Enthalpy and heat capacity parameters of ammonium bromide interaction with hexamethylphosphotriamide in water

Thermal effects of solution of ammonium bromide in hexamethylphosphotriamide (HMPT)-water mixtures at 4° C, 15°C, and 46°C in the region of minor additions of the nonaqueous component were investigated for the first time by calorimetry. Variations of heat capacity in the course of solution were calculated using previous data for 25°C and 40°C. The curves of the heat capacity of solution (transfer) and temperature variation of transfer entropy versus composition have a minimum near the region of the existence of the 1:20 HMPT:water clathrate. The enthalpy, heat capacity, and temperature variations of the entropy coefficients of electrolyte amide pair and three-particle interactions in water were calculated in terms of MacMillan-Mayer theory. Within the framework of the group additivity method it is shown that the heat capacity coefficient of the bromide ion-HMPT pair interaction is negative and dictates temperature variations of enthalpy for ammonium bromide and tetraalkylammonium interactions with HMPT in water.