A New Strategy To Form Multicompartment Micelles: Fluorocarbon

Jun 9, 2009 - (1-5) Multicompartment micelles are aggregates of surfactants, which ..... of C12E23N+SO3−F8 are examined at a concentration range of ...
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A New Strategy To Form Multicompartment Micelles: Fluorocarbon-Hydrocarbon Ion-Pair Surfactant Hande Unsal and Nihal Aydogan* 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 distinct subdomains which make them different from the classical micelles. Owing to these properties multicompartment micelles have a great potential to be used as solubilization agents and carriers for a wide variety of applications where it is important to prevent the uncontrolled interaction of the solubilizates before reaching the target and to convey them to the specified point simultaneously. Here we show that effective compartmentalization inside the micelle and high solubilization capacity for the two immiscible water-insoluble materials in cases of both simultaneous and separate solubilization can be achieved by newly designed ion-pair hybrid surfactant CH3(CH2)11(OCH2CH2)23N+(C2H5)3SO3-(CF2)7CF3 (C12E23N+SO3-F8) through the agency of favorable molecular design. Molecular structure is tailored by the approach of using a balance of forces to obtain compartmentalization, which is without precedent. This new molecule also has the properties of quite low critical micelle concentration and an extensive surfactant concentration range for solubilization which are additional important advantageous features.

Introduction It is of great importance to transport several compounds to the same place at a desired proportion simultaneously for a wide variety of processes. As a result, the studies in the field of multicompartment micelles have been accelerated recently.1-5 Multicompartment micelles are aggregates of surfactants, which are composed of a hydrophilic shell and a multidomain hydrophobic core with each domain having different properties.6 Existence of different subdomains in a micellar core makes it possible to cosolubilize and transport several different and immiscible materials in different subdomains selectively, preventing any undesired interactions before reaching the target. Because of this property, multicompartment micelles have high potential to 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 to make multicompartment micelles hydrocarbon-fluorocarbon hybrid systems were used. Fluorocarbons have unique properties such as extraordinary thermal, chemical and biological inertness, low surface tension, high hydrophobicity, high fluidity, high rigidity, and high gas dissolving capacities caused by their strong intramolecular and weak intermolecular interactions.7 These characteristics of fluorocarbons increase the interest for utilization of them in recent researches especially in biomedicine.8,9 For the scope of multicompartment micelles, high tendency *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 2005, 21, 7214–7219. (3) Weberskirch, R.; Preuschen, J.; Spiess, H. W.; Nuyken, O. Macromol. Chem. Phys. 2000, 201, 995–1007. (4) Kotzev, A.; Laschewsky, A.; Adriaensens, P.; Gelan, J. Macromolecules 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.

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to self-aggregate and lipophobicity along with hydrophobicity are the key features of fluorocarbons. So, hydrocarbon-fluorocarbon hybrid systems are considered as appropriate structures for preparing multicompartment micelles. In this study, the novel hydrocarbon-fluorocarbon ionpair hybrid surfactant CH3(CH2)11(OCH2CH2)23N+(C2H5)3SO3-(CF2)7CF3 (C12E23N+SO3-F8), which is thought to be able to form multicompartment micelles with high solubilization capacity, is designed and the aggregation and the solubilization properties of this new molecule have been investigated with the hope of introducing a new approach to multicompartment micelles. To our knowledge, this is the first time in literature of utilization of ion-pair amphiphiles in order to obtain multicompartment micelles although they have been used and studied for a long while. We aimed to reduce some problems faced in previous studies related to multicompartment micelles such as low content of hydrophobic groups in the aggregate, absence of common interface between two distinct hydrophobic cores, low solubilization capacity, and overlapping of hydrophobic subdomains which is critical for cosolubilization by making the appropriate molecular design. This new molecule has gratifying results such as quite low CMC (critical micelle concentration) and superior surface activity, high solubilization capacities for hydrocarbon-based and fluorocarbonbased probes both separately and simultaneously, which leads to the deduction of achieving efficient compartmentalization inside 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 (10) Aydogan, N.; Abbott, N. L. Langmuir 2002, 18, 7826–7830.

