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Langmuir 1999, 15, 8590-8601
Phase Behavior and Microstructure of Aqueous Mixtures of Cetyltrimethylammonium Bromide and Sodium Perfluorohexanoate Daniel J. Iampietro and Eric W. Kaler* Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received April 8, 1999. In Final Form: August 6, 1999 The phase behavior and microstructure of aqueous mixtures of cetyltrimethylammonium bromide (C16TAB) and sodium perfluorohexanoate (FC5) were investigated as a function of total surfactant concentration, mixing ratio, and temperature. The aggregation behavior of both the pure components and several mixtures was studied via surface tension experiments, and analysis of the results using the regular solution model yielded a β value of -19.4, indicating highly nonideal mixing. The evolution of phase behavior from the FC5-water binary axis with the addition of C16TAB was of particular interest. Below the critical micelle concentration or cmc of FC5 (5.70 wt %), the addition of small amounts of C16TAB leads to the formation of large structures, including vesicles, between surfactant concentrations of 2 and 4 wt %. Above the cmc, FC5 forms small globular micelles in solution. The addition of C16TAB to the FC5 micelles leads to an increase in viscosity and eventually phase separation into two rodlike micellar solutions, one enriched in FC5 and the other enriched in C16TAB. The microscopic structure of the vesicles and micelles was probed using small-angle neutron scattering, and the experimental data was analyzed with two complimentary methods. The vesicles were found to be polydisperse with average sizes ranging from 500 to 800 Å (determined from light scattering) and a bilayer thickness of about 29 Å. Consistent with the viscosity results, neutron-scattering measurements showed that the rodlike micelles undergo one-dimension growth and become more flexible near the phase boundary. Results from an indirect transform of the scattered intensity indicate that near the center of the aggregate cross section there is a region consisting solely of the hydrocarbon surfactant.
Introduction Above the critical micellar concentration of a single surfactant in water, the surfactant monomers aggregate to form a variety of microstructures ranging from spherical and elongated micelles to flat lamellar sheets. The type of equilibrium structure depends on a balance between factors which favor aggregation (e.g., the hydrophobic effect) and those which oppose aggregation (e.g., electrostatic, chain packing, and surface interactions). There are a number of ways to shift this balance and drive transitions in the microstructure, but of particular interest is the addition of a second surfactant. This is beneficial from an industrial standpoint because synergistic interactions between surfactants typically make formulations containing surfactant mixtures more effective and less expensive than those with a single pure component. The synergy is enhanced in mixtures of oppositely charged surfactants in which strong electrostatic interactions between the head groups alter the aggregation process and lead to new states of aggregation which are not formed by either of the single surfactants, such as equilibrium vesicles. Unlike lipid systems where the formation of vesicles requires the input of mechanical energy1 or chemical treatment,2 the unique aspect of these vesicles is that the formation proceeds spontaneously with only simple mixing. Their spontaneous formation along with the fact that physical properties such as the vesicle size and polydispersity become independent of time and are not a function of the method of preparation support * To whom correspondence should be addressed. (1) Szoka, F.; Papahadjopoulos, D. Annu. Rev. Biophys. Bioeng. 1980, 9, 467-508. (2) Kagawa, Y.; Racker, E. J. Biol. Chem. 1971, 246, 5477-5487.
the claim that these vesicles are equilibrium structures, although there are other views.3 It is obvious that electrostatics play an important role in setting the equilibrium structure, but it is also useful to understand the relationship between the observed phase behavior and microstructure transitions with other experimentally controllable variables such as the surfactant architecture and solution properties such as the ionic strength and pH. The stability of the vesicle phase has been of special interest,4-6 with a variety of work showing that the composition range over which vesicles form can be expanded by increasing the asymmetry between the two surfactant chains7-9 or by branching one of the surfactant chains.10 Furthermore, varying the effective potential or charge density on the vesicle surface either through the addition of a simple electrolyte11,12 or by adjusting the pH in mixtures containing fatty acids13,14 also strongly influences vesicle stability. Another inter(3) Laughlin, R. G. Colloids Surf., A 1997, 128, 27-38. (4) Lasic, D. D. J. Colloid Polym. Sci. 1990, 140, 302-304. (5) Porte, G.; Ligoure, C. J. Chem. Phys. 1995, 102, 4290-4298. (6) Safran, S. A.; Pincus, P. A.; Andelman, D.; MacKintosh, F. C. Phys. Rev. A 1991, 43, 1071-1078. (7) Herrington, K. L., University of Delaware, 1994. (8) Yatcilla, M. T.; et al. J. Phys. Chem. 1996, 100, 5874-5879. (9) Yuet, P. K.; Blankschtein, D. Langmuir 12, 1996, 3819-3827. (10) Kaler, E. W.; Herrrington, K. L.; Murthy, A. K.; Zasadzinski, J. A. J. Phys. Chem. 1992, 96, 6698-6707. (11) Brasher, L. L.; Herrington, K. L.; Kaler, E. W. Langmuir 1995, 11, 4267. (12) Papahadjopoulos, D.; et al. Biochim. Biophys. Acta 1977, 465, 579-598. (13) Gebicki, J. M.; Hicks, M. Chem. Phys. Lipids 1976, 16, 142160. (14) Huang, J.-B.; Zhoa, G.-X. Colloid Polym. Sci. 1995, 273, 156164.
