Precipitation in aqueous mixtures with addition of a strongly hydrophilic or hydrophobic solute

We examine phase separation in aqueous mixtures due to preferential solvation with a low-density solute (hydrophilic ions or hydrophobic particles). For hydrophilic ions, preferential solvation can stabilize water domains enriched with ions. This precipitation occurs above a critical solute density n_p in wide ranges of the temperature and the average composition, where the mixture solvent would be in a one-phase state without solute. The volume fraction of precipitated domains tends to zero as the average solute density n̄ is decreased to n_p or as the interaction parameter χ is decreased to a critical value χ_p. If we start with one-phase states with n̄ > n_p or χ> χ_p, precipitation proceeds via homogeneous nucleation or via heterogeneous nucleation, for example, around suspended colloids. In the latter case, colloid particles are wrapped by thick wetting layers. We also predict a first-order prewetting transition for n̄ or χ slightly below n_p or χ_p for neutral colloids.