(608e) Phase Behavior and Salt Partitioning in Polyelectrolyte Complexes

Polyelectrolyte complexation, initiated by associative phase separation of oppositely charged polyelectrolytes, constitutes the underlying physical mechanism of various biological processes in nature and has been applied in myriad technological settings. However, the true phase behavior of polyelectrolyte complexes (PEC) remains poorly characterized and understood. In this talk, we will present a complete description of the phase behavior of PECs by combining experimental measurements on model polymer systems with complementary insight from well-defined simulations. First, oppositely charged pairs of poly(lysine) and poly(glutamic acid) of matched chain lengths were used to establish binodal phase diagrams for polyelectrolytes with hydrophilic backbones. Contrary to the widely accepted Voorn-Overbeek (VO) theory for complexation, we find preferential partitioning of salt ions into the supernatant phase. Upon increasing the salt concentrations, the salt partitioning underwent a unique trend exhibiting a distinct minimum. These trends were revealed by simulations to be strongly influenced by excluded volume effect, which was overlooked in the earlier PEC theories. Phase behavior for polyelectrolytes with hydrophobic backbones exhibited starker deviations from the Voorn-Overbeek theory predictions. For a solution comprising poly(acrylic acid) and poly(allylamine hydrochloride), the PEC phase behavior agreed qualitatively with the expectations from the VO theory at low ionic strengths. However, contribution of hydrophobicity from the polyelectrolyte backbone became increasingly significant at high polymer and salt concentrations, leading to non-trivial deviations from the expected results. This work is expected to provide a framework for constructing more sophisticated designs of polyelectrolyte complex-based materials for enabling biomedical and technological applications.