10.1021/la990407l CCC: $18.00 © 1999 American Chemical Society Published on Web 10/02/1999
Phase Behavior and Microstructure of Aqueous Mixtures
esting microstructural transition is that from spherical to elongated micelles. This can be driven, for example, by decreasing the effective size of the surfactant head group by screening the electrostatic interaction through the addition of salt.15-18 This transition is also influenced by changes in the surfactant structure19 or by using surfactants that contain two hydrophobic tail groups (phospholipids) instead of just a single one.20,21 Most surfactants have hydrophobic portions made of hydrocarbons, but surfactants with either partially or completely fluorinated hydrophobic regions are common. Fluorination of the hydrophobe increases the surface activity of the surfactant over that of the analogous hydrocarbon surfactant. For a given chain length, fluorocarbon surfactants have a lower critical micellar concentration and are more effective at lowering the surface tension of water than hydrocarbon surfactants.22 In addition to an enhanced surface activity, the surfactant chain is both hydrophobic and oleophobic, so fluorocarbon surfactants are used in coating applications where both water and oil repellency are required. Furthermore, the ability of fluorinated surfactants to solubilize a variety of compounds has led to their proposed use in emulsion polymerizations23 and in vivo oxygen transport.24 Combining a hydrocarbon and fluorocarbon surfactant provides a new approach for controlling the aggregation process. Hydrocarbon/fluorocarbon surfactant mixtures are of interest because of physical and chemical differences between the two hydrophobic moieties. Physically, the fluorocarbon chain is bulkier (the volume of the CF2 group is 50% larger than that of the corresponding CH2 group) and considerably more rigid than the hydrocarbon chain,25,26 while chemical differences between the chains can lead to miscibility gaps which are well documented for mixtures of alkanes and perfluoroalkanes.27-29 The interplay of these specific physical and chemical processes is expected to influence the aggregation process and the type of equilibrium microstructure formed and potentially lead to new ways to control transitions between microstructures. Previous work on hydrocarbon/fluorocarbon systems has focused on mixed micelles in solutions containing surfactants of the same charge. These mixed micelles have been studied using a variety of experimental tech(15) Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1976, 80, 1075-1085. (16) Ikeda, S.; Hayashi, S.; Imae, T. J. Phys. Chem. 1981, 85, 106112. (17) Porte, G.; Appell, J. J. Phys. Chem. 1981, 85, 2511-2519. (18) Kumar, S.; David, S. L.; Aswal, V. K.; Goyal, P. S.; Din, K.-u. Langmuir 1997, 13, 6461-6464. (19) Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1983, 87, 1264-1277. (20) Lin, T.-L.; Chen, S.-H.; Roberts, M. F. J. Phys. Chem. 1987, 91, 406-413. (21) Lin, T.-L.; Tseng, M.-Y.; Chen, S.-H.; Roberts, M. F. J. Phys. Chem. 1990, 94, 7239-7243. (22) Kissa, E. Fluorinated Surfactant Synthesis, Properties, Applications; Marcel Dekker: New York, 1994. (23) Rahl, F. J.; Evanco, M. A.; Fredericks, R. J.; Reimschuessel, A. C. J. Polym. Sci. 1972, 10, 1337-1349. (24) Riess, J. G.; Krafft, M. P. Chem. Phys. Lipids 1995, 75, 1-14. (25) Bates, T. W.; Stockmayer, W. H. J. Chem. Phys. 1966, 45, 23212322. (26) Mattice, W. L.; Suter, U. W. Conformational Theory of Large Molecules: The Rotational Isomeric State Model in Macromolecular Systems; John Wiley & Sons: New York, 1994. (27) Scott, R. L. J. Phys. Chem. 1958, 62, 136-145. (28) Gilmour, J. B.; Zwicker, J. O.; Katz, J.; Scott, R. L. J. Phys. Chem. 1967, 71, 3259-3270. (29) Hicks, C. P.; Hurle, R. L.; Toczylkin, L. S.; Young, C. L. Aust. J. Chem. 1978, 31, 19-25.
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niques,30-35 and the possibility of coexistence of two types of micelles in solution was proposed on the basis of the mutual phobicity of the two surfactant chains.36-39 Surprisingly, mixtures of oppositely charged hydrocarbon and fluorocarbon surfactants have received little attention,38-42 and in general, phase behavior results indicate that these mixtures tend to form bilayer structures over a wide range of compositions, with vesicles forming in a few cases.40,41 Here we investigate the phase behavior and microstructure of the mixture of water, the cationic hydrocarbon surfactant cetyltrimethylammonium bromide, and the anionic fluorocarbon surfactant sodium perfluorohexanoate at dilute concentrations (
