Nonelectrolyte solvation in aqueous dimethyl sulfoxide - A calorimetric and infrared spectroscopic study

We report enthalpies of solution of formamide, N,N-dimethylformamide, N,N-dimethylacetamide, acetic acid, methyl acetate, and acetonitrile in water + dimethylsulfoxide mixed solvents. These, along with literature data for additional solutes, are analyzed in terms of the extended coordination model of solvation. We also analyze infrared data for several of these solutes. These analyses show that N,N-dimethylformamide and N,N-dimethylacetamide are preferentially hydrated, while the other solutes appear to be preferentially solvated by dimethylsulfoxide. In all cases, the extent of preferential solvation is relatively small. It is also found that the degrees of preferential solvation recovered from analyses of the enthalpy data correspond closely to those recovered from the infrared data, although the latter refer only to the polar chromophores on the solute molecules. It is found that the extent to which the solutes disrupt the solvent-solvent interactions varies systematically with the area of the nonpolar surfaces of the solute molecules.