EXCESS PARTIAL MOLAR ENTHALPIES, ENTROPIES, GIBBS ENERGIES, AND VOLUMES IN AQUEOUS DIMETHYLSULFOXIDE

The excess partial molar enthalpies, the vapor pressures, and the dens ities of dimethylsulfoxide (DMSO) - H2O mixtures were measured and the excess partial molar Gibbs energies and the partial molar volumes wer e calculated for DMSO and for H2O. The values of the excess partial mo lar Gibbs energies for both DMSO and H2O are negative over the entire composition range. The results for the water-rich region indicated tha t the presence of DMSO enhances the hydrogen bond network of H2O. Unli ke monohydric alcohols, however, the solute-solute interaction is repu lsive in terms of the Gibbs energy. This was a result of the fact that the repulsion among solutes in terms of enthalpy surpassed the attrac tion in terms of entropy. The data in the DMSO-rich region suggest tha t DMSO molecules form clusters which protect H2O molecules from exposu re to the nonpolar alkyl groups of DMSO.