Network-forming phase separation of oppositely charged polyelectrolytes forming coacervates in a solvent

TL;DR

This study uses simulations to reveal that oppositely charged polyelectrolytes form percolated networks with unique coarsening dynamics, challenging the traditional droplet model of coacervates and providing new insights into their morphology.

Contribution

It demonstrates that polyelectrolyte coacervates form percolated networks with distinct growth laws, influenced by charge inhomogeneity and solvent quality, which differs from neutral polymer behavior.

Findings

Polyelectrolytes form a percolated network in semi-dilute solutions.

Coarsening follows a $ au^{1/2}$ growth law, different from neutral polymers.

Initial irregular droplet shapes transition to spherical forms over time.

Abstract

The formation of coacervates through phase separation of oppositely charged polyelectrolytes (PEs) is critical for understanding biological condensates and developing responsive materials. Traditionally, coacervates are viewed as spherical droplets with growth dynamics resembling liquid-liquid phase separation. However, our fluid particle dynamics simulations incorporating hydrodynamic and electrostatic interactions challenge this perspective. Here, we find that oppositely charged PEs form a percolated network even in semi-dilute solutions, coarsening with a unique growth law, $\ell \propto t^{1/2}$. This self-similarity, absent for neutral polymers in poor solvents, arises because PEs in good solvents exhibit weaker, longer-range attractions due to spatial charge inhomogeneity under global charge neutrality. This results in a lower density of the PEs-rich phase and reduced interfacial tension. Increased charge asymmetry further slows network coarsening. Additionally, coacervate droplets initially display irregular shapes due to weak interfacial tension, transitioning slowly to spherical forms. Our research provides new insights into coacervate morphology and coarsening dynamics.