S.; Th€unemann, A. F.; Weberskirch, R.; M€ohwald, H. Langmuir
2002, 35, 1091–1101.(5) Lodge, T. P.; Rasdal, A.; Li, Z.; Hillmyer, M. A. J. Am. Chem. Soc. 2005,
127, 17608–17609.(6) Lutz, J. F.; Laschewsky, A. Macromol. Chem. Phys. 2005, 206, 813–817.(7) Krafft, M. P.; Riess, J. G. Biochimie 1998, 80, 489–514.(8) Krafft, M. P. Adv. Drug Delivery Rev. 2001, 47, 209–228.(9) Abe, M. Curr. Opin. Colloid Interface Sci. 1999, 4, 354–356. (10) Aydogan, N.; Abbott, N. L. Langmuir 2002, 18, 7826–7830.
SO3-(CF2)7CF3] was synthesized with a yield of approximately
90% in our laboratory. For the synthesis of C12E23N+SO3
-F8,Brij 35 (0.005 mol), triethylamine (0.0125 mol), and perfluoro-1-octanesulfonyl fluoride (0.005 mol) were reacted in ether withvigorous stirring at 40 �C for 24 h. Ether was removed from themixture by evaporation in a vacuum and C12E23N
+SO3-F8 was
taken by 40:1 (v/v) ether:ethanol. After evaporation of 40:1 (v/v)ether:ethanol the product was purified by washing with first 1:1 (v/v)ether:hexane and then with hexane several times.11,12
-(CF2)7CF3).Aqueous surfactant solutions were prepared fresh for each
experiment with ultrapure water (18.3 mΩ 3 cm). The equilibriumsurface tensions of aqueous C12E23N
+SO3-F8 solutions were
measured by the pendant drop method (Kruss DSA10-MK2).The CMC value is found from the plot of surface tension vs.surfactant concentration as one sharp break in the graph.
The size and shape of the aggregates were determined bysimultaneous dynamic and static light scattering measurements,using CGS-3 with a 632.8 nm laser (Malvern, UK). The ultrapurewater used in the preparation of aqueous solutions filteredthrough a 0.22 μm filter for the removal of dust and the solutionswere filtered again througha 0.45μmcellulose acetate filter beforemeasurement. Themeasurements were performed at the angles of30-150� with 10� increment. The dn/dc value (0.089mL/g) of thenew ion-pair hybrid surfactant was determined with BI-DNDC(Brookhaven Inst., USA) at 25 �C.
In the dynamic light scattering (DLS) method, the measuredintensity time correlation function is used to obtain translationaldiffusion coefficient, D, by utilizing scattering vector q, whereq= (4πn/λ) sin(θ/2). Here n is the refractive index of the solvent,λ is thewavelengthof the light, andθ is the scattering angle.Whenthe translational diffusion coefficient is known the hydrodynamicradius, Rh, can be obtained from the Stokes-Einstein equation
Rh ¼ kBT
6πηD
where kB is the Boltzmann constant, T is absolute temperature,and η is the solvent viscosity.13
The radius of gyration and form factor can be determined fromstatic light scattering experiments and are used to evaluate theshape of the aggregates.
