Local theory for ions in binary liquid mixtures

The influence of ions on the bulk phase behavior of binary liquid mixtures acting as their solvents and on the corresponding interfacial structures close to a planar wall is investigated by means of density functional theory based on local descriptions of the effective interactions between ions and their solvents. The bilinear coupling approximation (BCA), which has been used in numerous previous related investigations, is compared with a novel local density approximation (LDA) for the ion-solvent interactions. It turns out that within BCA the bulk phase diagrams, the two-point correlation functions, and critical adsorption exhibit qualitative features which are not compatible with the available experimental data. These discrepancies do not occur within the proposed LDA. Further experimental investigations are suggested which assess the reliability of the proposed LDA. This approach allows one to obtain a consistent and rather general understanding of the effects of ions on solvent properties. From our analysis we infer, in particular, that there can be an experimentally detectable influence of ions on binary liquid mixtures due to steric effects but not due to charge effects.