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surfactant C12E23N+SO3-F8 [CH3(CH2)11(OCH2CH2)23N+(C2H5)3SO3-(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-1octanesulfonyl fluoride (0.005 mol) were reacted in ether with vigorous stirring at 40 °C for 24 h. Ether was removed from the mixture 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 1 H NMR (400 MHz, CDCl3) δ 3.3-3.8 (92H, OCH2CH2), 3.3 (2H, N+CH2CH2O), 3.2 (6H, N+CH2CH3), 1.6 (2H, OCH2CH2CH2), 1.4 (9H, N+CH2CH3), 1.2 (18H, CH2), 0.9 (3H, CH3); 13 C NMR (CDCl3) δ (ppm) 7.5 (N+CH2CH3), 14 (CH3), 22.631.8 (CH2), 53.9-56.8 (N+CH2CH3), 60.16 (N+CH2CH2O), 70-72.6 (OCH2CH2), 107.8-113.9 (CF2), 118.5 (CF3), 114.3 (CF2CF2SO3); LC-MS (-) (m/z 499.1, SO3-(CF2)7CF3). Aqueous surfactant solutions were prepared fresh for each experiment with ultrapure water (18.3 mΩ 3 cm). The equilibrium surface 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 by simultaneous dynamic and static light scattering measurements, using CGS-3 with a 632.8 nm laser (Malvern, UK). The ultrapure water used in the preparation of aqueous solutions filtered through a 0.22 μm filter for the removal of dust and the solutions were filtered again through a 0.45 μm cellulose acetate filter before measurement. The measurements were performed at the angles of 30-150° with 10° increment. The dn/dc value (0.089 mL/g) of the new ion-pair hybrid surfactant was determined with BI-DNDC (Brookhaven Inst., USA) at 25 °C. In the dynamic light scattering (DLS) method, the measured intensity time correlation function is used to obtain translational diffusion coefficient, D, by utilizing scattering vector q, where q = (4πn/λ) sin(θ/2). Here n is the refractive index of the solvent, λ is the wavelength of the light, and θ is the scattering angle. When the translational diffusion coefficient is known the hydrodynamic radius, Rh, can be obtained from the Stokes-Einstein equation kB T Rh ¼ 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 from static light scattering experiments and are used to evaluate the shape of the aggregates. The opportunity for determining the particle shape is provided by the angular dependence of scattering intensity. Scattering intensity is expressed as14 Rθ ¼ KcMw Pθ Sθ where Pθ and Sθ are form and structure factors, respectively, and K is the optical constant, which is   4π2 n2 dn 2 K ¼ NA λ4 dc Here n is solvent refractive index, λ is incident light wavelength, NA is Avogadro’s number, and (dn/dc) is the refractive index increment.15 Molecular weight, radius of gyration, Rg, and second (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: Polym. Phys. 2006, 44, 3406–3417. (14) Zhou, S.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackler, U. E.; Isobe, H.; Nakamura, E. Science 2001, 291, 1944–1947.

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viral coefficient, A2, are calculated by using Kc/Rθ with the Guinier method: 0 1   Kc 1 B n o þ 2A2 cC ¼ ln@ ln A 1 2 2 R Mw exp - = Rg q 3

when Rg is known, form factor can be determined by using the specific equations for different shapes.16,17 For spherical aggregates ! rffiffiffi  2 5 3 Pθ u ¼ qRs ¼ ðsinðuÞ - u cosðuÞ qRg ¼ 3 u3 For monodisperse coils Pθ ðu ¼ qRg Þ ¼



2 expð -u2 Þ - 1 þ u2 u4

2

Atomic force microscopy (PSIA Corporation, XE-100E) measurements in the noncontact mode were also performed to image the aggregates. An aqueous solution of C12E23N+SO3-F8 was dropped and spread on a previously cleaned glass substrate and the solution on the glass was frozen. Then the frozen solution on the glass substrate was lyophilized for at least 24 h to remove the water without deforming the aggregates.13 The measurements were done by using 910M-NCHR type silicon cantilevers with a resonance frequency of 303.65 kHz and 1 Hz scanning speed. To measure the solubilization capacity for hydrocarbon-based water-insoluble dye (orange OT), first an excess amount of orange OT was dissolved in ethanol and ethanol was evaporated to minimize the suspended portion of orange OT. After aqueous surfactant solutions were added to dye, the resulting solutions were kept at 25 °C for 17 h. Determination of fluorocarbonbased water-insoluble material solubilizing capacity was done by using 1H,1H,2H,2H-tridecafluor-1-octyliodid (FCI). FCI was added to aqueous surfactant solutions in excess and these solutions were kept at 25 °C for 17 h. In simultaneous solubilization experiments, FCI was added to orange OT after the evaporation of ethanol. Then surfactant solutions were added to the orange OT/FCI mixture to prevent competition. Solubilized amounts were determined by UV-vis spectrometer (Labomed Spectro Double&Auto Cell UV-vis spectrophotometer). All of the water insolubles separated with mild centrifugation to remove suspended dye particles.

