Phase Diagrams of Binary Mixtures of Oppositely Charged Colloids

TL;DR

This paper presents a theoretical framework for phase diagrams of binary mixtures of oppositely charged colloids, bridging high-temperature hard-sphere behavior and low-temperature colloidal crystal structures, with validation against simulations.

Contribution

It introduces a mean-field-like formalism that accurately predicts phase diagrams of oppositely charged colloids, considering Coulomb screening effects and guiding experimental crystal preparation.

Findings

Semi-quantitative agreement with computer simulations.

Topology of phase diagrams depends on Coulomb screening.

Predictions for conditions to form specific colloidal crystals.

Abstract

Phase diagrams of binary mixtures of oppositely charged colloids are calculated theoretically. The proposed mean-field-like formalism interpolates between the limits of a hard-sphere system at high temperatures and the colloidal crystals which minimize Madelung-like energy sums at low temperatures. Comparison with computer simulations of an equimolar mixture of oppositely charged, equally sized spheres indicate semi-quantitative accuracy of the proposed formalism. We calculate global phase diagrams of binary mixtures of equally sized spheres with opposite charges and equal charge magnitude in terms of temperature, pressure, and composition. The influence of the screening of the Coulomb interaction upon the topology of the phase diagram is discussed. Insight into the topology of the global phase diagram as a function of the system parameters leads to predictions on the preparation conditions for specific binary colloidal crystals.