Micellization behavior of a catanionic surfactant with high solubility mismatch Composition, temperature, and salt effects

Abstract

While cationic/anionic surfactant mixtures have been reported to form strongly non-ideal mixed aggregates in aqueous solution the micellization of true catanionic surfactants i e the salt free ion paired compounds obtained from the mixtures has been much less investigated However if there is a significant solubility mismatch between the paired chains these surfactants may show less conventional aggregation features in water such as a temperature driven vesicle-micelle transition and the coexistence of two lamellar phases in equilibrium In this work we carry out micellization studies for a catanionic surfactant of the type C(16)(+)C(8)(-) cetyltrimethylammonium octylsulfonate (TASo) by surface tension and conductivity measurements For comparison purposes the catanionic mixture cetyltrimethylammonium bromide (CTAB)/sodium octylsulfonate (SOSo) at different mixing ratios and the neat SOSo have also been studied For the non ideal CTAB/SOSo mixture Rubingh s model has been applied to determine the beta interaction parameter and the mixed micelle composition The cmc of TASo has been investigated as a function of temperature salt and excess ionic surfactant displaying some differences with respect to conventional ionic and nonionic surfactants The trends in the thermodynamic parameters of micellization of the three types of systems are comparatively rationalized