Results and Discussion Molecular Design of the New Ion-Pair HydrocarbonFluorocarbon Hybrid Surfactant. The new molecule designed (CH3(CH2)11(OCH2CH2)23N+(C2H5)3SO3-(CF2)7CF3 (C12E23N+SO3-F8)) in this study has several structural features which separate it from the previous molecules which are used in the formation of multicompartment micelles (see Figure 1). FHUB ((11- hydroxundecyl)tridecafluorooctane diethylammonium iodide), the first cationic hydrocarbon-fluorocarbon hybrid molecule presented in the literature, showed that when hydrocarbon and fluorocarbon chains are attached together at a point, solubilization capacity is supposed to be low due to small volumes of the subdomains.18 Similarly, in another study in which Yshaped polymeric molecules are used, segmented aggregates such as hamburger-type micelles, worms, and vesicles were obtained for which the aggregate shape is dependent upon (15) Aydogan, N.; Aldis, N. Langmuir 2006, 22, 2028–2033. (16) Kazuo Onuma, K.; Kanzaki, N. J. Cryst. Growth 2005, 284, 530–537. (17) Gilanyi, T.; Varga, I.; Meszaros, R.; Filipcsei, G.; Zrinyi, M. Phys. Chem. Chem. Phys. 2000, 2, 1973–1977. (18) Aydogan, N.; Aldis, N.; Guvenir, O. Langmuir 2003, 19, 10726–10731.

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Figure 1. Molecular structure of newly designed fluorocarbonhydrocarbon hybrid ion-pair surfactant CH3(CH2)11(OCH2CH2)23N+(C2H5)3SO3-(CF2)7CF3 (C12E23N+SO3-F8).

chain lengths and volume fractions of blocks. Because of molecular structure, there is the possibility of being arranged one after another for hydrocarbon and fluorocarbon-based compartments in some of these aggregates, especially for segmented worms resulting in small-volume micellar subdomains.19 Fluorocarbons and hydrocarbons tend to demix with each other but it has been earlier stated that the free energy of transfer of a -CH2- from the hydrocarbon to the fluorocarbon phase is approximately one-third of free energy of transfer of a -CH2group from alkane to water. This means that the lipophobicity of fluorocarbons is not as strong as the hydrophopicity of fluorocarbons and hydrocarbons. Thus, fluorocarbons and hydrocarbons may have partial solubilities in each other under some circumstances.20 For this reason it comes into question to minimize the boundary area between fluorocarbon and hydrocarbon domains in the micellar core by maximizing the subdomain volume. Thus, separating hydrocarbon and fluorocarbon chains with a flexible spacer in the molecule is thought as the appropriate method to provide effective compartmentalization of the hydrophobic subdomains in the micellar core. Because of its flexibility, the ability of having different configurations in aqueous medium, and biocompatibility, the polyoxyethylene chain is chosen as the spacer between fluorocarbon and hydrocarbon chains. Micelle size and the solubilization capacity increases with the increase of hydrophobic chain length.21,22 Therefore, hydrocarbon and fluorocarbon chain lengths must be long enough. Increasing chain lengths of the hydrophobic tails causes the hydrophobicity of the molecule to increase but a surfactant molecule must have enough hydrophilicity to form micelles in aqueous solutions. In the case of providing the hydrophilicity only by the PEO chain, there would be the risk of separate hydrocarbon and fluorocarbon based aggregates existing instead of compartmentalization in a micelle due to the excessive length of the chain. To prevent this situation it was decided to incorporate hydrophilic ionic groups in the molecule. This molecule is decided to be in ion-pair structure to benefit from the electrostatic attraction since having a low CMC value and large aggregate sizes are of great importance for solubilization applications. At the same time it is thought that holding the ionic groups together by ionic forces instead of covalent bonds facilitates effective compartmentalization by bringing in extra flexibility and it is anticipated to prevent formation of individual hydrocarbon and fluorocarbon-based aggregates. (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 Biological Sciences; Chapman and Hall: London, UK, 1968.

