Ginzburg-Landau theory of solvation in polar fluids: Ion distribution around an interface.

We present a Ginzburg-Landau theory of solvation of ions in polar binary mixtures. The solvation free energy arising from the ion-dipole interaction can strongly depend on the composition and the ion species. Most crucial in phase separation is then the difference in the solvation free energy between the two phases, which is the origin of the Galvani potential difference known in electrochemistry. We also take into account an image potential acting on each ion, which arises from inhomogeneity in the dielectric constant and is important close to an interface at very small ion densities. Including these solvation and image interactions, we calculate the ion distributions and the electric potential around an interface with finite thickness. In particular, on approaching the critical point, the ion density difference between the two phases becomes milder. The critical temperature itself is much shifted even by a small amount of ions. We examine the surface tension in the presence of ions in various cases.