When Solvent Matters: Fundamental Phase Reversals in Polyelectrolyte Coacervation

Polyelectrolyte coacervation underpins a wide range of biological and technological processes, from underwater adhesion and drug delivery to the formation of membraneless organelles. Despite this importance, most theoretical descriptions treat the solvent implicitly through a uniform dielectric constant. However, contrary to what its name implies, the dielectric constant is not constant. Its decrease with increasing temperature in most solvents has been linked to the strengthening of electrostatic interactions that drive the experimentally observed lower critical solution temperatures (LCSTs) in polyelectrolyte coacervates. The dielectric constant also varies with system composition: introducing charged species into a polar solvent induces an additional dielectric decrement. Implicit-solvent models, which assume a composition-independent dielectric, cannot rigorously capture this effect. To address this limitation, we develop the first analytical theory to treat the solvent explicitly in polyelectrolyte coacervates, accounting for ion–ion, dipole–dipole, and ion–dipole interactions without invoking a dielectric continuum. This approach predicts a complete reversal of the phase behavior: instead of an LCST, the system exhibits an upper critical solution temperature (UCST) driven by the dielectric mismatch between the coacervate and dilute phases. Molecular dynamics simulations confirm these predictions and further reveal that the two-body potential of mean force, often used to rationalize coacervation trends, fails to reproduce this reversal because it neglects collective many-body correlations that dominate the macroscopic response. The results thus highlight how explicit treatment of the solvent fundamentally alters the thermodynamic behavior of electrostatically driven phase separation.