The diffusive dynamics and electrochemical regulation of weak polyelectrolytes across liquid interfaces

TL;DR

This paper develops a framework to understand how pH-dependent charge regulation affects the phase separation, structure, and electrochemical properties of weak polyelectrolytes at liquid interfaces, revealing new behaviors distinct from strong polyelectrolytes.

Contribution

It introduces a novel simulation-based analysis of weak polyelectrolytes, highlighting the impact of charge regulation and pH gradients on coacervate formation and stability.

Findings

pH gradients are maintained across coacervate interfaces

Charge regulation influences electric double layer formation

Weak polyelectrolytes exhibit distinct behaviors from strong ones

Abstract

We propose a framework to study the spatio-temporal evolution of liquid-liquid phase separation of weak polyelectrolytes in ionic solutions. Unlike strong polyelectrolytes, which carry a fixed charge, the charge state of weak polyelectrolytes is modulated by the electrochemical environment through protonation and deprotonation processes. Leveraging numerical simulations and analysis, our work reveals how solution acidity (pH) influences the formation, interactions, and structural properties of phase-separated coacervates. We find that pH gradients can be maintained across coacervate interfaces resulting in a clear distinction in the electro-chemical properties within and outside the coacervate. By regulating the charge state of weak polyelectrolytes, pH gradients interact and modulate the electric double layer forming at coacervate interfaces eventually determining how they interact. Further linear and nonlinear analyses of stationary localised solutions reveal a rich spectrum of behaviours that significantly distinguish weak from strong polyelectrolytes. Overall, our results demonstrate the importance of charge regulation on phase-separating solutions of charge-bearing molecules and the possibility of harnessing charge-regulated mechanisms to control coacervates and shape their stability and spatial organisation.