Coacervation of Oppositely Charged Polyelectrolytes: Effects of Composition Asymmetry

Using a simple liquid-state theory that accounts for electrostatic correlation by the mean-spherical approximation and chain connectivity by the first-order thermodynamic perturbation theory, we study the phase behaviors of a concentration-asymmetric mixture of polycation and polyanion solutions. We construct the full three-dimensional polycation--polyanion--cation concentration phase diagram at a fixed Bjerrum length and use this phase diagram to study the titration behavior of a polycation solution by varying amount of an equal-concentration polyanion solution, both in the absence and presence of added salts. We find that coacervation takes place within a window of volume ratios between the two solutions, when the concentration of the polyelectrolytes (PEs) and/or small ions in the initial solutions is not too high. Furthermore, the partition of the extra PE of the major components and of excess small ions shows qualitatively different behaviors for slightly asymmetric mixtures than for highly asymmetric mixtures. In addition, for salt-free or low salt concentration mixtures, the Galvani potential exhibits an abrupt jump across the symmetry point along the titration path.