Surface Charge Density and Steric Repulsion in Polyelectrolyte–Surfactant Coacervation

Abstract

Solutions of oppositely charged polyelectrolytes and surfactants can undergo phase separation, in a charge-driven process known as complex coacervation. These materials are widely used in a variety of applications because of their useful rheological and structural properties. It is understood that these properties are related to the assembly of the surfactants into micelles, which then undergo complexation with the oppositely charged polyelectrolytes to form the coacervate phase. However, there remain challenges in understanding how the molecular features of the components give rise to this useful phase behavior, with a still-nascent understanding of how electrostatics, micelle structure, composition, and steric interactions interplay to govern coacervation. In this paper, we used a combination of experiment and a recently developed hybrid simulation/theory model to understand polyelectrolyte–surfactant coacervates. We used mixtures of ionic and neutral surfactants to systematically vary the micelle surface charge density, along with PEG side-chains on the neutral surfactants to vary the steric repulsions between nearby micelles. Finally, we altered the polyelectrolyte charge density to tune the polymer-mediated attractions between micelles. By mapping the phase behavior of these solutions, we showed that higher charge density on the polymer or micelle, or decreasing steric repulsion, facilitates coacervation. We considered analogous quantities in our simulation/theory model, which makes predictions for both the thermodynamics and the structure of the micelle–polyelectrolyte rich coacervate and micelle–polyelectrolyte poor supernatant phases. By varying the micelle surface charge density and the correlation-based polymer–micelle interaction energy, we showed phase separation behaviors consistent with experiments.