Statics and Dynamics of Colloid-Polymer Mixtures Near Their Critical Point of Phase Separation: A Computer Simulation Study of a Continuous AO Model

TL;DR

This study introduces a new coarse-grained simulation model for colloid-polymer mixtures that accurately reproduces phase behavior and enables detailed analysis of particle dynamics near the critical point.

Contribution

The paper develops a computationally efficient model replacing hard-sphere interactions with softer potentials, suitable for molecular dynamics, and investigates the static and dynamic critical phenomena.

Findings

Coexistence curve matches the AO model and Ising critical behavior.

Polymer self-diffusion slightly increases near criticality.

Colloid self-diffusion decreases significantly at the critical point.

Abstract

We propose a new coarse-grained model for the description of liquid-vapor phase separation of colloid-polymer mixtures. The hard-sphere repulsion between colloids and between colloids and polymers, which is used in the well-known Asakura-Oosawa (AO) model, is replaced by Weeks-Chandler-Anderson potentials. Similarly, a soft potential of height comparable to thermal energy is used for the polymer-polymer interaction, rather than treating polymers as ideal gas particles. It is shown by grand-canonical Monte Carlo simulations that this model leads to a coexistence curve that almost coincides with that of the AO model and the Ising critical behavior of static quantities is reproduced. Then the main advantage of the model is exploited - its suitability for Molecular Dynamics simulations - to study the dynamics of mean square displacements of the particles, transport coefficients such as the self-diffusion and interdiffusion coefficients, and dynamic structure factors. While the self-diffusion of polymers increases slightly when the critical point is approached, the self-diffusion of colloids decreases and at criticality the colloid self-diffusion coefficient is about a factor of 10 smaller than that of the polymers. Critical slowing down of interdiffusion is observed, which is qualitatively similar to symmetric binary Lennard-Jones mixtures, for which no dynamic asymmetry of self-diffusion coefficients occurs.