The role of salt–macroion van der Waals interactions in the colloid–colloid potential of mean force

In an aqueous electrolyte solution, the potential of mean force for two macroions is affected not only by the size and charge of each electrolyte ion but also by the ionʹs polarizability. The mean force between two colloidal particles in a saline solution can be calculated by augmenting electrostatic interactions with salt-specific dispersion (van der Waals) interactions between small ions and between small ions and macroions. Results of Monte Carlo simulations are qualitatively consistent with specific-salt effects observed experimentally by numerous investigators, including those of Hofmeister for aqueous proteins over 100 years ago. In agreement with theoretical predictions of Ninham et al., our calculations show that the potential of mean force (and hence the phase behavior of colloidal or protein solution) is sensitive to the van der Waals interaction that depends on salt identity.