A density--functional study of interfacial properties of colloid--polymer mixtures

TL;DR

This paper develops a density-functional theory to study interfacial properties of colloid-polymer mixtures, incorporating polymer interactions and comparing results with ideal polymer models, revealing significant effects on surface tensions.

Contribution

It introduces a new density-functional approach that accounts for polymer interactions in colloid-polymer mixtures, extending previous ideal polymer models.

Findings

Wall surface tension is higher with interacting polymers.

Fluid-fluid interfacial tension decreases with polymer interactions.

The theory aligns well with two-component mixture predictions.

Abstract

Interfacial properties of colloid--polymer mixtures are examined within an effective one--component representation, where the polymer degrees of freedom are traced out, leaving a fluid of colloidal particles interacting via polymer--induced depletion forces. Restriction is made to zero, one and two--body effective potentials, and a free energy functional is used which treats colloid excluded volume correlations within Rosenfeld's Fundamental Measure Theory, and depletion--induced attraction within first--order perturbation theory. This functional allows a consistent treatment of both ideal and interacting polymers. The theory is applied to surface properties near a hard wall, to the depletion interaction between two walls, and to the fluid--fluid interface of demixed colloid--polymer mixtures. The results of the present theory compare well with predictions of a fully two--component representation of mixtures of colloids and ideal polymers (the Asakura--Oosawa model), and allow a systematic investigation of the effects of polymer--polymer interactions on interfacial properties. In particular, the wall surface tension is found to be significantly larger for interacting than for ideal polymers, while the opposite trend is predicted for the fluid--fluid interfacial tension.