  We calculate the ion and composition distributions around colloid particles
in an aqueous mixture, accounting for preferential adsorption, electrostatic
interaction, selective solvation among ions and polar molecules, and
composition-dependent ionization. On the colloid surface, we predict a
precipitation transition induced by strong preference of hydrophilic ions to
water and a prewetting transition between weak and strong adsorption and
ionization. These transition lines extend far from the solvent coexistence
curve in the plane of the interaction parameter \chi (or the temperature) and
the average solvent composition. The colloid interaction is drastically altered
by these phase transitions on the surface. In particular, the interaction is
much amplified upon bridging of wetting layers formed above the precipitation
line. Such wetting layers can either completely or partially cover the colloid
surface depending on the average solvent composition.
