Thermodynamics of micellar systems: activity and entropy of sodium decanoate and n-alkylamine hydrobromides in water

The freezing-point depressions of aqueous solutions of the homologous series RNH 3 Br, where R varies from ethyl to n-octyl, and of sodium decanoate were determined and used to calculate osmotic and mean activity coefficients. In the case of the surfactants octylamine hydrobromide and sodium decanoate, the pre- and post-micellar regions were covered. Activity data at higher temperatures were calculated using previously published thermochemical data. Some difficulties arise in the critical micellar region of sodium decanoate at high temperatures as a result of the large concentration and temperature dependence of the heat capacity data. The activities from freezing point data are compared with those obtained from specific electrodes.