  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.
