Miscibility and Immiscibility in Multiphase Complex Coacervate Droplets

Solutions of polymers of opposite charges, upon mixing, undergo liquid-liquid demixing as a natural consequence of chain connectivity and a low entropy of polymer mixing and leading to phase separation from the bulk. The phase-separated droplets formed by such polyelectrolyte blends are called "coacervates" and find their place in the biological world in numerous processes inside cells. Recently, there has been intense interest in understanding the unique physical features shown by multicomponent droplets that arrange in a layered core-shell morphology in-vivo as shown by Brangwynne and coworkers. Such morphologies sometimes give rise to critical sequential reaction pathways in organisms. These "multiphase" morphologies also arise when more than two polyelectrolytes are mixed, sometimes leading to immiscible, coexisting droplets of different thermodynamic phases that are in equilibrium. Using polymers with identical backbones but varying charge densities, we elucidated how differences in polymer chemistry can govern the immiscibility between different coacervate phases. Using the Flory–Huggins theory, modified for incorporating charged polymers, we found how backbones are responsible for the multiphase separation in polyelectrolyte solution blends. Finally, we calculated the free energy of the formation of these multiphase morphologies and their variation with polymer chemistry and salt concentration. These findings help uncover hidden facets of intracellular phase separation.