Phase Separation in Symmetric Mixtures of Oppositely Charged Rodlike Polyelectrolytes

TL;DR

This study investigates phase separation in salt-free mixtures of oppositely charged rodlike polyelectrolytes, revealing how electrostatic interactions influence nematic ordering, counterion distribution, and phase behavior, with implications for complexation.

Contribution

It provides a quasi-analytical analysis of phase transitions and counterion partitioning in rodlike polyelectrolyte mixtures, highlighting differences from flexible polyelectrolytes.

Findings

Electrostatic interactions favor nematic ordering.

Counterions distribute uniformly at low electrostatic strength.

Higher electrostatic strength causes counterions to partition into concentrated phases.

Abstract

Phase separation in salt-free symmetric mixtures of oppositely charged rodlike polyelectrolytes is studied using quasi-analytical calculations. Stability analyses for the isotropic-isotropic and the isotropic-nematic phase transitions in the mixtures are carried out and demonstrate that electrostatic interactions favor nematic ordering. Coexistence curves for the symmetric mixtures are also constructed and are used to examine the effects of linear charge density and electrostatic interaction strength on rodlike polyelectrolyte complexation. It is found that the counterions are uni- formly distributed in the coexisting phases for low electrostatic interaction strengths dictated by the linear charge density of the polyelectrolytes and Bjerrum's length. However, the counterions also partition along with the rodlike polyelectrolytes with an increase in the electrostatic interaction strength. It is shown that the number density of the counterions is higher in the concentrated (or "coacervate") phase than in the dilute (or supernatant) phase. In contrast to such rodlike mixtures, flexible polyelectrolyte mixtures can undergo only isotropic-isotropic phase separation. A comparison of the coexistence curves for weakly-charged rodlike mixtures with those of analogous flexible polyelectrolyte mixtures reveals that the electrostatic driving force for the isotropic-isotropic phase separation is stronger in the flexible mixtures.