``Looping-back'' complexation in stoichiometrically assymmetric polyelectrolyte solutions

The complexation of ionic polymers is typically studied in model systems lacking compositional heterogeneity that is often unavoidable in realistic systems. We address this by examining the coacervation behavior of synthetic, homologous polyions over a range of stoichiometric asymmetry between polyanions and polycations. An unexpected, narrow ``looping-back'' coexistence window is observed at low salt concentrations, and is captured by a theoretical model incorporating reversible ion-binding and chain connectivity, revealing the importance of the competition between charge neutrality and mixing entropy. Partitioning of both small ions and polyions between the supernatant and coacervate phases is measured, and theoretically analyzed.