Structure and Dynamics of Coacervates Formed by Oppositely Charged Polyelectrolytes*

We develop a scaling theory and perform molecular dynamic simulations of weakly interacting coacervates with electrostatic interaction energy per charge less than thermal energy kT. Such liquid coacervates formed by oppositely charged polyelectrolytes can be asymmetric in charge density and number of charges per chain. We predict that these coacervates form interpenetrating solutions with two correlation lengths and two qualitatively different types of conformations of polyelectrolytes with lower and higher charge densities, which are analogous to chain conformations in quasi-neutral and in polyelectrolyte solutions, respectively. Weaker charged chains are attracted to and adsorbed on stronger charged chains forming a screening “coat” around the stronger charged polyelectrolytes. We present scaling prediction for polymeric diffusion coefficient, viscosity and stress relaxation function of these coacervates. Salt added at lower concentrations screens the repulsion between stronger charged chains, thereby reducing the thickness of the screening coat and resulting in the non-zero net polymer charge in the coacervate. At higher salt concentrations salt screens the attraction between oppositely charged chains, decreasing the coacervate concentration and its polymeric charge density. Thus, we predict a non-monotonic salt concentration dependence of polymeric charge density for asymmetric coacervates. Phase diagram for a mixture of oppositely charged polyelectrolytes at various compositions is proposed for different salt concentrations.