Unconventional Coacervate Formation between Neutral Polymer and Inorganic Polyanions in Salted Aqueous Solution

Conventional coacervate complexes are formed between oppositely charged polyelectrolytes via ion pairing with the release of counterions. Yet other intermolecular interactions, such as cation-π interaction and hydrogen bonding, have been employed to produce polymer coacervate complexes. In this work, we report an unconventional coacervate formation between uncharged poly(ethylene glycol) (PEG) and inorganic polyoxometalate (POM) polyanions in LiCl aqueous solution. Composition analysis has confirmed that the dense coacervate consists of PEO and POM with little amount of LiCl in contrast to the higher concentration of LiCl in the supernatant than its bulk concentration, suggesting the entropic contribution to the liquid-liquid separation. More interestingly, the measured electric potential of PEG indicates that PEG is positively charged in the presence of multivalent POM polyanions. Additionally, considerable amounts of heat release are measured upon PEG-POM coacervation by isothermal titration calorimetry, indicating a highly exothermic process. As combined, we speculate a cation-mediated coacervation between neutral PEG and POM polyanions, in which both enthalpy and entropy contribution are significant, distinct from conventional entropy-driven polyelectrolyte coacervation.