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Figure 2. Change of interfacial tension with C12E23N+SO3-F8 concentration at 20 °C.

Interfacial Properties. The change of interfacial tension with concentration for the C12E23N+SO3-F8 molecule is shown in Figure 2. The minimum equilibrium surface tension and CMC values of C12E23N+SO3-F8 at 20 oC are determined as 29 mN/m and 0.0125 mM, respectively. It is obvious from Figure 2 that there is only one break representing the presence of CMC that could be important for ion-pair surfactants. In the situation of no strong binding between counter parts of the molecule, both parts are in the surfactant structure and are supposed to have different CMCs. Thus, the existence of only one break point in the graph of interfacial tension versus surfactant concentration means that there is only one CMC value and the molecule and its counterion are bound in aqueous solution, which is an expected result according to previous studies revealing high binding degree for quaternary ammonium containing surfactants and fluorocarbon-based counterions. It is also previously stated that the increase in the chain length and thus the hydrophobicity of the fluorocarbon-based counterion increases its binding degree.23 In another study, the counterion binding degree for dodecylammonium pentafluoropropionate is found as 98-100%.24 Thus, our fluorocarbon-based counterion, which has a fairly longer chain length than pentafluoropropionate, is expected to possess a high binding degree. The interfacial properties of C12E23N+SO3-F8 are given in Table 1 in comparison with other types of surfactants which have structural similarities to C12E23N+SO3-F8. Solubilization by surface active agents starts at concentrations higher than CMC. For this reason, it is preferred for a surfactant molecule that is designed to be used for solubilization to have a low CMC value. 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 new hybrid molecule consists of a polyoxyethylene (POE) chain and it is well-known in literature that as the POE chain length increases the minimum area occupied by a surfactant molecule at the air-water interface and thus the minimum interfacial tension increases.25 The minimum area per molecule of C12E23N+SO3-F8 is calculated from the Gibbs adsorption equation as 87 A˚2/molecule parallel to our expectations.24 The POE chain may have different conformations at the air-water interface. That is why predicting the arrangement of POE is (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. (25) Wang, X.; Yan, F.; Li, Z.; Zhang, L.; Zhao, S.; An, J.; Yu, J. Colloids Surf., A 2007, 302, 532–539.

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Article 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)

C12E23N+SO3-F8 (water, 20 °C) PFOS (CF3(CF2)7SO3- N+(CH2CH3)4)32 L (CF3(CF2)3CH2O(CH2CH2O)3CH3)31 B (CH3(CH2CH2O)3OCH2(CF2)10CH2O(CH2CH2O)3CH3)31 Y (CF3(CF2)5(CH2)2N[(CH2CH2O)3H]2)31 FHUB (HO(CH2)11N+(CH2CH3)2(CH2)2(CF2)5CF3 I-)17 HTAB (OH (CH2)11N+(CH2CH3)3 Br-)17 DTAB (100 mM LiBr, 40 °C)17 DTAB (CH3(CH2)11N+(CH3)3Br-, water)24 SOS (water, 25 °C)25 SDS (water, 30 °C)23 C12E2S (water, 30 °C)23

