Complexation, phase separation, and redissolution in polyelectrolyte-macroion solutions

The complexation, phase separation, and redissolution of polyelectrolyte-macroion solutions have been studied by means of Monte Carlo simulations. A simple model system with focus on the electrostatic interactions has been used to examine the properties of the macroion solutions at different amounts of oppositely charged polyelectrolytes. As oppositely charged polyelectrolytes are added, the stable macroion solution with repelling macroions becomes successively less stable. The strong electrostatic attraction brings macroions and polyelectrolytes closely together, and slightly before macromolecular charge equivalence, distinct and repelling complexes formed by macroions and polyelectrolytes are established. At macromolecular charge equivalence, the system becomes unstable, and a large and loose cluster of macroions and polyelectrolytes is formed. Finally, in excess of polyelectrolytes, the large cluster is broken up and the macroions are dispersed again-a redissolution has occurred. The effect of the macroion radius, the chain length, and the chain flexibility on the phase separation is also investigated. A semiflexible chain displayed a smaller tendency to promote phase instability as compared to flexible and stiff chains, the origin most likely arising from the similar chain persistence length and macroion radius.