Calculations on ionic solvation. Part 4.—Further calculations in solvation of gaseous univalent ions using one-layer and two-layer continuum models

Abstract

The equation for the electrostatic free energy of solvation of a gaseous ion, based on a one-layer model, contains two disposable parameters, the dielectric constant of the layer ε₁ and the thickness of the layer. If ε₁ is taken as 1.05n², where n is the solvent refractive index, results are no better than our previous calculations when ε₁ was set equal to 2.0. However, if ε₁= 1.05n² and if (b–a) is used as an adjustable parameter, excellent agreement with observational values is obtained for both aprotic and hydroxylic solvents; the required values of (b—a) are quite close to the solvent radii used previously. In order to calculate the corresponding entropy of solvation, a value of δε/δT is required. For aprotic solvents the necessary δε/δT values to obtain agreement with experiment are close to values of δn²/δT and close to the value of –1.6 – 10^–3 K^–1 used before. However, for hydrogen bonded solvents rather unrealistic values of δε/δT are needed to reproduce the observed entropies of solvation and it is concluded that for these solvents an extra positive entropic contribution is necessary. It is shown also that using the one-layer model, the variations of ΔG^o_s with solvent, for a given ion, arise both from effects within the layer and effects in the bulk solvent, but that the corresponding variation of ΔG^o_s is almost entirely due to effects beyond the layer. The observations of Criss and Abraham that the entropy of transfer of alkali metal cations and halide anions between solvents depends only on the solvents and not on the ion transferred is interpreted satisfactorily using the one-layer model.

Calculations have also been carried out using a model in which an ion is surrounded by two concentric solvent layers and then the bulk solvent. Although this model contains extra disposable parameters, it is shown that results using the two-layer model are not significantly better than the latest results using the one-layer model. In particular, it is demonstrated that the extra entropic effects in hydrogen bonded solvents are still necessary even using the two-layer model to calculate the electrostatic entropies of solvation.