Field theory description of ion association in re-entrant phase separation of polyampholytes

TL;DR

This paper develops a field theory model to understand ion association effects on phase separation in polyampholytes, revealing re-entrant phase behavior influenced by ion concentration, sequence charge pattern, and volumetric effects.

Contribution

It introduces a comprehensive field theory framework incorporating electrostatic and excluded volume interactions, analyzing re-entrant phase separation in polyampholytes with ion pairing.

Findings

Re-entrant phase separation occurs at high ion concentrations.

Ion association leads to volume decrease and influences phase behavior.

Sequence charge pattern affects phase separation dynamics.

Abstract

Phase separation of several different overall neutral polyampholyte species (with zero net charge) is studied in solution with two oppositely charged ion species that can form ion-pairs through an association reaction. A field theory description of the system, that treats polyampholyte charge sequence dependent electrostatic interactions as well as excluded volume effects, is hereby given. Interestingly, analysis of the model using random phase approximation and field theoretic simulation consistently show evidence of a re-entrant polyampholyte phase separation at high ion concentrations when there is an overall decrease of volume upon ion-association. As an illustration of the ramifications of our theoretical framework, several polyampholyte concentration vs ion concentration phase diagrams under constant temperature conditions are presented to elucidate the dependence of phase separation behavior on polyampholyte sequence charge pattern as well as ion-pair dissociation constant, volumetric effects on ion association, solvent quality, and temperature.