Demixing of colloid-polymer mixtures in poor solvents

TL;DR

This paper uses density functional theory to study how poor solvent conditions influence the phase separation of colloid-polymer mixtures, revealing shifts in demixing behavior and reentrant transitions at low colloid concentrations.

Contribution

It introduces a geometry-based density functional theory model that captures the effects of poor solvents on colloid-polymer demixing, including reentrant phase transitions.

Findings

Demixing occurs at lower colloid concentrations with worsening solvent quality.

Reentrant demixing transition predicted at low colloid concentrations.

Poor solvents induce effective attractions leading to phase separation.

Abstract

The influence of poor solvent quality on fluid demixing of a model mixture of colloids and nonadsorbing polymers is investigated using density functional theory. The colloidal particles are modelled as hard spheres and the polymer coils as effective interpenetrating spheres that have hard interactions with the colloids. The solvent is modelled as a two-component mixture of a primary solvent, regarded as a background theta-solvent for the polymer, and a cosolvent of point particles that are excluded from both colloids and polymers. Cosolvent exclusion favors overlap of polymers, mimicking the effect of a poor solvent by inducing an effective attraction between polymers. For this model, a geometry-based density functional theory is derived and applied to bulk fluid phase behavior. With increasing cosolvent concentration (worsening solvent quality), the predicted colloid-polymer binodal shifts to lower colloid concentrations, promoting demixing. For sufficiently poor solvent, a reentrant demixing transition is predicted at low colloid concentrations.