Temperature dependence of the pair interaction between hydrophobic and hydrophilic solutes: A calorimetric study

The enthalpies of tetraethyl- and tetrabutylammonium bromides solution in water and highly aqueous water-formamide (FA) mixtures were measured at 277.15, 288.15, 313.15, 319.15 and 328.15 K. The standard enthalpies of solution are obtained and compared with previously determined values at 298.15 K. Heat capacities of solution were computed. Enthalpic and heat capacity coefficients of solute-FA pair interactions were calculated. The heat capacity of methylene group interaction with FA was found to be negative, whereas the heat capacity of bromide ion-FA interaction appeared to be positive. The free energy and entropic coefficients of tetrabutylammonium bromide-FA interaction coefficients were estimated and compared with those for other non-electrolytes. It has been found that the interaction between two hydrophobic species is the entropic origin which is in agreement with classical models of hydrophobic interaction. In contrast, the interaction of tetrabutylammonium bromide with FA is the enthalpic origin, it being repulsive at all temperatures studied.