Effect of many-body interactions on the bulk and interfacial phase behavior of a model colloid-polymer mixture

Abstract

We study a model suspension of sterically stabilized colloidal particles and nonadsorbing ideal polymercoils, both in bulk and adsorbed against a planar hard wall. By integrating out the degrees of freedom of thepolymer coils, we derive a formal expression for the effective one-component Hamiltonian of the colloids. Weemploy an efficient Monte Carlo simulation scheme for this mixture based on the exact effective colloidHamiltonian; i.e., it incorporates all many-body interactions. The many-body character of the polymermediatedeffective interactions between the colloids yields bulk phase behavior and adsorption phenomena thatdiffer substantially from those found for pairwise simple fluids. We determine the phase behavior for size ratiosq=p /c=1, 0.6, and 0.1, where c and p denote the diameters of the colloids and polymer coils, respectively.For q=1 and 0.6, we find both a fluid-solid and a stable colloidal gas-liquid transition with an anomalouslylarge bulk liquid regime caused by the many-body interactions. We compare the phase diagrams obtainedfrom simulations with the results of the free-volume approach and with direct simulations of the truebinary mixture. Although we did not simulate the polymer coils explicitly, we are able to obtain the threepartial structure factors and radial distribution functions. We compare our results with those obtained fromdensity functional theory and the Percus-Yevick approximation.We find good agreement between all results forthe structure. We also study the mixture in contact with a single hard wall for q=1. Upon approach of thegas-liquid binodal, we find far from the triple point, three layering transitions in the partial wetting regime.