MICELLE FORMATION IN AQUEOUS NABR SOLUTIONS OF ALKYLTRIMETHYLAMMONIUMBROMIDES

Sound velocity in aqueous solutions of alkyltrimethylammonium bromides with alkyl chains of octyl, decyl, dodecyl and tetradecyl has been me asured as a function of surfactant and NaBr concentrations at 25 degre es C. The critical micelle concentrations, CMC, were determined from i ntersections of two straight line portions of the surfactant concentra tion dependence of sound velocity in the solutions. CMC decreases with the increase of ionic strength, following the Corrin-Harkins equation . The standard free energy of micelle formation was estimated from the CMC at different NaBr concentrations. It was presented as a sum of in crements of the methylene and the polar head groups at each NaBr conce ntration. The methylene group contribution varies with NaBr concentrat ion from -1.81 kJ/mol in water to -2.95 kJ/mol at salt concentrations higher than 0.5 m NaBr.