Energetics for preferential solvation of a chromium complex in aqueous organic solvent

Abstract

Line broadening of NMR signals of solvent components induced by a paramagnetic chromium complex has been measured at varying temperatures to reveal the energetics for preferential solvation of the complex by a component of a mixture of an organic solvent (acetonitrile or acetone) and water. At water mole fractions lower than 0.8, [Cr(SCN)₆]^3– is preferentially solvated by the organic solvent molecules, mostly owing to lowering of the enthalpy of the organic solvent in the solvation layer compared to that of the bulk solution. However, at water mole fractions higher than 0.8, the population of water molecules in the solvation layer increases markedly because of the increase in entropy. Such thermodynamic properties of the solvation are correlated with the microscopic structure of mixed solvents. In contrast, in an acetonitrile (or aceteone)–benzene mixture, no preferential solvation was observed for Cr(acac)₃.