The opportunity for determining the particle shape is providedby the angular dependence of scattering intensity. Scatteringintensity is expressed as14
Rθ ¼ KcMwPθSθ
where Pθ and Sθ are form and structure factors, respectively, andK is the optical constant, which is
K ¼ 4π2n2
NAλ4
dn
dc
� �2
Here n is solvent refractive index, λ is incident light wavelength,NA is Avogadro’s number, and (dn/dc) is the refractive indexincrement.15Molecularweight, radius of gyration,Rg, and second
viral coefficient, A2, are calculated by using Kc/Rθ with theGuinier method:
lnKc
R
� �¼ ln
1
Mw exp -1 = 3Rg2q2
n o þ 2A2c
0B@
1CA
when Rg is known, form factor can be determined by using thespecific equations for different shapes.16,17
For spherical aggregates
Pθ u ¼ qRs ¼ffiffiffi5
3
rqRg
!¼ 3
u3ðsinðuÞ-u cosðuÞ
� �2
For monodisperse coils
Pθðu ¼ qRgÞ ¼ 2
u4expð-u2Þ-1 þ u2
� �2
Atomic force microscopy (PSIA Corporation, XE-100E) mea-surements in the noncontact mode were also performed to imagethe aggregates. An aqueous solution of C12E23N
+SO3-F8 was
dropped and spread on a previously cleaned glass substrate andthe solution on the glass was frozen. Then the frozen solutionon the glass substrate was lyophilized for at least 24 h to removethe water without deforming the aggregates.13 The measure-ments were done by using 910M-NCHR type silicon cantileverswith a resonance frequency of 303.65 kHz and 1 Hz scanningspeed.
To measure the solubilization capacity for hydrocarbon-basedwater-insoluble dye (orange OT), first an excess amount oforange OT was dissolved in ethanol and ethanol was evaporatedto minimize the suspended portion of orange OT. After aqueoussurfactant solutions were added to dye, the resulting solutionswere kept at 25 �C for 17 h. Determination of fluorocarbon-based water-insoluble material solubilizing capacity was done byusing 1H,1H,2H,2H-tridecafluor-1-octyliodid (FCI). FCI wasadded to aqueous surfactant solutions in excess and thesesolutions were kept at 25 �C for 17 h. In simultaneous solubiliza-tion experiments, FCI was added to orange OT after the eva-poration of ethanol. Then surfactant solutions were added tothe orange OT/FCI mixture to prevent competition. Solubilizedamounts were determined by UV-vis spectrometer (LabomedSpectro Double&Auto Cell UV-vis spectrophotometer). Allof the water insolubles separated with mild centrifugation toremove suspended dye particles.
Results and Discussion
Molecular Design of the New Ion-Pair Hydrocarbon-Fluorocarbon Hybrid Surfactant. The new molecule designed(CH3(CH2)11(OCH2CH2)23N
+(C2H5)3SO3-(CF2)7CF3 (C12E23-
N+SO3-F8)) in this study has several structural features which
separate it from the previous molecules which are used in theformation of multicompartment micelles (see Figure 1). FHUB((11- hydroxundecyl)tridecafluorooctane diethylammonium io-dide), the first cationic hydrocarbon-fluorocarbon hybrid mole-cule presented in the literature, showed that when hydrocarbonand fluorocarbon chains are attached together at a point, solu-bilization capacity is supposed to be low due to small volumesof the subdomains.18 Similarly, in another study in which Y-shaped polymeric molecules are used, segmented aggregatessuch as hamburger-type micelles, worms, and vesicles wereobtained for which the aggregate shape is dependent upon(11) Li, Y.; Chen, Z.; Tian, J.; Zhou, Y.; Chen, Z.; Liu, Z. J. Fluorine Chem.
