Pathway and Driving Force 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, little is known about the pathway for the coacervate from the dilute phase, and there is considerable controversy regarding the thermodynamic driving force. Theory and simulation have suggested that the polycations and polyanions exist as pairs in the supernatant phase. Here, using umbrella sampling, we perform MD simulations to study the potential of mean force (PMF) between two pairs of polycation and polyanion in dilute solution. Our result shows that the merging of the two pairs is slightly dominated by energy in salt-free condition, and becomes entropy-driven in the presence of added salts. We further estimate the polymer concentration in the dilute phase from the calculated PMF.