Role of Surface Electrostatics in Surfactant Micelle-Polyelectrolyte Complex Coacervates

Particle-polyelectrolyte (PE) complex coacervation is a liquid–liquid phase separation that occurs when polyelectrolytes undergo associative phase separation with an oppositely charged nonpolymeric species, such as surfactant micelles, colloids, and proteins. While this phenomenon has been extensively studied because of its benefits in biological and industrial applications, a molecular understanding of particle-PE coacervation remains mostly limited to models best suited for low-linear charge density components. Recently, we introduced a hybrid Monte Carlo and self-consistent field theory (MC-SCFT) approach that suggests the importance of polyelectrolyte-mediated correlation attractions between charged micelles for coacervation to occur. In this work, we investigate these correlation attractions by exploring how the surface charge density of surfactant micelles affects the phase behavior of particle-polyelectrolyte complex coacervation using a combination of molecular simulation and polymer field theory. Potentials of mean force of various micelle surface charge fractions were obtained by employing thermodynamic integration on Brownian dynamic simulations. Our results show that electrostatic correlation interactions are short-ranged and closely linked to local charge environments, with an observed interplay between electrostatic interactions and steric repulsion. These were then implemented into our MC-SCFT model to produce free energy landscape and phase diagram as a function of surface charge fraction.