0.0125 1.05 2 0.12 0.38 0.45 21 2.5 15 0.46 8.20 2.88

29.0 23.6 24.4 38.7 23.7 25.0 48.0 37.0 41.8 32.7 39.6 41.4

87 ( 5 35 43 25 55 88 ( 5 68 ( 5 41 77 81 53 63

a complex discussion.26 However, by considering the area per molecule value for C12E23N+SO3-F8 it is believed to be possible to claim that the POE chain may take a more horizontal position rather than a vertical one at the interface because of the electrostatic attraction between quaternary ammonium and sulfonate groups.27 Nevertheless, despite having a large area C12E23N+SO3-F8 is quite effective in reducing interfacial tension due to the synergistic effect of its hydrocarbon-fluorocarbon hybrid and ion-pair structure. DTAB (dodecyltrimethylammonium bromide) and HTAB (11-hydroxyundecyl) trimethylammonium bromide) are cationic and SOS (sodium oleicsulfonate) and SDS (sodium dodecyl sulfate) are anionic well-known hydrocarbon-based single-tailed surfactants. CMC values of these surfactants are much higher than that of C12E23N+SO3-F8 .25,28,29 Hydrocarbon-fluorocarbon hybrid surfactants have higher surface activity and lower CMC values than that of classical hydrocarbon-based surfactants.30 The hybrid structure of C12E23N+SO3-F8 is one of the reasons for it to have lower CMC and limiting surface tension values than that 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-fluorocarbon hybrid, cationic, bolaform surfactant.18 The cationic and bolaform character of FHUB causes configurational and electrostatic constraints for arrangement of the molecules in micellar form or at the air-water interface. The CMC value of C12E23N+SO3-F8 is much smaller than that of most ionic hybrid surfactants not only than that of FHUB.30 Fluorocarbon-based surfactants are known to have low CMC and limiting surface tension due to their high hydrophobicity.8 L, B, Y, and PFOS (tetraethylammonium perfluorooctane sulfonate) have interfacial properties appropriate to this expectation.31,32 However, the CMC value of C12E23N+SO3-F8 is smaller even than those of fluorocarbonbased surfactants. The ion-pair nature of C12E23N+SO3-F8 increases its propensity to form micelles due to electrostatic attractions between charged groups. Having a considerably higher CMC value than C12E23N+SO3-F8 for PFOS, which has a high structural similarity with the fluorocarbon-based counterion, is also another indicator for the synergistic effect of the 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 activity at the interface. In light of the findings related to the interfacial behavior of C12E23N+SO3-F8 such as starting to form aggregates at a quite low concentration and being bound with its counterion in solution, it is decided that C12E23N+SO3-F8 is worth being investigated with regard to aggregation and solubilization properties. Aggregation Properties. The size and the shape of aggregates are important for systems where the high solubilization capacity is the target property. Packing parameter, which is defined as p = vo/aelo, is used for predicting the type of aggregates formed by a surfactant.33 An increase in the packing parameter value reveals formation of aggregates with lower curvature such as cylinders, bilayers, or vesicles while smaller packing parameter values are indicative of aggregates with higher curvature such as globular micelles.33 For ion-pair surfactants, because of electrostatic attraction, headgroup area gets smaller and this indicates that ion-pair surfactants have a higher tendency to form bilayer or vesicle-type aggregates instead of globular micelles.34 There are several studies supporting this hypothesis.35,36 It is also known that hydrocarbon-based surfactants generally form globular or cylindrical micelles and fluorocarbon-based surfactants generally form aggregates of lower curvature such as vesicles and rods because of high hydrophobicity and rigidity of fluorocarbons.37 Aggregation behaviors of hybrid surfactants lie between those of hydrocarbon and fluorocarbon-based surfactants and since C12E23N+SO3-F8 is both an ion-pair and a hybrid surfactant, it can be thought to form aggregates with low curvature. Moreover, C12E23N+SO3-F8 contains a PEO chain with 23 EO groups which cannot be classified as a short chain. The existence of the PEO chain causes an increase in the headgroup area and a decrease in the value of the packing parameter. Panels a and b of Figure 3 show the DLS measurements of 2 and 5 mM aqueous C12E23N+SO3-F8 solutions at the angle of 90°, respectively. Two different sized aggregates with hydrodynamic radius of 2 ( 1 nm and 100 ( 50 nm exist as a mixture in solution. However, F8C2SO3-Na+, which is structurally similar to the fluorocarbon-based counterion of C12E23N+SO3-F8, forms aggregates whose hydrodynamic radius values are 50 and 300 nm. This difference between the aggregate sizes of C12E23N+SO3-F8 and F8C2SO3-Na+ is evidence for the absence of separate fluorocarbon-based aggregates in C12E23N+SO3-F8

(26) Tsukanova, V.; Salesse, C. J. Phys. Chem. B 2004, 108, 10754–10764. (27) Colin, A.; Giermanska-Kahn, J.; Langevin, D.; Desbat, B. Langmuir 1997, 13, 2953–2959. (28) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley and Sons: Hoboken, NJ, 2004. (29) Shalaby, M. N. Chem. Eng. Commun. 2007, 194, 464–476. (30) Kondo, Y.; Yoshino, N. Curr. Opin. Colloid Interface Sci. 2005, 10, 88–93. (31) Eastoe, J.; Rogers, S. E.; Martin, L. J.; Paul, A. Langmuir 2006, 22, 2034– 2038. (32) Gente, G.; La Mesa, C.; Muzzalupo, R.; Ranieri, G. A. Langmuir 2000, 16, 7914–7919.

