Electrostatic Correlations Based on Increasing Bjerrum Length with Temperature Sufficient to Model Lower Critical Solution Temperature in Polyelectrolyte Complex Coacervates*

The liquid-liquid phase separation of aqueous solutions of polyelectrolyte and salt is responsible for fascinating phenomena ranging from membraneless organelles to targeted drug delivery. Recently, Ali et al. (2019) demonstrated lower critical solution temperature (LCST) liquid-liquid phase separation in these systems, which has not previously been modeled or observed. Adhikari et al. (2019) demonstrated that LCST could be modeled with solvent-polymer and dipole-dipole interactions if the temperature-dependence of water's dielectric constant is incorporated. Here, we show that electrostatic correlations based on a Bjerrum length that increases with temperature is sufficient to model LCST in a symmetric polyelectrolyte and salt solution, using both Voorn-Overbeek and a liquid state theory. We show qualitative agreement with the experimental trends in the binodal, but found that the observed widening of the binodal with polymer on polymer-temperature axis requires a negatively sloped tie line, contrary to experimental findings. We also demonstrate UCST and LCST as in Ye et al. (2020) and suggest further validation experiments.