J. Phys. Chem. 1980, 84, 2191-2194
2191
Micelle Formation of Ionic Surfactants in Polar Nonaqueous Solvents H. N. Singh,” Syed MI. Saleem,+ R. P. Singh, Department of Chemistry, Aligarh Muslim University, Aligarh 20200I, India
and K. S. Birdi Fyslsk-Kemisk Institut, Technical Unlversity of Denmark, DK 2800 Lyngby, Denmark (Received: August 20, 1979)
Micelle formation of two ionic surfactants, viz. sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB),has been studied in organic solvents of various dielectric constant and intermolecular H-bonding capability, viz. N-msthylacetamide (NMA), formamide (FA), dimethyl sulfoxide (Me2SO), and N,N’-dimethylformamide (DMF), at different temperatures by conductance measurements. AH,’ and AS,”, standard enthalpies and entropies of micelle formation, respectively, have been determined by studying the variation of the critical micelle concentration (cmc) with temperature. Compensation plots have been obtained for both the surfactants in the above solvents. The value of slope of these plots are almost identical with each other and also with that obtained in aqueous medium. Micelle formation in these solvents has been explained on the basis of several factors such as dielectric constant of the medium, its intermolecular H-bonding capability, and also its bulk “structuredness”.
Introduction Micelle for mation in nonaqueous solvents has attracted only little attention as compared to the vast number of extensive studies that have been replorted in the literature dealing with the formation of micelles in aqueous surfactant solutions. Recently, to differentiate from the “inverted” micelles which are formed in nonpolar organic solvents, the lkrm “solvophobic interaction” has been coined to describe the micelle formation in polar organic solvents, in analogy with “hydrophobic interactions” causing micellization in aqueous medium.l The micelles formed due to “solvophobic interactions” are similar in many respects to the micelles that are formed in aqueous medium, although, in ,general, micelle formation is not as favored in nonaqueous solvents as in water for a given surfactant2 Notwithstanding the recognition of the importance of “water structure” in the micelle formation in aqueous surfactant soliutions, no significant attempt seems to have been made to investigate the role of “structuredness” which may exist in many polar organic solvents due to the presence of one or more potential hydrogen bonding centers in their molecules. In continuation of our work on micelle formation in nonaqueous solvent^,^ we have studied the micellization of ionic surfactants in various organic solvents of varying dielectric constant and possessing different number of potential hydrogen bonding centers, thus offering the possibility of subst