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(33) Nagarajan, R. Langmuir 2002, 18, 31-38. (34) Fukuda, H.; Kawata, K.; Okuda, H. J. Am. Chem. Soc. 1990, 112, 1635– 1637. (35) Bhattacharya, S.; Haldar, J. Colloids Surf., A 2002, 205, 119–126. (36) Bordes, R.; Vedrenne, M.; Coppel, Y.; Franceschi, S.; Perez, E.; RicoLattes, I. ChemPhysChem 2007, 8, 2013–2018. (37) Matsuoka, K.; Moroi, Y. Curr. Opin. Colloid Interface Sci. 2003, 8, 227– 235.

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Figure 3. Size analysis of C12E23N+SO3-F8 aggregates obtained from DLS, SLS (static light scattering) and AFM measurements: (a) DLS analysis of 2 mM aqueous solution. (b) DLS analysis of 5 mM aqueous solution. (c) AFM error image for 2 mM aqueous solution. Lengths of the aggregates 1, 2, and 3 are 200, 146, and 80 nm while the diameters are 146, 75, and 35 nm, respectively. (d) Shape analysis of 2 mM aqueous solution.

solutions in addition to having only one CMC. In the AFM image presented (Figure 3c), larger aggregates with a diameter of almost 150 nm and smaller aggregates at various sizes are shown and these sizes which are obtained from AFM images are coherent with the light scattering results. Formation of large 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 for two different-sized micelles do not change significantly with concentration in this range of concentration. From shape analysis of the micelles carried out by static light scattering using the Guinier method, it is proposed that spherical micelles with 2 nm and coil type micelles with 100 nm radii coexist in the solution for all concentrations studied. From AFM measurements, it is seen that smaller aggregates are globular in shape and larger aggregates have a semicoil structure or have the tendency of bending. For 2 mM C12E23N+SO3-F8 solutions the radius of gyration is calculated as 116.9 nm at 25 °C from static light scattering studies. The shape analysis of C12E23N+SO3-F8 for the case solubilization of fluorocarbon and hydrocarbon probes has been performed as indicated below. Solubilization Properties. Simultaneous and individual solubilization capacities of C12E23N+SO3-F8 for water-insoluble hydrocarbon and fluorocarbon-based materials are determined by UV-visible spectroscopy. First, individual solubilization capacities for orange OT (hydrocarbon-based solubilizate) and 7888 DOI: 10.1021/la900456t

FCI (CF3(CF2)5CH2CH2I) (fluorocarbon-based solubilizate) are investigated. Orange OT is chosen for its structural similarity to some commonly used water-insoluble drugs, ease of determination by UV-visible spectroscopy due to its characteristic absorbance peak at a distinct wavelength, and opportunity to make comparisons with previous studies since it is a widely used solubilizate. Reasons for using FCI to examine fluorocarbon solubilization capacity include the following: having a linear fluorocarbon chain which is shorter than that of C12E23N+SO3-F8 to make incorporation of solubilizate in fluorocarbonrich domain probable, its high fluorocarbon content, and its characteristic absorbance peak at a distinct wavelength in the UV-visible spectrum. Solubilization capacities for orange OT and FCI by C12E23N+SO3-F8 in the concentration range of 0.1-5 mM are given in Table 2 in comparison with different classical single tailed surfactants such as DTAB (CH3(CH2)11N+(CH3)3Br-), CTAB (CH3(CH2)15N+(CH3)3Br-), and CpyC (C21H38ClN 3 H2O).38 It is seen that orange OT solubilization capacities of DTAB and C12E23N+SO3-F8 in terms of mol (solubilized orange OT)/ mol (micellized surfactant) are nearly the same and have an approximate value of 0.011, which is considered a constant value. Solubilization capacities of DTAB solutions in water and in 100 mM NaBr are identical; the only difference is in CMC values. It is also seen from Table 2 that the amounts of (38) Schott, H. J. Phys. Chem. 1967, 71, 3611–3617.

