Lower Critical Solution Temperature in Polyelectrolyte Complex Coacervates

A model linear oppositely charged polyelectrolyte complex exhibits phase separation upon heating consistent with lower critical solution temperature (LCST) behavior. The LCST coexistence curves narrow with increasing monovalent salt concentration (Cs) that reduces the polymer concentration (Cp) in the polymer-rich phase. The polymer-rich phase exhibits less hydration with increasing temperature, while an increase in Cs increases the hydration extent. The apparent critical temperature, taken as the minimum in the phase diagram, occurs only for a narrow range of Cs. Mean field theory suggests an increasing Bjerrum length with temperature can lead to an electrostatic-driven LCST; however, the temperature dependence of the Flory–Huggins interaction parameter and solvation effects must also be considered.