Composition inversion in mixtures of binary colloids and polymer

TL;DR

This study investigates the phase behavior of a simple binary colloid mixture with polymer, revealing a single liquid-vapor transition and composition inversion phenomena through experiments and simulations.

Contribution

It demonstrates that a minimal colloid-polymer mixture exhibits complex phase behavior, including composition inversion, challenging previous assumptions of multiple phase separations.

Findings

Only a single liquid-vapor transition occurs.

Composition inversion is observed with increasing polymer concentration.

Density fluctuations near criticality are dominated by larger colloids.

Abstract

Understanding the phase behaviour of mixtures continues to pose challenges, even for systems that might be considered "simple". Here we consider a very simple mixture of two colloidal and one non-adsorbing polymer species which can be simplified even further to a size-asymmetrical binary mixture, in which the effective colloid-colloid interactions depend on the polymer concentration. We show that this basic system exhibits surprisingly rich phase behaviour. In particular, we enquire whether such a system features only a liquid-vapor phase separation (as in one-component colloid-polymer mixtures) or whether, additionally, liquid-liquid demixing of two colloidal phases can occur. Particle-resolved experiments show demixing-like behaviour, but when combined with bespoke Monte Carlo simulations, this proves illusory, and we reveal that only a single liquid-vapor transition occurs. Progressive migration of the small particles to the liquid phase as the polymer concentration increases gives rise to composition inversion - a maximum in the large particle concentration in the liquid phase. Near criticality the density fluctuations are found to be dominated by the larger colloids.