Capillary Condensation and Interface Structure of a Model Colloid-Polymer Mixture in a Porous Medium

TL;DR

This study uses density functional theory to analyze capillary condensation and interface structures in a colloid-polymer mixture within porous media, revealing how matrix properties influence phase behavior and interface characteristics.

Contribution

It introduces a detailed theoretical analysis of colloid-polymer mixtures in porous matrices, highlighting the effects of matrix size and packing fraction on phase behavior and interfaces.

Findings

Matrix size and packing fraction significantly affect capillary condensation.

Results can be mapped onto matrix-free systems via simple rescaling.

Capillary condensation in colloidal suspensions is experimentally accessible.

Abstract

We consider the Asakura-Oosawa model of hard sphere colloids and ideal polymers in contact with a porous matrix modeled by immobilized configurations of hard spheres. For this ternary mixture a fundamental measure density functional theory is employed, where the matrix particles are quenched and the colloids and polymers are annealed, i.e. allowed to equilibrate. We study capillary condensation of the mixture in a tiny sample of matrix as well as demixing and the fluid-fluid interface inside a bulk matrix. Density profiles normal to the interface and surface tensions are calculated and compared to the case without matrix. Two kinds of matrices are considered: (i) colloid-sized matrix particles at low packing fractions and (ii) large matrix particles at high packing fractions. These two cases show fundamentally different behavior and should both be experimentally realizable. Furthermore, we argue that capillary condensation of a colloidal suspension could be experimentally accessible. We find that in case (ii), even at high packing fractions, the main effect of the matrix is to exclude volume and, to high accuracy, the results can be mapped onto those of the same system without matrix via a simple rescaling.