Colloids and polymers in random colloidal matrices: demixing under good-solvent conditions

TL;DR

This study models colloid-polymer mixtures within random colloidal matrices, revealing how matrix disorder influences phase separation and critical points, with notable differences from classical Asakura-Oosawa-Vrij predictions.

Contribution

It introduces a simplified coarse-grained model for colloid-polymer mixtures in disordered matrices, highlighting novel effects on phase behavior and criticality compared to traditional models.

Findings

Critical colloid volume fraction remains unchanged by the matrix.

Polymer volume fraction at criticality increases with matrix disorder.

No capillary condensation or evaporation observed in the presence of the matrix.

Abstract

We consider a simplified coarse-grained model for colloid-polymer mixtures, in which polymers are represented as monoatomic molecules interacting by means of pair potentials. We use it to study polymer-colloid segregation in the presence of a quenched matrix of colloidal hard spheres. We fix the polymer-to-colloid size ratio to 0.8 and consider matrices such that the fraction f of the volume that is not accessible to the colloids due to the matrix is equal to 40%. As in the Asakura-Oosawa-Vrij (AOV) case, we find that binodal curves in the polymer and colloid volume-fraction plane have a small dependence on disorder. As for the position of the critical point, the behavior is different from that observed in the AOV case: while the critical colloid volume fraction is essentially the same in the bulk and in the presence of the matrix, the polymer volume fraction at criticality increases as f increases. At variance with the AOV case, no capillary colloid condensation or evaporation is generically observed.