Effects of High Salt Concentrations on the Micellization of Octyl Glucoside: Salting-Out of Monomers and Electrolyte Effects on the Micelle-Water Interfacial Tension 1 Pasupati Mukerjee* and Chun C. Chan School of Pharmacy, University of Wisconsin, Madison, Wisconsin 53705 Received January 18, 2002. In Final Form: April 24, 2002 The effects of two added electrolytes up to high concentrations, 0-4 M NaCl and 0-6.6 M LiCl, on the critical micellization concentration (cmc) of octyl glucoside (OG), a nonionic surfactant, in aqueous solution have been measured. The fluorescence probe 6-p-toluidino-2-naphthalene sulfonate was used to determine the cmc values. Log cmc values were found to vary linearly with the molar electrolyte concentration, CS, up to the highest concentrations. A theoretical approach based on the salting-out of the monomeric chain, presented in 1965, has been expanded by incorporating an additional effect of added electrolytes on the interfacial tension of the micelle-water interface. A group additivity relationship for the salting-out of hydrocarbon chains, also proposed in 1965, has been shown to give a good account of the salting-out coefficients of some hydrocarbons, primary alcohols, and methyl esters of carboxylic acids of different chain lengths obtained from literature data. Salting-out coefficients for the octyl group of OG in NaCl and LiCl, calculated on this basis, significantly overestimated the effects of electrolytes on the cmc of OG. Electrolyte effects on the interfacial tension of the estimated surface areas of OG micelles where hydrocarbons are exposed to water were calculated based on literature data on the effects of NaCl and LiCl on the dodecane- water interfacial tension. When these latter effects were combined with the estimated salting-out of the chains, the electrolyte effects on the cmc of OG could be explained nearly quantitatively up to the highest C S. The results of some earlier studies in the literature have been shown to be compatible with the approaches presented. For long-chain surfactants in concentrated brine associated with some petroleum-oil recovery operations, the salting-out effects may have pronounced influences on their activities and how they change with salt concentration. Introduction Inorganic electrolytes can have pronounced effects on the formation of micelles by uncharged surfactants, nonionic and zwitterionic, in aqueous solution 2-12 resulting in significant reduction of their critical micellization concentrations (cmc). In 1965, a theoretical treatment was developed based on the application of the principles of salting-out of nonelectrolytes by electrolytes. 13 This ap- proach was shown to be superior to some earlier explana- tions. 3,4,13 The activity coefficient, f, of a nonelectrolyte in electrolyte solutions can be represented by the Setchenow relationship, 14,15 where C s is the molar electrolyte concentration and k s is the salting-out coefficient. 14,15 For monomer-micelle equilibria, it was suggested that the salting-out of the hydrocarbon chain of the surfactant monomer is of primary importance and that the salt effects on the hydrophilic headgroups of both the monomeric and micellized sur- factants exposed to water were likely to cancel to a great extent. 8,13,16 The equation derived from this model, 13 where cmc(0) is the cmc value in the absence of electrolytes and k is a constant, has been found to be moderately successful in describing the cmc values of many nonionic and zwitterionic systems. 8,13 The experimental values of k were in reasonable accord with some calculated estimates of the salting-out coefficients, k s , for many electrolytes. 8,9,13 For some zwitterionic systems, there seems to be an imperfect cancellation of the expected salting-in of the zwitterionic headgroups of the surfactants in the mono- meric and micellized states. 13 Salting-out of monomers has also been shown to be important for ionic surfac- tants. 13,17,18 No effect of added electrolytes on the exposed hydro- carbon portions of the micelles was considered in the above model used in 1965. 13 Later, in 1970, a proposal was made that the micelle-water interfacial tension is substantial and that this tension leads to a high Laplace pressure inside the micelles. 19 The interfacial tension of the micelle- * To whom correspondence should be addressed. 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(14) McDevitt, W. F.; Long, F. A. J. Am. Chem. Soc. 1952, 74, 1773. (15) Long, F. A.; McDevitt, W. F. Chem. Rev. 1952, 51, 119. (16) Mukerjee, P. J. Phys. Chem. 1970, 74, 3824. (17) Mukerjee, P. Adv. Colloid Interface Sci. 1967, 1, 241. (18) Franchini, M. K.; Carstensen, J. T. J. Pharm. Sci. 1996, 85, 220. (19) Mukerjee, P. Kolloid Z. Z. Polym. 1970, 236, 76. log f ) k s C s (1) log cmc ) log cmc(0) - kC s (2) 5375 Langmuir 2002, 18, 5375-5381 10.1021/la020059e CCC: $22.00 © 2002 American Chemical Society Published on Web 06/17/2002