2005, 126, 888–891.(12) Li, Y.; Chen, Z.; Tian, J.; Zhou, Y.; Chen, Z. J. Fluorine Chem. 2004, 125,
1077–1080.(13) Lu, C.; Guo, S.; Liu, L.; Zhang, Y.; Li, Z.; Gu, J. J. Polym. Sci., Part B:
chain lengths and volume fractions of blocks. Because of mole-cular structure, there is the possibility of being arranged oneafter another for hydrocarbon and fluorocarbon-based compart-ments in some of these aggregates, especially for segmentedworms resulting in small-volume micellar subdomains.19 Fluor-ocarbons and hydrocarbons tend to demix with each otherbut it has been earlier stated that the free energy of transfer ofa -CH2- from the hydrocarbon to the fluorocarbon phase isapproximately one-third of free energy of transfer of a -CH2-group from alkane to water. This means that the lipophobicityof fluorocarbons is not as strong as the hydrophopicity offluorocarbons and hydrocarbons. Thus, fluorocarbons andhydrocarbons may have partial solubilities in each other undersome circumstances.20 For this reason it comes into questionto minimize the boundary area between fluorocarbon andhydrocarbon domains in the micellar core by maximizing thesubdomain volume. Thus, separating hydrocarbon and fluoro-carbon chains with a flexible spacer in the molecule is thoughtas the appropriate method to provide effective compartmen-talization of the hydrophobic subdomains in the micellarcore. Because of its flexibility, the ability of having differentconfigurations in aqueous medium, and biocompatibility, thepolyoxyethylene chain is chosen as the spacer between fluoro-carbon and hydrocarbon chains. Micelle size and the solubiliza-tion capacity increases with the increase of hydrophobic chainlength.21,22 Therefore, hydrocarbon and fluorocarbon chainlengths must be long enough. Increasing chain lengths of thehydrophobic tails causes the hydrophobicity of the moleculeto increase but a surfactant molecule must have enoughhydrophilicity to form micelles in aqueous solutions. In thecase of providing the hydrophilicity only by the PEO chain,there would be the risk of separate hydrocarbon and fluoro-carbon based aggregates existing instead of compartmentaliza-tion in a micelle due to the excessive length of the chain. Toprevent this situation it was decided to incorporate hydrophilicionic groups in the molecule. This molecule is decided to be inion-pair structure to benefit from the electrostatic attractionsince having a low CMC value and large aggregate sizes areof great importance for solubilization applications. At thesame time it is thought that holding the ionic groups togetherby ionic forces instead of covalent bonds facilitates effectivecompartmentalization by bringing in extra flexibility and it isanticipated to prevent formation of individual hydrocarbonand fluorocarbon-based aggregates.
Interfacial Properties. The change of interfacial tensionwith concentration for the C12E23N
+SO3-F8 molecule is
shown in Figure 2. The minimum equilibrium surface tensionand CMC values of C12E23N
+SO3-F8 at 20
oC are determined as29mN/mand0.0125mM, respectively. It is obvious fromFigure 2that there is only one break representing the presence ofCMC that could be important for ion-pair surfactants. In thesituation of no strong binding between counter parts of themolecule, both parts are in the surfactant structure and aresupposed to have different CMCs. Thus, the existence of onlyone break point in the graph of interfacial tension versussurfactant concentration means that there is only one CMCvalue and the molecule and its counterion are bound inaqueous solution, which is an expected result according toprevious studies revealing high binding degree for quaternaryammonium containing surfactants and fluorocarbon-basedcounterions. It is also previously stated that the increase inthe chain length and thus the hydrophobicity of the fluorocar-bon-based counterion increases its binding degree.23 In anotherstudy, the counterion binding degree for dodecylammoniumpentafluoropropionate is found as 98-100%.24 Thus, our fluor-ocarbon-based counterion, which has a fairly longer chainlength than pentafluoropropionate, is expected to possess ahigh binding degree.
The interfacial properties of C12E23N+SO3
-F8 are given inTable 1 in comparison with other types of surfactants whichhave structural similarities to C12E23N
+SO3-F8. Solubilization
by surface active agents starts at concentrations higher thanCMC. For this reason, it is preferred for a surfactant moleculethat is designed to be used for solubilization to have a low CMCvalue. As an advantageous property, the C12E23N
+SO3-F8
molecule has the lowest CMC value among different anionic,cationic, and nonionic surfactants given in Table 1. The newhybrid molecule consists of a polyoxyethylene (POE) chainand it is well-known in literature that as the POE chainlength increases the minimum area occupied by a surfactantmolecule at the air-water interface and thus the minimuminterfacial tension increases.25 The minimum area per moleculeof C12E23N
+SO3-F8 is calculated from the Gibbs adsorption
equation as 87 A2/molecule parallel to our expectations.24 ThePOE chain may have different conformations at the air-waterinterface. That is why predicting the arrangement of POE is
Figure 2. Change of interfacial tension with C12E23N+SO3
-F8
concentration at 20 �C.
