Fluid-fluid demixing curves for colloid-polymer mixtures in a random colloidal matrix

TL;DR

This study investigates how fluid-fluid phase separation in colloid-polymer mixtures is affected by a colloidal porous matrix, revealing that demixing behavior largely depends on a single volume fraction parameter and showing phenomena like capillary condensation.

Contribution

The paper introduces a detailed analysis of the influence of a quenched colloidal matrix on phase separation in colloid-polymer mixtures, highlighting the dominant role of a single parameter and new insights into critical point shifts.

Findings

Demixing curves depend mainly on a single volume fraction parameter.

Critical colloid packing fraction increases with matrix volume fraction.

Capillary condensation occurs for certain parameters, indicating phase coexistence within the matrix.

Abstract

We study fluid-fluid phase separation in a colloid-polymer mixture adsorbed in a colloidal porous matrix close to the \theta -point. For this purpose we consider the Asakura-Oosawa model in the presence of a quenched matrix of colloidal hard spheres. We study the dependence of the demixing curve on the parameters that characterize the quenched matrix, fixing the polymer-to-colloid size ratio to 0.8. We find that, to a large extent, demixing curves depend only on a single parameter f, which represents the volume fraction which is unavailable to the colloids. We perform Monte Carlo simulations for volume fractions f equal to 40% and 70%, finding that the binodal curves in the polymer and colloid packing-fraction plane have a small dependence on disorder. The critical point instead changes significantly: for instance, the colloid packing fraction at criticality increases with increasing f. Finally, we observe for some values of the parameters capillary condensation of the colloids: a bulk colloid-poor phase is in chemical equilibrium with a colloid-rich phase in the matrix.