We have measured osmotic coefficients of aqueous solutions of decyltrimethylammonium bromide, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and hexadecyltrimethylammonium bromide at 55 °C and at concentrations ranging from 0.01 to 0.30 m. The measurements were made with a vapor pressure osmometer/molecular weight apparatus. We have also measured the osmotic coefficients of aqueous mixtures of sodium bromide and these same surfactants. The ratio of molalities of surfactant to added salt was held constant at 1:1. A mass-action model that includes activity coefficients of all species in solution was used to analyze the data.

We have used a flow microcalorimeter and a vibrating-tube densimeter to measure heat capacities and densities of aqueous solutions of decyltrimethylammonium bromide, dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and hexadecyltrimethylammonium bromide at 10, 25, 40, and 55 °C. We have calculated apparent molar heat capacities from the measured heat capacities. From the temperature dependence of parameters obtained from previously measured enthalpy data, we have derived relative apparent molar heat capacities. The apparent molar heat capacities calculated from our measurements are consistent within experimental error with those calculated from the enthalpy data. The application of simple thermodynamic relationships has allowed us to combine the apparent molar heat capacities and the relative apparent molar heat capacities to obtain Cp2°v alues for each surfactant at each temperature.

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