Figure 1. Molecular structure of newly designed fluorocarbon-hydrocarbon hybrid ion-pair surfactant CH3(CH2)11(OCH2-CH2)23N
+(C2H5)3SO3-(CF2)7CF3 (C12E23N
+SO3-F8).
(19) Li, Z.; Hillmyer, M. A.; Lodge, T. P. Langmuir 2006, 22, 9409–9417.(20) Binks, B. P.; Fletcher, P. D. I.; Kotsev, S. N.; Thompson, R. L. Langmuir
1997, 13, 6669–6682.(21) Myers, D. Surfactant Science and Technology; VCH: New York, 1988.(22) Elworthy, P. H.; Florence, A. T.; Macfarlane, C. B. Solubilization By
Surface-Active Agents And Its Applications In Chemistry And The BiologicalSciences; Chapman and Hall: London, UK, 1968.
(23) Yoshida, N.; Matsuoka, K.; Moroi, Y. J. Colloid Interface Sci. 1997, 187,388–395.
(24) Sugihara, G.; Era, Y.; Funatsu, M.; Kunitake, T.; Lee, S.; Sasaki, Y. J.Colloid Interface Sci. 1997, 187, 435–442.
a complex discussion.26 However, by considering the area permolecule value for C12E23N
+SO3-F8 it is believed to be possi-
ble to claim that the POE chain may take a more horizontalposition rather than a vertical one at the interface because ofthe electrostatic attraction between quaternary ammoniumand sulfonate groups.27 Nevertheless, despite having a largearea C12E23N
+SO3-F8 is quite effective in reducing interfacial
tension due to the synergistic effect of its hydrocarbon-fluor-ocarbon hybrid and ion-pair structure. DTAB (dodecyltrimethy-lammonium bromide) and HTAB (11-hydroxyundecyl)trimethylammonium bromide) are cationic and SOS (sodiumoleicsulfonate) and SDS (sodium dodecyl sulfate) are anionicwell-known hydrocarbon-based single-tailed surfactants. CMCvalues of these surfactants are much higher than that ofC12E23N
+SO3-F8 .25,28,29 Hydrocarbon-fluorocarbon hybrid
surfactants have higher surface activity and lower CMC valuesthan that of classical hydrocarbon-based surfactants.30 Thehybrid structure of C12E23N
+SO3-F8 is one of the reasons for
it to have lower CMC and limiting surface tension values thanthat of many classical hydrocarbon-based surfactants. However,C12E23N
+SO3-F8 also have higher surface activity and lower
CMC value than FHUB, which is a hydrocarbon-fluorocarbonhybrid, cationic, bolaform surfactant.18 The cationic and bola-form character of FHUB causes configurational and electrostaticconstraints for arrangement of the molecules in micellar form orat the air-water interface. The CMC value of C12E23N
+SO3-F8
is much smaller than that of most ionic hybrid surfactants notonly than that of FHUB.30 Fluorocarbon-based surfactants areknown to have lowCMCand limiting surface tension due to theirhigh hydrophobicity.8 L, B, Y, and PFOS (tetraethylammoniumperfluorooctane sulfonate) have interfacial properties appropri-ate to this expectation.31,32 However, the CMC value ofC12E23N
+SO3-F8 is smaller even than those of fluorocarbon-
based surfactants. The ion-pair nature of C12E23N+SO3
-F8
increases its propensity to form micelles due to electrostaticattractions between charged groups. Having a considerablyhigher CMC value than C12E23N
+SO3-F8 for PFOS, which
has a high structural similarity with the fluorocarbon-basedcounterion, is also another indicator for the synergistic effect ofthe ion-pair nature of C12E23N
+SO3-F8. The tailored structure
of this novel surfactant gives us the opportunity of using
balance of forces to obtain low CMC and good surface activityat the interface. In light of the findings related to the interfacialbehavior of C12E23N
+SO3-F8 such as starting to form aggre-
gates at a quite low concentration and being bound with itscounterion in solution, it is decided that C12E23N
+SO3-F8
is worth being investigated with regard to aggregation andsolubilization properties.Aggregation Properties. The size and the shape of aggre-
