With size between simple ions and large colloidal suspensions, macroions exhibit completely different solution behaviors. The size disparity between macroions and their counterions, and the consequent counterion association and counterion-mediated attraction, are essential features of macroions and continuously surprising us by demonstrating new phenomena. Previously, hollow spherical blackberry structures from various macroions, e.g., polyoxometalates, metal-organic cages, functionalized fullerenes and dendrimers, are well reported. Nowadays, two unexpected phases, hydrogel and coacervate, are observed in dilute aqueous solution of fully hydrophilic nanoscale uranyl peroxide clusters (U60 and U24Pp12) in presence of di- or tri-valent salts. Full phase diagrams are constructed and the mechanisms behind them are clarified mainly as a result of strong counterion-mediated attraction. The imbalance between macroions and small counterions leads to counterion association around macroions and the resulted counterion-mediated attraction as well as anisotropic 2-D single-layer supramolecular sheets. The nanosheets further bend and enclose to form hollow, spherical blackberry-type structures with weak counterions; while when introducing stronger counterions, the rigid nanosheets stand along in dilute aqueous solution, leading gelation with significant excluded volume. At elevated ionic strength, such electrostatic-based gels further collapse into coacervate phase, due to the closer stacking of nanosheets. The fascinating phase behavior of nanoscale hydrophilic macroions are expected to be general for macroions, which indicates the unique features of soluble ions with nanoscale sizes, distinguishing them from simple ions and large colloids.

Hydrogel and coacervate formation from uranyl peroxide clusters through counterion-mediated attraction

ACS Spring 2022

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