Micelle Formation and Phase Equilibria in a Water-Trifluoroethanol-Fluorocarbon Surfactant System Giacomo Gente, † Camillo La Mesa,* ,† Rita Muzzalupo, ‡ and Giuseppe Antonio Ranieri ‡ Dipartimento di Chimica, Universita ` di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy, and Dipartimento di Chimica, Universita ` della Calabria, Arcavacata di Rende, 87030 Rende (Cs), Italy Received January 19, 2000. In Final Form: July 11, 2000 The solution behavior of the fluorinated surfactant tetraethylammonium perfluorooctane-sulfonate, PFOS, in water-trifluoroethanol, TFE, mixtures has been investigated by surface tension, electrical conductance, and PGSE (pulsed gradient spin-echo) NMR self-diffusion methods. Addition of progressive amounts of TFE in the solvent has little influence on the critical micellar concentration, cmc. Conversely, self-diffusion, counterion binding, and the surface pressure at the cmc are significantly affected by added fluoroalkanol. The above effects have been explained in terms of the solvent viscosity, dielectric permittivity, and surface activity, respectively. The complete phase behavior of the above system has been drawn, and the phase boundaries were determined. According to the above findings, added surfactant promotes the separation of the homogeneous solvent mixture into two coexisting fluid phases. The observed behavior was rationalized on thermodynamic grounds. Introduction Micelle formation in fluorinated surfactant solutions has been a classical field of investigation in surfactant sciences since Klevens 1 and Shinoda. 2-5 The reasons for so much interest lie in the fact that fluorinated surfactants are more hydrophobic than hydrocarbon ones. 6 In fact, the Gibbs energy of transfer of a fluoromethylene unit from water to the corresponding micelle interior, ΔG tr , is 1.5 times higher than the CH 2 one. 7 Such an effect is related to cmc values and indicates the occurrence of strongly unfavorable interactions of CF 2 groups with water. Studies reported so far deal with micelle formation, 8-11 aggregate size and shape, 12 phase diagrams, 13 phase transitions, 14 and emulsion 15 and microemulsion 16 forma- tion. Other papers report on the effect of counterions or simple electrolytes 17 and of alkanols 18 on the solution behavior of fluorocarbon surfactants. Almost nothing is known of the effect played by a fluorinated alcohol on the solution properties of fluoro- carbon surfactants. Apart from the possible technological applications of the above mixtures in refrigerators 19 and biotechnological applications (for instance, in the prepa- ration of blood substitutes 20 and in protein stabilization), 21 there are fundamental aspects of the above systems, which deserve investigation. In particular, interactions between fluoroalkanols and fluorinated surfactants are very poorly investigated. For this purpose we report on micelle formation, phase equilibria, and other physicochemical properties in ternary systems containing water, a fully fluorinated surfactant, tetraethylammonium perfluorooctane-sulfonate, hereafter termed as PFOS, and 2,2,2-trifluoroethanol, TFE, as cosolvent. Experimental data by surface tension, electrical conductance, self-diffusion, and, in part, viscosity are reported and discussed. The above data are supported by a detailed investigation of the phase diagram for the water-TFE-PFOS system at 25 °C. Knowledge of the phase diagram is useful to quantify the observed surfactant partitioning between solution phases and to discuss relevant physicochemical aspects of this system. Experimental Section A. Materials. Tetraethylammonium perfluorooctane-sul- fonate, PFOS, (from Riedel) was used as received. No surface- active impurities were detected in surface tension versus log [molal] plots. 2,2,2-Trifluoroethanol (from Riedel) was distilled and stored as indicated elsewhere. 22 Water was doubly distilled, deionized, and degassed before use: its electrical conductance, κ, is about 10 -6 s cm -1 at room * Corresponding author. E-mail: lamesa@uniroma1.it. † Dipartimento di Chimica, Universita ` di Roma “La Sapienza”. ‡ Dipartimento di Chimica, Universita ` della Calabria. (1) Klevens, H. B. J. Phys. Colloid Chem. 1950, 54, 1012. (2) Shinoda, K.; Soda, T. J. Phys. Chem. 1963, 67, 2072. (3) Shinoda, K.; Katsura, K. J. Phys. Chem. 1964, 68, 1594. (4) Kunieda, H.; Shinoda, K. J. Phys. Chem. 1976, 80, 2468. (5) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980, 84, 365. (6) Tanford, C. The Hydrophobic Effect. Formation of Micelles, Vesicles and Membranes; Academic Press: New York, 1980. (7) Shinoda, K.; Nakagawa, T.; Tamamushi, B.; Isemura, T. In Colloidal Surfactants; Academic Press: New York, 1963; p 37. (8) Mukerjee, P.; Yang, A. Y. S. J. Phys. Chem. 1976, 80, 1388. (9) Mukerjee, P.; Korematsu, K.; Okawauki, M.; Sugihara, G. J. Phys. Chem. 1985, 89, 5308. (10) Muzzalupo, R.; Ranieri, G. A.; La Mesa, C. Colloids Surf., A 1995, 104, 327. (11) La Mesa, C.; Sesta, B. J. Phys. Chem. 1987, 91, 1450. (12) Fontell, K. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 1, p 69. (13) Chidichimo, G.; Coppola, L.; La Mesa, C.; Ranieri, G. A.; Saupe, A. Chem. Phys. Lett. 1988, 145, 85. (14) Monduzzi, M. Curr. Opin. Colloid Interface Sci. 1998, 3, 467. (15) Gambi, C. M. C.; Senatra, D. Curr. Opin. Colloid Interface Sci. 1999, 4, 88. (16) Bongiovanni, R.; Borgarello, E.; Carlini, F. M.; Fisicaro, E.; Pellizzetti, E. Colloids Surf. 1990, 48, 277. (17) Hoffmann, H.; Schorr, W. J. Phys. Chem. 1981, 85, 3160. (18) Carlfors, J.; Stilbs, P. J. Colloid Interface Sci. 1985, 103, 332. (19) Sanyo Electric Corp. Ltd., Japanese Patent 58,21982,1,877. (20) Lo Nostro, P.; Monici, M.; Baglioni, P.; Fossombroni, V.; Bernabei, P. A. Colloid & Interface Group Ital. Chem. Soc. Meeting; Rome, Italy, October, 1999. (21) Sonnichsen, F. D.; Van Eik, J. E.; Hodges, R. S.; Sykes, B. D. Biochemistry 1992, 31, 8790. (22) Gente, G.; La Mesa, C. J. Solution Chem. 2000, 29, 859. 7914 Langmuir 2000, 16, 7914-7919 10.1021/la000074o CCC: $19.00 © 2000 American Chemical Society Published on Web 09/13/2000