gates are important for systems where the high solubilizationcapacity is the target property. Packing parameter, which isdefined as p= vo/aelo, is used for predicting the type of aggregatesformed by a surfactant.33 An increase in the packing parametervalue reveals formation of aggregates with lower curvature suchas cylinders, bilayers, or vesicles while smaller packing parametervalues are indicative of aggregates with higher curvature such asglobular micelles.33 For ion-pair surfactants, because of electro-static attraction, headgroup area gets smaller and this indicatesthat ion-pair surfactants have a higher tendency to form bilayeror vesicle-type aggregates instead of globular micelles.34 Thereare several studies supporting this hypothesis.35,36 It is also knownthat hydrocarbon-based surfactants generally form globularor cylindrical micelles and fluorocarbon-based surfactants gen-erally form aggregates of lower curvature such as vesiclesand rods because of high hydrophobicity and rigidity of fluor-ocarbons.37 Aggregation behaviors of hybrid surfactants liebetween those of hydrocarbon and fluorocarbon-based surfac-tants and sinceC12E23N
+SO3-F8 is both an ion-pair and a hybrid
surfactant, it can be thought to form aggregates with lowcurvature. Moreover, C12E23N
+SO3-F8 contains a PEO chain
with 23 EO groups which cannot be classified as a shortchain. The existence of the PEO chain causes an increase in theheadgroup area and a decrease in the value of the packingparameter.
Panels a and b of Figure 3 show the DLS measurements of2 and 5 mM aqueous C12E23N
+SO3-F8 solutions at the angle
of 90�, respectively. Two different sized aggregates with hydro-dynamic radius of 2 ( 1 nm and 100 ( 50 nm exist as a mixturein solution. However, F8C2SO3
-Na+, which is structurallysimilar to the fluorocarbon-based counterion of C12E23N
+-SO3
-F8, forms aggregates whose hydrodynamic radius valuesare 50 and 300 nm. This difference between the aggregate sizes ofC12E23N
+SO3-F8 andF8C2SO3
-Na+ is evidence for the absenceof separate fluorocarbon-based aggregates in C12E23N
+SO3-F8
Table 1. Comparison of Interfacial Properties of C12E23N+SO3
-F8 with Those of Other Surfactants
surfactant CMC (mM) γlim (mN/m) Alim (A�2/molecule)
solutions in addition to having only one CMC. In the AFMimage presented (Figure 3c), larger aggregates with a diameterof almost 150 nm and smaller aggregates at various sizes areshown and these sizes which are obtained from AFM imagesare coherent with the light scattering results. Formation oflarge sized aggregates is a parallel result to our expectations.The aggregation and solubilization properties of C12E23N
+-SO3
-F8 are examined at a concentration range of 0.1-5 mM.As can be seen from Figure 3 hydrodynamic radius values fortwo different-sized micelles do not change significantly withconcentration in this range of concentration. From shape analysisof the micelles carried out by static light scattering using theGuinier method, it is proposed that spherical micelles with2 nm and coil type micelles with 100 nm radii coexist in thesolution for all concentrations studied. From AFM measure-ments, it is seen that smaller aggregates are globular in shapeand larger aggregates have a semicoil structure or have thetendency of bending. For 2 mM C12E23N
+SO3-F8 solutions
the radius of gyration is calculated as 116.9 nm at 25 �Cfrom static light scattering studies. The shape analysis ofC12E23N
+SO3-F8 for the case solubilization of fluorocarbon
and hydrocarbon probes has been performed as indicated below.Solubilization Properties. Simultaneous and individual so-
lubilization capacities of C12E23N+SO3
-F8 for water-insolublehydrocarbon and fluorocarbon-based materials are determinedby UV-visible spectroscopy. First, individual solubilizationcapacities for orange OT (hydrocarbon-based solubilizate) and
FCI (CF3(CF2)5CH2CH2I) (fluorocarbon-based solubilizate) areinvestigated. Orange OT is chosen for its structural similarity tosome commonly used water-insoluble drugs, ease of determina-tion by UV-visible spectroscopy due to its characteristic absor-bance peak at a distinct wavelength, and opportunity to makecomparisons with previous studies since it is a widely usedsolubilizate. Reasons for using FCI to examine fluorocarbonsolubilization capacity include the following: having a linearfluorocarbon chain which is shorter than that of C12E23N
+-SO3
-F8 to make incorporation of solubilizate in fluorocarbon-rich domain probable, its high fluorocarbon content, and itscharacteristic absorbance peak at a distinct wavelength in theUV-visible spectrum.
