Molecular association in solution: A Kirkwood-Buff analysis of sodium chloride, ammonium sulfate, guanidinium chloride, urea, and 2,2,2-trifluoroethanol in water

Molecular association has been investigated as a function of solution composition for sodium chloride (NaCl), guanidinium chloride (GdmCl), ammonium sulfate ((NH4)(2)SO4), urea, and 2,2,2-trifluoroethanol (TFE) mixtures with water. A Kirkwood-Buff analysis of the available experimental data provided information concerning the degree of excess association of the various species in solution. The results indicate that increasing concentrations of NaCl have no effect on the excess self-association of water, while GdmCl, (NH4)(2)SO4, and urea slightly increase the excess self-association of water. The presence of TFE results in composition dependent effects with a strong increase in the self-association of water at low TFE concentrations. The consequences of these observations in the context of water structure making properties are discussed. The excess self-association of cosolvents (except urea) displayed an initial increase which proceeded through a maximum at low cosolvent concentration and then decreased steadily with concentration. Analysis of the excess solvation of the cosolvents indicated substantially desolvated urea and GdmCl molecules at all concentrations, a constant desolvation of NaCl, and a variation from highly solvated ions at low concentration to zero excess solvation for (NH4)(2)SO4.