Investigating Phase Boundaries of Crowded Polyelectrolyte Complex CoacervatesSystems

Polyelectrolyte complex (PEC) coacervates are macro phase-separated systems where polyelectrolytes self-assemble to form a dense complex phase, leaving behind a polymer-lean supernatant. The local environment may heavily influence complexation, including crowder molecules that limit polyelectrolyte configurations but otherwise do not participate in phase separation. Understanding the role of crowders in complexation and coacervation is important for elucidating and controlling related phenomena, such as the formation and aging of biomolecular condensates within biological cells, and industrial processes creating shelf-stable food and cosmetics. We present an investigation of a PEC coacervate system including neutral polyethylene glycol (PEG) of varying lengths as a crowding agent. The effects of crowders on PEC coacervates are hypothesized to be largely entropic and lead to stabilization and densification of the liquid state through depletion forces. We observe crowding effects to be non-monotonically dependent on PEG molecular weight; below a specific size, PEG infiltrates the complex phase and does not induce strong entropic effects on the PEC, while larger molecules are excluded from the PEC volume and have a more pronounced stabilizing effect for the liquid.