Effect of excluded volume interactions on the interfacial properties of colloid-polymer mixtures

TL;DR

This study uses simulations to explore how excluded volume interactions influence the phase behavior and interfacial properties of colloid-polymer mixtures, revealing differences from ideal polymer models and effects of confinement.

Contribution

It provides a detailed numerical analysis of the impact of polymer interactions on phase diagrams and interfacial phenomena in colloid-polymer systems, extending beyond idealized models.

Findings

Interacting polymers require more polymers to induce demixing.

Interfacial tension is reduced with polymer interactions.

Excluded volume interactions suppress capillary condensation.

Abstract

We report a numerical study of equilibrium phase-diagrams and interfacial properties of bulk and confined colloid-polymer mixtures using grand canonical Monte Carlo simulations. Colloidal particles are treated as hard spheres, while the polymer chains are described as soft repulsive spheres. The polymer-polymer, colloid-polymer, and wall-polymer interactions are described by density-dependent potentials derived by Bolhuis and Louis [Macromolecules, 35 (2002), p.1860]. We compared our results with those of the Asakura-Oosawa-Vrij model, that treats the polymers as ideal particles. We find that the number of polymers needed to drive the demixing transition is larger for the interacting polymers, and that the gas-liquid interfacial tension is smaller. When the system is confined between two parallel hard plates, we find capillary condensation. Compared with the AOV model, we find that the excluded volume interactions between the polymers suppress capillary condensation. In order to induce capillary condensation, smaller undersaturations and smaller plate separations are needed in comparison with ideal polymers.