Solubilization capacities for orangeOTandFCI byC12E23N+-
SO3-F8 in the concentration range of 0.1-5 mM are given in
Table 2 in comparison with different classical single tailedsurfactants such as DTAB (CH3(CH2)11N
+(CH3)3Br-), CTAB
(CH3(CH2)15N+(CH3)3Br
-), and CpyC (C21H38ClN 3H2O).38
It is seen that orange OT solubilization capacities of DTABand C12E23N
+SO3-F8 in terms of mol (solubilized orange OT)/
mol (micellized surfactant) are nearly the same and have anapproximate value of 0.011, which is considered a constantvalue. Solubilization capacities of DTAB solutions in waterand in 100 mM NaBr are identical; the only difference is inCMC values. It is also seen from Table 2 that the amounts of
Figure 3. Size analysis of C12E23N+SO3
-F8 aggregates obtained fromDLS, SLS (static light scattering) and AFMmeasurements: (a) DLSanalysis of 2mMaqueous solution. (b)DLS analysis of 5mMaqueous solution. (c)AFMerror image for 2mMaqueous solution. Lengths ofthe aggregates 1, 2, and 3 are 200, 146, and 80nmwhile the diameters are 146, 75, and 35nm, respectively. (d) Shape analysis of 2mMaqueoussolution.
(38) Schott, H. J. Phys. Chem. 1967, 71, 3611–3617.
orange OT solubilized by 1 mol of micellized CTAB or CpyC,which have the same hydrocarbon chain length, are close to eachother and higher than those of DTAB and C12E23N
+SO3-F8.
Similarly, the solubilized amounts of orange OT by 1 mol ofsurfactant for 0.01 N aqueous sodium decyl sulfate, sodiumdodecyl sulfate, and sodium tetradecyl sulfate solutions are0.00023, 0.0028, and 0.026, respectively.39 This case states thathydrocarbon chain length is quite effective in solubilization ofhydrocarbon-based materials and solubilization capacity in-creases with increased chain length. Since the orange OT solubi-lization capacity of C12E23N
+SO3-F8 is almost the same as that
of DTAB, which has the same length hydrocarbon chain andhigher than most hydrocarbon-based classical anionic surfac-tants, it can be stated that C12E23N
+SO3-F8 is efficient in
hydrocarbon solubilization and effective compartmentalizationwithin the micellar core is achieved.
