Ionic-group-dependent phase behavior of polyelectrolyte coacervates

Complex coacervates are polymer-rich phases of liquid-liquid phase separation when two oppositely charged polyelectrolytes are mixed in aqueous solutions. Previously, Voorn-Overbeek model(VO model), simple and intuitive combination of the entropy of mixing and electrostatic interaction, was proposed to capture the behavior of complex coacervates. Since the VO model doesn't account for the chain connectivity of polyelectrolytes and chemistry-specific details, advanced models have been suggested up to now. However, experimental data of model system is rare to compare with the theoretical description. In this study, 4 polyelectrolytes are prepared (e.g.,strong/weak and polyanion/cation)and thus 4 pairs of polyelectrolyte complex coacervates are investigated to map out the phase diagrams as a function of the pair of the ionic group. It is found that the phase diagram shows distinctive features including (1)the salt resistance and the area of two-phase region are significantly dependent on the pairs of polyelectrolytes, and (2)the tie lines in the binodal curve show negative slope. We believe chemical-specific parameters play an important role to control phase behavior, and this observation shed a new light on the fascinating and biologically important topic of complex coacervation.