  We briefly review the effects of selective solvation of ions in aqueous
mixtures, where the ion densities and the composition fluctuations are strongly
coupled. We then examine the surface tension \gamma of a liquid-liquid
interface in the presence of ions. We show that \gamma can be decreased
drastically due to the electrostatic and solvation interactions near the
interface. We calculate how the free energy is changed due to small surface
undulations in the presence of an electric double layer. A surface instability
occurs for negative \gamma, which can easily be realized for antagonistic ion
pairs near the solvent criticality. Three-dimensional simulation shows how the
surface instability is induced.