The amounts of solubilized FCI by C12E23N+SO3
-F8 atthe same surfactant concentrations are approximately 15 timeshigher than that of orange OT. FCI solubilization in termsof (moles of solubilized FCI)/(moles of micellized surfactant)is found as 0.23, which is also constant at the range of surfactantconcentration studied. It is observed in previous studies utilizinghybrid surfactants that fluorocarbon-based materials have ahigher tendency to be solubilized in the micelles than hydrocar-bon-based ones and the tendency increases with increased fluor-ocarbon content of the surfactant. The solubilized amountof perfluorobenzene by 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-hexane sulfonate is found as four times that of 2-naphthol.40
The difference between the solubilized amounts of hydrocarbon
and fluorocarbon-based solubilizates is higher for C12E23N+-
SO3-F8 owing to the structural difference between orange OT
and FCI in addition to the trend of high solubilization abilityof fluorocarbons in micelles as seen in the literature. The cyclicstructure of orange OT is believed to make incorporation of itinto the micellar core different. It is also seen from the tablethat 15 mM DTAB cannot solubilize FCI and on the otherhand 25 mM DTAB solution can solubilize FCI slightly. Thisis because the lipophobicity of fluorocarbons is not as strongas their hydrophobicity. Fluorocarbon-based single-tailed anio-nic F8C2SO3
-Na+ (CF3(CF2)7(CH2)2SO3-Na+) surfactant is the
most effective surfactant given in Table 2 for FCI solubilizationbut it is unable to solubilize orange OT. DTAB is effectivein solubilization of hydrocarbon-based materials and micellesof F8C2SO3
-Na+ can solubilize only fluorocarbon-based materialwhile the micelles of C12E23N
+SO3-F8 are effective for solubiliza-
tion of both hydrocarbon and fluorocarbon-based materials.Further investigations were made for determination of simul-
taneous solubilization capacity. Figure 4 shows the relation
Table 2. Individual Solubilization Capacities for Orange OT and FCI
In Comparison with Some Other Surfactants (Molar Absorptivity
Values for Orange OT and FCI are 11655 and 353.3 L/(mol 3 cm),
Figure 4. Change of separately and simultaneously solubilizedamounts of orange OT and FCI with C12E23N
+SO3-F8 concen-
tration: (a) for only orange OT solubilization; (b) for only FCIsolubilization; and (c) for simultaneous solubilization of orangeOT and FCI (0, FCI;[, orange OT).
(39) Merrill, R. C.; McBain, J. W. Eighteenth Colloid Symposium, New York,1941.(40) Saeki, A.; Sakai, H.; Kamogawa, K.; Kondo, Y.; Yoshino, N.; Uchiyama,
H.; Harwell, J. H.; Abe, M. Langmuir 2000, 16, 9991–9995.
between solubilized amounts of orange OT and FCI separately,and simultaneously.
It is seen from Figure 4 that solubilized amounts of solubili-zates change linearly with surfactant concentration. The reasonfor this linearity could be the indication of changelessness ofaggregate properties in the concentration range studied. In thecase of cosolubilization of orange OT and FCI, solubilizedamounts are found to be almost equal to 90% of separatelysolubilized amounts for both orange OT and FCI, which demon-strates that C12E23N
+SO3-F8 is able to solubilize hydrocarbon
and fluorocarbon-basedmaterials effectively both simultaneouslyand separately. However, there are some difficulties in determin-ing solubilized FCI concentration at low surfactant concentra-tions due to the low molar absorptivity value of FCI and thecontribution of orange OT to the absorbance value, which isutilized for FCI quantification. This is the reason for the ine-quality of simultaneously solubilized FCI amounts per 1 mol ofmicellized surfactant.
The small decrease in the case of cosolubilization is believed tobe due to the exclusion of orange OT from fluorocarbon-basedsubdomain, and FCI from hydrocarbon-based subdomain owingto the competition between orangeOT andFCI.A previous studyin which a triblock amphiphilic copolymer is used for solubiliza-tion of pyrene and 1-naphtyl perfluoroheptanyl ketone (NFH)supports our findings. In that study the difference betweenseparately and simultaneously solubilized amounts of pyrene isignorable butNFHdecreases to 63%of its separate value.5Whenperfluorobenzene by 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2-hexane sulfonate is utilized for adsolubilization of 2-naphtholand perfluorobenzene, cosolubilized amounts of 2-naphthol andperfluorobezene are one-half and one-fifth of separately adsolu-bilized amounts. In those studies, the decrease in simultaneoussolubilization is much more than that of C12E23N
+SO3-F8,
causing the suggestion that hydrocarbon- and fluorocarbon-based solubilization sites overlap each other in the micellar core.
