Ion-specific effects in the colloid-colloid or protein-protein potential of mean force: Role of salt-macroion van der Waals interactions

In an aqueous electrolyte solution, the potential of mean force (PMF) for two macroions is affected not only by the size and charge of each electrolyte ion but also by the ion's polarizability. The Lifshitz theory provides a basis for calculating the van der Waals interaction between cation-colloid, anion-colloid, cation-cation, and anion-anion pairs. Monte Carlo simulations are used to determine how salt identity affects the PMF between colloidal particles or globular proteins in a saline solution, a phenomenon observed experimentally by Hofmeister for aqueous proteins more than 100 years ago. The calculations show that the PMF and, hence, solution phase behavior are sensitive to the van der Waals interaction between an ion and a macroion. The calculations described here may be useful for interpretation of experimental phase diagrams and for guiding design of separation processes where a salt is used to induce colloid or protein precipitation.