(352c) Ion Ejection and Dynamic Arrest in Pdda-ATP Coacervates Transferred to Deionized Water

Complex coacervates, formed through associative phase separation of oppositely charged macromolecules, play essential roles in biological compartmentalization and synthetic materials design. While the driving forces underlying coacervate formation are well-characterized, the mechanisms that stabilize coacervate droplets against coalescence and fusion remain less understood. In this work, we investigate the physicochemical changes that occur when PDDA-ATP coacervates are transferred from their native supernatant to deionized (DI) water — a process that unexpectedly stabilizes the coacervates against further coalescence.

Using molecular dynamics simulations, we quantify ion dynamics and structural organization within the coacervate phase under both conditions. Our results reveal a rapid ejection of more than 50% of small ions (Na⁺ and Cl⁻) from the coacervate into the surrounding DI water, accompanied by a pronounced slowdown in the mobility of the remaining ions within the coacervate phase. We propose that this ion depletion leads to the formation of a kinetically arrested interfacial layer, which acts as a physical barrier to further molecular exchange and coalescence.

This study provides new molecular-level insight into the stabilization mechanisms of complex coacervates in low-salt environments, with implications for understanding phase-separated biological systems and designing robust coacervate-based materials under nonequilibrium conditions.