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

Abstract

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 in wide ranges of the temperature and the average composition above a critical solute density $n_p$, 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 $\bar n$ is decreased to $n_p$ or as the interaction parameter $\chi$ is decreased to a critical value $\chi_p$. If we start with one-phase states with ${\bar n}>n_p$ or $\chi>\chi_p$, precipitation proceeds via homogeneous nucleation or via heterogeneous nucleation, for example, around suspended colloids. In the latter case, colliod particles are wrapped by thick wetting layers. We also predict a first-order prewetting transition for $\bar n$ or $\chi$ slightly below $n_p$ or $\chi_p$.