On the fluid-fluid phase separation in charged-stabilized colloidal suspensions

TL;DR

This paper develops a thermodynamic model for charged colloidal suspensions, examining conditions for fluid-fluid phase separation and assessing the impact of salt and charge renormalization on stability.

Contribution

It introduces a fixed surface potential framework and analyzes the role of volume terms and salt in phase stability, providing new insights into colloidal suspension behavior.

Findings

Charge renormalization stabilizes suspensions without salt.

Salt induces instability leading to potential phase separation.

The Jellium-like approximation aligns well with simulations.

Abstract

We develop a thermodynamic description of particles held at a fixed surface potential. This system is of particular interest in view of the continuing controversy over the possibility of a fluid-fluid phase separation in aqueous colloidal suspensions with monovalent counterions. The condition of fixed surface potential allows in a natural way to account for the colloidal charge renormalization. In a first approach, we assess the importance of the so called ``volume terms'', and find that in the absence of salt, charge renormalization is sufficient to stabilize suspension against a fluid-fluid phase separation. Presence of salt, on the other hand, is found to lead to an instability. A very strong dependence on the approximations used, however, puts the reality of this phase transition in a serious doubt. To further understand the nature of the instability we next study a Jellium-like approximation, which does not lead to a phase separation and produces a relatively accurate analytical equation of state for a deionized suspensions of highly charged colloidal spheres. A critical analysis of various theories of strongly asymmetric electrolytes is presented to asses their reliability as compared to the Monte Carlo simulations.