Fluid-fluid demixing transitions in colloid--polyelectrolyte star mixtures

TL;DR

This paper models the effective interactions in colloid-polyelectrolyte star mixtures to predict phase separation, revealing increased instability with higher star functionality and size ratio.

Contribution

It introduces a Derjaguin-like approximation to derive interaction potentials and calculates demixing binodals for the first time in this system.

Findings

Mixture becomes unstable at moderate concentrations.

Higher star functionality increases demixing tendency.

Larger size ratios promote phase separation.

Abstract

We derive effective interaction potentials between hard, spherical colloidal particles and star-branched polyelectrolytes of various functionalities and smaller size than the colloids. The effective interactions are based on a Derjaguin-like approximation, which is based on previously derived potentials acting between polyelectrolyte stars and planar walls. On the basis of these interactions we subsequently calculate the demixing binodals of the binary colloid--polyelectrolyte star mixture, employing standard tools from liquid-state theory. We find that the mixture is indeed unstable at moderately high overall concentrations. The system becomes more unstable with respect to demixing as the star functionality and the size ratio grow.