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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), Respectively) surfactant

surfactant solubilizate mol (solubilizate)/mol concn (mM) concn (mM) (micellized surf.) orange OT solubilization

SO3-F8

+

C12E23N

DTAB35(water) DTAB35(100 mM NaBr) CpyC35(17.5 mM NaCl) CTAB35(13 KBr)

mM

0.1 0.5 1 2 5 19 22.7 6.5

0.0028 0.0083 0.0136 0.0215 0.0559 0.0461 0.0800 0.0243

0.031 0.017 0.014 0.011 0.011 0.0115 0.0104 0.0105

9.75 1.4

0.0607 0.0267

0.0109 0.0226

2.79 2.75

0.0607 0.0358

0.0236 0.0229

5.5

0.0890

0.0206

FCI solubilization SO3-F8

+

C12E23N

F8C2SO3-Na+ DTAB

0.1 0.5 1 2 5 0.5 15 25

0.085 0.116 0.224 0.280 1.042 0.212 0.125

0.97 0.24 0.23 0.14 0.21 0.42 0.005

orange OT solubilized by 1 mol of micellized CTAB or CpyC, which have the same hydrocarbon chain length, are close to each other and higher than those of DTAB and C12E23N+SO3-F8. Similarly, the solubilized amounts of orange OT by 1 mol of surfactant for 0.01 N aqueous sodium decyl sulfate, sodium dodecyl sulfate, and sodium tetradecyl sulfate solutions are 0.00023, 0.0028, and 0.026, respectively.39 This case states that hydrocarbon chain length is quite effective in solubilization of hydrocarbon-based materials and solubilization capacity increases with increased chain length. Since the orange OT solubilization capacity of C12E23N+SO3-F8 is almost the same as that of DTAB, which has the same length hydrocarbon chain and higher than most hydrocarbon-based classical anionic surfactants, it can be stated that C12E23N+SO3-F8 is efficient in hydrocarbon solubilization and effective compartmentalization within the micellar core is achieved. The amounts of solubilized FCI by C12E23N+SO3-F8 at the same surfactant concentrations are approximately 15 times higher than that of orange OT. FCI solubilization in terms of (moles of solubilized FCI)/(moles of micellized surfactant) is found as 0.23, which is also constant at the range of surfactant concentration studied. It is observed in previous studies utilizing hybrid surfactants that fluorocarbon-based materials have a higher tendency to be solubilized in the micelles than hydrocarbon-based ones and the tendency increases with increased fluorocarbon content of the surfactant. The solubilized amount of 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 (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.

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Figure 4. Change of separately and simultaneously solubilized amounts of orange OT and FCI with C12E23N+SO3-F8 concentration: (a) for only orange OT solubilization; (b) for only FCI solubilization; and (c) for simultaneous solubilization of orange OT and FCI (0, FCI; [, orange OT).

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 ability of fluorocarbons in micelles as seen in the literature. The cyclic structure of orange OT is believed to make incorporation of it into the micellar core different. It is also seen from the table that 15 mM DTAB cannot solubilize FCI and on the other hand 25 mM DTAB solution can solubilize FCI slightly. This is because the lipophobicity of fluorocarbons is not as strong as their hydrophobicity. Fluorocarbon-based single-tailed anionic F8C2SO3-Na+ (CF3(CF2)7(CH2)2SO3-Na+) surfactant is the most effective surfactant given in Table 2 for FCI solubilization but it is unable to solubilize orange OT. DTAB is effective in solubilization of hydrocarbon-based materials and micelles of F8C2SO3-Na+ can solubilize only fluorocarbon-based material while the micelles of C12E23N+SO3-F8 are effective for solubilization of both hydrocarbon and fluorocarbon-based materials. Further investigations were made for determination of simultaneous solubilization capacity. Figure 4 shows the relation DOI: 10.1021/la900456t

