(493h) Driving Force and Pathway in Polyelectrolyte Complex Coacervation

Polyelectrolyte complex coacervation is a liquid–liquid phase separation when mixing a polycation solution with a polyanion solution. There has been extensive research on this problem in recent years due to its fundamental role in biology and in materials and biomedical applications. However, considerable controversy exists regarding the thermodynamic driving force. While experiments usually find the coacervation to be strongly entropy driven, simulations generally report significant energetic contribution in driving the coacervation. Here, using umbrella sampling and thermodynamics analysis, we perform MD simulations to study the potential of mean force (PMF) of the two key stages on the pathway in coacervation: the polycation–polyanion complexation and the condensation of two pairs each containing one polycation and polyanion. By accounting for the entropy contribution in electrostatic interaction, our results show that both stages are strongly entropy driven with negligible energetic contribution, consistent with experiments. We further estimate the polymer concentration in the dilute phase from the calculated PMF and investigate the detailed molecular events on the coacervation pathway.