Role of cosolvent in hydrophobic interactions. Calorimetric studies of alkanols in concentrated aqueous solutions of urea at 298 K

Calorimetric measurements have been carried out at 298.15 K on aqueous solutions of 7 mol kg-1 urea containing alkan-m-ols and alkan-m,n-diols in order to obtain more information on the influence of urea on the hydrophobic solvation and interactions. The pairwise interaction coefficients of the virial expansion of the excess enthalpies were found to be positive and generally lower than the corresponding values in water. The only exception concerns pentan- and hexan-2- and -3-ols, whose coefficients in the mixed solvent are higher. These coefficients were rationalized according to the preferential configuration model. Urea mainly solvates the hydroxy groups. Hydrophobic interactions are attenuated by the presence of such a large amount of urea, thus indicating that the hydrophobic cosphere of the alkanols must be reduced with respect to that in pure water. The interaction mechanism in water and in concentrated urea solutions does not seem to be alterated. The data show that in 7 mol kg-1 urea the nature of the hydration cosphere of the alkyl chain does not change, so that water still plays the major role in this interaction process between two hydrated molecules.