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between solubilized amounts of orange OT and FCI separately, and simultaneously. It is seen from Figure 4 that solubilized amounts of solubilizates change linearly with surfactant concentration. The reason for this linearity could be the indication of changelessness of aggregate properties in the concentration range studied. In the case of cosolubilization of orange OT and FCI, solubilized amounts are found to be almost equal to 90% of separately solubilized amounts for both orange OT and FCI, which demonstrates that C12E23N+SO3-F8 is able to solubilize hydrocarbon and fluorocarbon-based materials effectively both simultaneously and separately. However, there are some difficulties in determining solubilized FCI concentration at low surfactant concentrations due to the low molar absorptivity value of FCI and the contribution of orange OT to the absorbance value, which is utilized for FCI quantification. This is the reason for the inequality of simultaneously solubilized FCI amounts per 1 mol of micellized surfactant. The small decrease in the case of cosolubilization is believed to be due to the exclusion of orange OT from fluorocarbon-based subdomain, and FCI from hydrocarbon-based subdomain owing to the competition between orange OT and FCI. A previous study in which a triblock amphiphilic copolymer is used for solubilization of pyrene and 1-naphtyl perfluoroheptanyl ketone (NFH) supports our findings. In that study the difference between separately and simultaneously solubilized amounts of pyrene is ignorable but NFH decreases to 63% of its separate value.5 When perfluorobenzene by 1-oxo-1-[4-(tridecafluorohexyl)phenyl]-2hexane sulfonate is utilized for adsolubilization of 2-naphthol and perfluorobenzene, cosolubilized amounts of 2-naphthol and perfluorobezene are one-half and one-fifth of separately adsolubilized amounts. In those studies, the decrease in simultaneous solubilization is much more than that of C12E23N+SO3-F8, causing the suggestion that hydrocarbon- and fluorocarbonbased solubilization sites overlap each other in the micellar core. Finally, the effect of solubilization on the aggregate size and shape is investigated since it is known that material solubilization may change aggregate properties. Shape analysis for the cases of separate and cosolubilizations of orange OT and FCI are shown in Figure 5. It is suggested from the shape analysis that globular and coil-type aggregates coexist as a mixture in all of the solutions given in Figure 5. When orange OT is solubilized, the shape of the micelles becomes closer to sphere although FCI solubilization and cosolubilization of orange OT and FCI do not make any observable change in shape analysis. However, in any case, material solubilization causes an increase in radius of the gyration value, which is highest for orange OT solubilization. In the case of OT solubilization, the hydrodynamic radius values also change to 2.5 ( 1 nm and 150 ( 50 nm. Observation of size change in both sized micelles is thought to indicate that orange OT is solubilized by both of them. Together with this result, being unable to solubilize orange OT for F8C2SO3-Na+ strengthens the conclusion that there are no separate fluorocarbon-rich and hydrocarbon-rich micelles in C12E23N+SO3-F8 solutions meaning all C12E23N+SO3-F8 micelles should have multicompartments. The effect of material solubilization on aggregate properties depends on the locus of solubilization. When an apolar material is solubilized inside the micellar core vo and thus packing parameter values increase indicating formation of more asymmetric aggregates of lower curvature, and when solubilizate is solubilized in the palisade layer it causes the ae value to increase and the packing parameter value to decrease which is an indicator of higher curvature.28 Considering this information it is suggested that orange OT might be solubilized 7890 DOI: 10.1021/la900456t

Unsal and Aydogan

Figure 5. Effect of solubilization on aggregate properties of 2 mM C12E23N+SO3-F8 at 25 oC: (a) 2 mM C12E23N+SO3-F8 + orange OT; (b) 2 mM C12E23N+SO3-F8 + FCI; and (c) 2 mM C12E23N+SO3-F8 + orange OT + FCI.

at a higher extent in the palisade layer because of its heteroatom such as the hydroxyl group. Linear fluoroalkyl chain solubilization may cause the lengthening of aggregates.2 The Rg/Rh ratio is also used for evaluation of aggregate geometry.41 Aggregates that are more asymmetric are thought to be formed with the increase in Rg/Rh ratio. In our study FCI solubilization does not alter the hydrodynamic radius of aggregates but causes (41) Brown, W. Y. N. Light Scattering Principles and Development; Clarendon Pres: Oxford, UK, 1996.

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an increase in radius of gyration value resulting in an increase in Rg/Rh value from 1.2 to 1.5 (longer aggregates). Therefore, it is claimed that FCI is solubilized inside the micellar core and more asymmetric aggregates are formed expectedly. Aggregate properties obtained by using only 2 mM C12E23N+SO3-F8 are given in Figure 5 but general tendencies are common for all surfactant concentrations studied.

Conclusion We conclude that it is possible to improve the compartmentalization inside micelles by a special molecular design in which hydrocarbon and fluorocarbon chains are provided to gain enough flexibility utilizing balance of forces. C12E23N+SO3-F8, which is the first multicompartment micelle forming ion-pair molecule in the literature, has a high solubilization capacity for

Langmuir 2009, 25(14), 7884–7891

hydrocarbon- and fluorocarbon-based materials both separately and simultaneously. Properties of having a quite low CMC value and producing stable micelles along with the high solubilization capacity 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 University research project (grant no. 0601602012). Supporting Information Available: TEM images of C12E23N+SO3-F8 aggregates and DLS analysis of F8C2SO3-Na+ solution. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la900456t

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