Fluid phase behavior of a model colloid-polymer mixture: Influence of polymer size and interaction strength

Abstract

We examine how the fluid-fluid phase behavior of a model colloid-polymer mixture evolves with variation of polymer size and/or interaction strength. Polymer-polymer interactions are approximated through Gaussian-core potentials while colloid-colloid and colloid-polymer interactions are assumed purely hard. Grand canonical transition matrix Monte Carlo simulation is used to construct binary liquid-liquid phase diagrams for a wide range of polymer sizes and interaction strengths. Overall, our results indicate that independently decreasing the polymer size or increasing the strength of repulsion between polymers have similar effects; these variations lead to an increase in the critical colloid concentration and decrease in the critical polymer concentration. An examination of the phase diagrams collected in this study reduced by their critical reservoir polymer concentration and critical colloid concentration reveals a single common phase envelope that is quantitatively similar to that for common models used to describe simple fluids. Finally, evaluation of the osmotic second virial coefficient in the vicinity of the critical point suggests that this quantity cannot be used as a reliable predictor for the conditions under which colloid-polymer mixtures phase separate.