Polyelectrolyte Complex Coacervation by Electrostatic Dipolar Interactions

To explain complex coacervation, the liquid-liquid phase separation of a solution of oppositely charged polyelectrolyte chains into a polyelectrolyte rich complex coacervate phase and a dilute aqueous phase, we propose a mean field theory based on the interplay of electrostatic dipolar attractions and hydrophobic interactions. We assume that polycations-polyanions complex even in the homogeneous phase and exist as dipolar chain pairs. Phase separation is caused by two driving forces: hydrophobicity and electrostatic dipolar attractions. We predict qualitatively different phase behaviors depending upon the strengths of the two driving forces. For moderately hydrophobic polyelectrolytes at room temperature, neither hydrophobicity nor electrostatics alone is strong enough to cause phase separation, but their combined effect results in phase separation. The constructed phase diagrams capture key experimental observations including the suppression of complex coacervation by salt, temperature and polycation-polyanion chain length asymmetry and its promotion by increasing chain length, and the preferential partitioning of salt into the polyelectrolyte dilute phase. When both driving forces are capable of causing phase instability, two unstable regions, one due to each, are obtained.