Fluorescence-probe study of anionic/cationic surfactant systems 2. Alkylsulfonate/alkyltrimethylammonium bromide

Abstract

Steady state fluorescence and time-resolved fluorescence techniques have been used to study the properties of two kinds of anionic surfactant systems and four kinds of mixed anionic/cationic surfactant systems. The two anionic surfactant systems are sodium dodecyl sulfate and sodium dodecylsulfonate (C 12As) aqueous solutions. The four alkylsulfonate/alkyltrimethylammonium bromide systems consist of C 12As/cetyltrimethylammonium bromide (CTAB), sodium octylsulfonate (C 8As)/CTAB, sodium hexadecylsulfonate (C 16As)/CTAB and C 12As/dodecyltrimethylammonium bromide (DTAB). We discuss mainly the variation of the micellar aggregation number of these systems with the NaCl concentration and temperature. Both the addition of anionic surfactant to cationic surfactant and of cationic surfactant to anionic surfactant result in a large increase in the micellar aggregation number. In addition, by a steady state fluorescence technique, the critical micelle concentration (CMC) for the C 8As/CTAB and C 12As/DTAB systems and the second CMC for the C 12As, C 12As/CTAB and C 12As/DTAB systems at which the micelles changed from spherical to rod shaped were determined. At the same time, the solubilization position of the fluorescence probe (pyrene) and the polarity of the probe microenvironment in the mixed anionic/cationic micelles were studied.