Dynamics of Polyelectrolyte Complex Coacervates: A Molecular Dynamics Study

Polyelectrolyte complex coacervates (PECCs) are formed by mixing oppositely charged polyelectrolytes. While it is observed experimentally that adding salts to coacervates accelerates their dynamics, the underlying mechanisms remain elusive since the addition of salts often leads
to the less concentrated PECCs, making it difficult to separate the effects of electrostatics and concentration. To solve this puzzle, we performed coarse-grained molecular dynamics simulations of PECCs with explicit solvent and analyzed the chain dynamics as a function of salt concentration. To discriminate the roles of salts and polyelectrolytes concentration, we compared the dynamics of coacervates with those of neutral polymer solutions at similar concentrations. In the concentration range studied (30~90% polymer volume fraction), the chain relaxation can be described by Rouse dynamics in both systems, while the monomer relaxation of coacervates is slower leading to longer Rouse time. As polymer concentration decreases, the relaxation times decrease similarly in both neutral and charged systems. This indicates that the change in coacervates dynamics induced by the addition of salts is primarily due to the dilution of polyelectrolytes in the coacervates.