Temperature dependence of the interaction between two hydrophobic solutes: a calorimetric study

The enthalpies of tetraethyl- and tetrabutylammonium bromides solution in water and its mixtures with hexamethyl phosphoric triamide (HMPT) in the “water-rich” region of the mixed solvent were measured at 277.15, 288.15 and 313.15 K. The standard enthalpies of solution are computed and compared with previously determined values at 298.15 K. The enthalpic and heat capacity coefficients of solute-HMPT pair interactions in water and the temperature changes of the entropic pair interaction coefficients were computed and compared with those for other organic non-electrolytes. It was shown that bromide-ion and cosolvent nature influenced strongly both enthalpic interaction parameters and their temperature dependence, the last one in particular. The enthalpy of methylene group interaction with HMPT was found to be large, positive and independent of the temperature. It was pointed out that the energetics of non-polar solute transfer from water to various highly aqueous water-organic mixtures at room temperature is defined to a great extent by dimensions of cosolvent molecules and packing effects of solvent molecules in the solute solvation shell.