Finally, the effect of solubilization on the aggregate size andshape is investigated since it is known that material solubilizationmay change aggregate properties. Shape analysis for the cases ofseparate and cosolubilizations of orange OT and FCI are shownin Figure 5. It is suggested from the shape analysis that globularand coil-type aggregates coexist as amixture in all of the solutionsgiven in Figure 5.When orangeOT is solubilized, the shape of themicelles becomes closer to sphere althoughFCI solubilization andcosolubilization of orange OT and FCI do not make anyobservable change in shape analysis. However, in any case,material solubilization causes an increase in radius of the gyrationvalue, which is highest for orangeOT solubilization. In the case ofOT solubilization, the hydrodynamic radius values also changeto 2.5 ( 1 nm and 150 ( 50 nm. Observation of size change inboth sized micelles is thought to indicate that orange OT issolubilized by both of them. Together with this result, beingunable to solubilize orange OT for F8C2SO3
-Na+ strengthensthe conclusion that there are no separate fluorocarbon-richand hydrocarbon-rich micelles in C12E23N
+SO3-F8 solutions
meaning all C12E23N+SO3
-F8 micelles should have multicom-partments. The effect of material solubilization on aggregateproperties depends on the locus of solubilization. When anapolar material is solubilized inside the micellar core vo and thuspacking parameter values increase indicating formation ofmore asymmetric aggregates of lower curvature, and whensolubilizate is solubilized in the palisade layer it causes theae value to increase and the packing parameter value to decreasewhich is an indicator of higher curvature.28 Considering thisinformation it is suggested that orange OT might be solubilized
at a higher extent in the palisade layer because of its heteroatomsuch as the hydroxyl group. Linear fluoroalkyl chain solubiliza-tion may cause the lengthening of aggregates.2 The Rg/Rh ratiois also used for evaluation of aggregate geometry.41 Aggregatesthat are more asymmetric are thought to be formed with theincrease in Rg/Rh ratio. In our study FCI solubilization doesnot alter the hydrodynamic radius of aggregates but causes
Figure 5. Effect of solubilization on aggregate properties of 2mMC12E23N
+SO3-F8 at 25
oC: (a) 2mMC12E23N+SO3
-F8+orangeOT; (b) 2mMC12E23N
+SO3-F8+FCI; and (c) 2mMC12E23N
+-SO3
-F8 + orange OT+ FCI.
(41) Brown, W. Y. N. Light Scattering Principles and Development; ClarendonPres: Oxford, UK, 1996.
an increase in radius of gyration value resulting in an increase inRg/Rh value from 1.2 to 1.5 (longer aggregates). Therefore, itis claimed that FCI is solubilized inside the micellar core andmore asymmetric aggregates are formed expectedly. Aggregateproperties obtained by using only 2 mM C12E23N
+SO3-F8
are given in Figure 5 but general tendencies are common forall surfactant concentrations studied.
Conclusion
We conclude that it is possible to improve the compartmenta-lization inside micelles by a special molecular design in whichhydrocarbon and fluorocarbon chains are provided to gainenough flexibility utilizing balance of forces. C12E23N
+SO3-F8,
which is the first multicompartment micelle forming ion-pairmolecule in the literature, has a high solubilization capacity for
hydrocarbon- and fluorocarbon-based materials both separatelyand simultaneously. Properties of having a quite low CMC valueand producing stable micelles along with the high solubilizationcapacity make us believe that this singular C12E23N
+SO3-F8
molecule have a great potential for use in various applications.
Acknowledgment. This work is supported by L’Oreal Turkey“Women in Science” program, Turkish Academy of Science“Distinguished Young Scientist” program, and Hacettepe Uni-versity research project (grant no. 0601602012).
Supporting InformationAvailable: TEM images ofC12E23-N+SO3
-F8 aggregates and DLS analysis of F8C2SO3-Na+
solution. This material is available free of charge via theInternet at http://pubs.acs.org.
