Electrical conductivity study of the self-association of ionic surfactants in solution in ethyleneglycol, formic acid and formamide

The self-association of several cationic surfactants has been investigated by means of solubility and electrical conductivityκ measurements in ethyleneglycol, formic acid and formamide. Operational critical micelle concentrations (CMCop) have been obtained from theκ vs. surfactant concentrationC plots by the usual interpolation procedure. It has been found that for the surfactant-solvent systems investigated the value of CMCop increases with the extent of theC-range investigated. This result together with others concerning the effect of temperature and surfactant chain length reveal that the CMCop-values have an ambiguous meaning concerning the surfactant self-association. Comparison of theκ vs.C plots for cetyltrimethylammonium chloride and tetramethylammonium chloride, for instance, reveals that some surfactant self-association takes place, probably leading to small, highly ionized surfactant aggregates, but the systems do not show true critical micelle concentration. Solubility measurements as a function of temperature lead to the same conclusion. The results are discussed according to our present understanding of the process of micellization and the necessity of using solvents of very high cohesive energy density in order to observe, with ionic surfactants, true CMCs corresponding to a highly cooperative association process. This appears to be the case only with water and hydrazine, not with formamide and other less cohesive solvents.