Hydrophobic hydration in mixtures of N,N-dimethylformamide and water enthalpies of solution and heat capacities of tetra- n-butylammonium bromide

Abstract

Enthalpies of solution of tetra- n-butylammonium bromide have been measured calorimetrically in mixtures of N,N-dimethylformamide ( dmf) + water from 278 to 328 K at 10 K intervals covering the whole mole fraction range. All profiles of enthalpy of solution against composition show endothermic maxima. From a simple hydration model it is possible to calculate the number n of water molecules which surround a hydrophobic alkyl group and the enthalpic effect of hydrophobic hydration in pure water. The latter effect appears to decrease strongly with temperature, while n is almost temperature independent. In addition partial molar heat capacities, C p,2 ∞, of n-Bu 4NBr have been calculated in the dmf + water mixtures; C p,2 ∞ hardly changes in the range from 0 to 0.6 mole fraction of water. In more water-rich regions C p,2 ∞ increases very rapidly towards its value in pure water.