Langmuir zyxwvu 1992,8, zyxwvut 23-26 23 Association of Alcohol with Cationic Micelles C. Gamboa,* A. Olea, H. Rios, and M. Henriquez Departamento de Quimica, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Casilla 653, Santiago, Chile Received November 2,1990. In Final Form: July 25, 1991 Partition constants for several aliphatic alcohols between water and micellar phases were determined using ultrafiltration and dialysis. Results indicated that the micellized concentrations of the long-chain alcohol followed a Langmuir adsorption isotherm type of profile. Association of pentanol and hexanol with cylindrical micelles induces an increase in size and in dissociation degree zyxw (a), followed by a micellar breakdown. These changes can be explained in terms of the number of alcohol molecules per micelle. Introduction The effect of alcohols on the micellar properties of sur- factants in aqueous solution is a matter of current intereut1-12 mainly in order to gain a better understanding of the role of alcohols in microemulsions. It is well established that short-chain alcohols have a small effect on the critical micelle concentration (cmc) or upon the micellar dissociation degree (a).2 The effect of alcohols on several physicochemical properties becomes important for butanol and longer chain al~ohols.'~ For instance, Vikholm et al.14 discussed the effect of hexanol in hexadecyltrhethylammonium bromide (CTAB) solutions. The solution viscosity increases abruptly above a certain hexanol content and then decreases at higher hexanol contents. This fact was interpreted as a shape transition from spherical to larger rod or disklike micelles, followed by a breakdown from large aggregates to spherical swollen micelles. For tetradecyltrimethylammonium bromide (TTAB) in brine (0.1 M KBr) in the presence of increasing concen- trations of 1-pentanol, Zana et ala2% reported an increase of the micelle hydrodynamic radius (RH) and in the viscosity of the solution, followed by a micellar breakdown as well. Zana explained these changes by suggesting that the amount of alcohol dissolved in the micellar phase decreases. However, there are only a few papers in the literature regarding the partitioning of alcohols between these phases. The usual approach is to determine the increase of the amount of alcohol dissolved in the presence of detergent as compared with that in water.15 Thus, for (1) Abuin, E.; Lissi, E. J. Colloid Interface Sci. 1983, 95, 198. (2) Zana, R.; Yiv, zyxwvutsrqpo S.; Strazielle, C.; Lianos, P. J. ColloidInterface Sci. 1981. zyxwvutsrqponml 80. 208. ~ --, ,- ~ (3) Yiv, S.; Zana, R.; Ulbricht, W.; Hoffmann, H. zyxwvutsrqp J. Colloid Interface (4) Candau, R.; Zana, R. J. Colloid Interface Sci. 1981,84,206. (5) Lianos, P.; Zana, R. Chem. Phys. Lett. 1980, 76, 62. (6) Hirsch, E.; Candau, S.: Zana, R. J. Colloid Interface Sci. 1984.97. Sci. 1981, 80, 224. 318. 43. (7) Zana, R.; Picot, C.; Duplessix, R. J. Colloid Interface Sci. 1983,93, (8) Marignan, J.; Basserau, P.; Delord, P. J. Phys. Chem. 1986,90,645. (9) Gomati, R.; Appell, J.; Basseteau, P.; Marignan, J.; Porte, G. J. (10) Marignan, J.; Gauthier-Foumier,F.; Appell, J.; Akoum,F. J. Phys. (11) Guerin, G.; Bellocq, A. M. J. Phys. Chem. 1988, 92, 2550. (12) Blokhus, A. M.; Hoiland, H.; Gilje, E.; Backlund, S. J. Colloid (13) Rao, I. V.; Ruckenstein, E. J. ColloidInterface Sci. 1987,119,211. (14) Vikholm, I.; Douheret, G.; Backlund, S.; Hoiland, H. J. Colloid (15) Muto, Y.; Yoda, K.; Yoshida, N.; Esumi, K.; Meguse, K.; Binana- Phys. Chem. 1987,91, 6203. Chem. 1988,92, 440. Interface Sci. 1988, 124, 125. Interface Sci. 1987, 116, 582. Limbele, W.; Zana, R. J. Colloid Interface Sci. 1989, 130, 165. CTAB in salt-free solutions, GettinslGobtained the fol- lowing partition constants: 55.5 for butanol, 205 for pen- tanol, and 566 for hexanol. On the other hand, Abuin and Lissil demonstrated that the partition constant values determined in sodium dodecyl sulfate solutions saturated with hexanol and heptanol are lower than the correspond- ing values at low alcohol concentration measured by fluorescence methods. Other researchers have developed an empirical equation relating the ability of an additive to depress the cmc and its distribution coefficient between micelles and water.17 The partition constant for a solute around the cmc can be obtained using that equation. For phenol in nonionic micelles of polyethoxylated no- nylphenol, the association constant was determined by several methodP and the results were interpreted in terms of a Langmuir isotherm. The knowledge of the micellar alcohol concentration seems to be a very important parameter which determines several physicochemical properties. The aim of this work is to study the solubilization pattern of alcohols in cylindrical cationic micelles by determining the respective partition constants. Two systems were choosen for this study: CTAB in brine (0.16 M NaBr) and cetyltrimeth- ylammonium tosylate (CTATOS). In the first case, solutions consist of cylindrical type mi~e1les.l~ CTATOS was chosen because it presents a well-defined second cmc attributed to a sphere-rod transition, in the absence of additives.20 Materials and Methods Viscosity measurements were carried out by extrapolating to zero flow rate, as previously described.19 Free tosylate (TOS-)concentration was measured using a to- sylate ion selective electrode with a calomel reference electrode and an Orion 701 A potentiometer. The tosylate electrode was constructed with a plastic membrane containing methyltrica- prylammonium tosylate.21 The electrode calibration was per- formed by measuring the potential (mV) of NaTOS solutions ranging between zyxw 5 x and 5 x 10+ M. The electrode response was Nernstian in this range. (16) Gettins, J.; Hall, D. J. Chem. SOC., Faraday Trans. 2 1978, 74, (17) Treiner, C. J. ColloidInterfaceSci. 1983,93,33. Abu-Hamdiyyah, (18) Kandori, K.; Greevy, R. Mc.; Schechter, R. J. Colloid Interface (19) Gamboa, C.; Sepulveda, L. J. Colloid Interface Sci. 1986, 113, (20) Gamboa, I. C.; Rios, H.; Sepulveda, L. J. Phys. Chem. 1989, 93, (21) Ion-Selectiue Electrodes in Analytical Chemistry; Freiser, H., 1957. M.; Rahman, I. J. Phys. Chem. 1987, 91, 1530. Sci. 1989, 132, 395. 566. 5540. Ed.; Plenum Press: New York, Vol. 1. 0743-7463/92/240S-0023$03.00/0 0 1992 American Chemical Society