Surface Charge Density and Steric Repulsion in Polyelectrolyte-Surfactant Coacervation

Solutions of oppositely charged polyelectrolytes andsurfactantscan undergo phase separation, in a charge-driven process known ascomplex coacervation. These materials are widely used in a varietyof applications because of their useful rheological and structuralproperties. It is understood that these properties are related tothe assembly of the surfactants into micelles, which then undergocomplexation with the oppositely charged polyelectrolytes to formthe coacervate phase. However, there remain challenges in understandinghow the molecular features of the components give rise to this usefulphase behavior, with a still-nascent understanding of how electrostatics,micelle structure, composition, and steric interactions interplayto govern coacervation. In this paper, we used a combination of experimentand a recently developed hybrid simulation/theory model to understandpolyelectrolyte-surfactant coacervates. We used mixtures ofionic and neutral surfactants to systematically vary the micelle surfacecharge density, along with PEG side-chains on the neutral surfactantsto vary the steric repulsions between nearby micelles. Finally, wealtered the polyelectrolyte charge density to tune the polymer-mediatedattractions between micelles. By mapping the phase behavior of thesesolutions, we showed that higher charge density on the polymer ormicelle, or decreasing steric repulsion, facilitates coacervation.We considered analogous quantities in our simulation/theory model,which makes predictions for both the thermodynamics and the structureof the micelle-polyelectrolyte rich coacervate and micelle-polyelectrolytepoor supernatant phases. By varying the micelle surface charge densityand the correlation-based polymer-micelle interaction energy,we showed phase separation behaviors consistent with experiments.