Lower Critical Solution Temperature in Polyelectrolyte Complex Coacervates

Abstract

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 (<i>C</i> _s) that reduces the polymer concentration (<i>C</i> _p) in the polymer-rich phase. The polymer-rich phase exhibits less hydration with increasing temperature, while an increase in <i>C</i> _s increases the hydration extent. The apparent critical temperature, taken as the minimum in the phase diagram, occurs only for a narrow range of <i>C</i> _s. 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.