Upper Critical Solution Temperature Phase Behavior of Polyguanidinium Simple Coacervation

Coacervation is liquid-liquid phase separation (LLPS) of polyelectrolytes in aqueous media, which is usually induced by electrostatic, cation-π and π-π interactions. The coacervation process is actively observed in bio-inspired phase-separating systems including cellular compartmentation of membraneless organelles. Recently, an arginine residue, containing a π-conjugated and positively charged guanidinium group, has drawn great attention to provide non-aromatic π-π stacking of like-charged guanidinium pairs in aqueous media. In this study, we demonstrate that the polyguadinium in aqueous media undergoes LLPS when the electrostatic repulsion is sufficiently reduced by monovalent or multivalent salts, and the simple coacervation shows the upper critical solution temperature (UCST) phase behavior. Remarkably, salt concentration plays a crucial role to regulate the phase behavior of polyguanidinium aqueous solutions. Using neutron and light scattering measurements, the thermodynamic properties including critical and theta temperature were extracted, resulting in understanding phase behavior of π-π interaction in aqueous media as a function of salt concentration, temperature, and polymer molecular weight. Our findings shed new light on the π-π association-induced phase transition to characterize thermodynamics of bio-inspired polymer solutions.