Micelle formation in aqueous NaBr solutions of alkyltrimethylammonium bromides

Sound velocity in aqueous solutions of alkyltrimethylammonium bromides with alkyl chains of octyl, decyl, dodecyl and tetradecyl has been measured as a function of surfactant and NaBr concentration at 25 degree C. The critical micelle concentrations, CMC were determined from intersection of two straight line portions of the surfactant concentration 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 increments of the methylene and the polar head groups at each NaBr concentration. The methylene group contribution varies with NaBr concentration from -1.81 kJ/mol in water to -2.95 kJ/mol at salt concentrations higher than 0.5 m NaBr.