Phase Behavior and Salt Partitioning in Polyelectrolyte Complexes*

Polyelectrolyte complexes, resulting from associative phase separation of oppositely charged polyelectrolytes, are omnipresent both in nature and technological world. However, the true phase behavior of complexes remains poorly understood. Here we demonstrate complementary experiments and a well-defined simulation model to unveil the complete description of the phase behavior of polyelectrolyte complexes, improving understanding of underlying physics of polyelectrolyte complexation. Experiments with model polypeptides lead to phase diagrams with compositions of the complex and the supernatant, which were in agreements with simulations predictions. Surprisingly, contrary to the widely accepted 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 the excluded volume interactions, which were overlooked in the earlier theories.