Charge Asymmetry Suppresses Coarsening Dynamics in Liquid–Liquid Phase Separation of Polymer Coacervates

Liquid–liquid phase separation (LLPS) of charged macromolecules in water typically involves the coarsening of small droplets. In experiments on biocondensate and polyelectrolyte complex coacervation (PEC), the coarsening process is found to be significantly slower than the known spinodal decomposition dynamics for neutral species. The suppressed coarsening in biocondensate formation has recently been attributed to the cellular environments, which are absent for the case of PEC. One factor that has not been considered is the net charges in the droplets arising from the charge imbalance of the constituent polymers, which can create additional free energy barrier that potentially slows down the coarsening.



Using dissipative particle dynamics simulation, we show that charge asymmetry fundamentally alters the LLPS dynamics in PEC. When the small droplets are charge-balanced, the coarsening follows the classical spinodal decomposition; However, when the small droplets carry net charges, the coarsening dynamics is significantly slower, with a reduced growth exponent that decreases with increasing charge asymmetry. We rationalize this slowing down by showing the free energy barrier in merging two net-charged coacervate droplets. Solvent quality has effects: Poorer solvent conditions accelerate the coarsening in charge-asymmetric systems, but slightly slows down that in symmetric systems. These results highlight the nontrivial effects of electrostatic interactions on the LLPS dynamics.