Calculations on ionic solvation. Part 2.—Entropies of solvation of gaseous univalent ions using a one-layer continuum model

Abstract

The electrostatic entropy of solvation of gaseous ions has been calculated using our previous model in which the ion is surrounded by a local solvent layer, immersed in the bulk solvent. The calculated electrostatic entropy is combined with the nonelectrostatic entropy of solvation, obtained from experimental data on entropies of solution of gaseous nonpolar solutes, to yield the total entropy of solvation of a gaseous ion. The only new parameters involved in the calculations are the variations with temperature of the solvent bulk dielectric constant (a known property) and the dielectric constant in the local solvent layer. We found that if the latter parameter is taken as –0.001 60 (a reasonable value for a region of low dielectric constant), there is excellent agreement with experiment for entropies of solvation of univalent cations and anions in a wide variety of aprotic solvents, and for entropies of transfer of these ions between aprotic solvents. Since no adjustable parameters are used in the calculations, the method can be used to predict entropies of solvation or of transfer in aprotic solvents. Agreement with experiment is not found for solvation entropies of ions in hydrogen bonded solvents.