Heat capacities,volumes and solubilities of pentanol in aqueous alkyltrimethylammonium bromides

Apparent molar heat capacities and volumes of pentanol, 0.05m in decyl-, tetradecyl- and hexadecyltrimethylammonium bromides micellar solutions, were measured at 25°C. They were assumed to approach the standard infinite dilution values and rationalized by means of previously reported equations following which the distribution constant between the aqueous and the micellar phase, heat capacity, and volume of pentanol in both phases are simultaneously derived. The present results show that the volume of the micellar core does not seem to have a significant effect on the apparent molar volume and heat capacity of pentanol in the micellar phase and on the free energy of transfer of pentanol from the aqueous to the micellar phase. We report an equation correlating the free energy of transfer of alcohols in alkyltrimethylammonium bromides as a function of the number of carbon atoms in the alcohol and surfactant alkyl chain. Also, the apparent molar heat capacities of pentanol in micellar solutions as a function of surfactant concentration show evidence of two maxima, which, by increasing the alkyl chain length of surfactant display an opposite dependence on concentration. The second maximum can be attributed to a sphere to rod transition. The second transition was also found in the case of butoxyethanol in hexadecyltrimethylammonium bromide. It is more difficult to explain the nature of the first maximum although an attempt is made.