Enthalpies of interaction of ammonium bromide with hexamethylphosphoramide and formamide in water at 25 and 40°C

Heats of solution of ammonium bromide in water and its mixtures with hexamethylphosphoramide (HMP) and formamide (FA) were measured at 25 and 40°C in the region of small additions of the nonaqueous component. The standardenthalpies of solution were determined in the Debye-Huckel second approximation. The enthalpy coefficients of pair and triad electrolyte-amide interactions in water were calculated using the McMillan-Mayer framework and compared to the amide-amide interaction coefficients. It was found that the electrolyte-amide interaction parameters and their temperature-dependent changes differ for FA and HMP: for FA, the pair interaction with ammonium bromide is enthalpy-favored and almost independent of temperature; for HMP, it is enthalpy-disfavored and strongly weakens with increasing temperature.