Closed-Loop Phase Behavior of Nonstoichiometric Coacervates in Coarse-Grained Simulations

Most studies of polyelectrolyte coacervate phase behavior focus on symmetric mixtures of oppositely charged polymers. Here we use a coarse-grained simulation, in which all bonded monomers and mobile counterions are represented as Lennard-Jones particles with unit charge and diameter equal to the Bjerrum length, to study the impact of charge asymmetry on coacervate phase behavior. We study the impact of salt on the concentration of polymers and mobile ions in each phase and qualitatively reproduce the closed-loop behavior observed in recent experiments on nonstoichiometric coacervates. We find that the counterions from added salt distribute unevenly, preferring the dilute phase to maximize their translational entropy, analogous to Donnan equilibrium for a charged membrane. The coacervate phase shrinks under osmotic pressure, leading to increased polymer concentration with small amounts of added salt.