Loop-in coacervation in non-stoichiometric polyelectrolyte mixtures

The response of coacervation window to salt addition in non-stoichiometric polyelectrolyte solutoins is examined by a joint experimental and theoretical effort. Polymer concentration in the coacervates is found to first increase with salt, then decrease, giving rise to a unique loop-in shape that is absent in symmetric mixtures. An asymptotic analysis in the low-salt regime shows that this loop-in behavior is caused by the osmotic pressure of the counterions confined in the coacervate due to electroneutrality, and that the effect is more prominent in solutions with stronger stoichiometric asymmetry. By labeling polycation and polyanion with distinct fluorescent labels, we resolved the partition coefficinets of polyions between the coexisting phases. The resutls showed that the polyions can only partially neutralize each other in the coacervates, and the excess are expelled to the supernatant.