Molecular Clusters in Dilute Aqueous Solution*

Comparing with simple ions and large colloidal suspensions, macroions exhibit completely different solution behaviors since the size disparity between them and their counterions, and the resulted counterion association and counterion-mediated attraction. 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 with di- or tri-valent salts. Full phase diagrams are constructed and the mechanisms behind them are given which mainly associated with counterion-mediated attraction. The imbalance between macroions and small counterions leads to counterion association around macroions, the consequent counterion-mediated attraction as well as anisotropic nanosheets. These nanosheets further bend and enclose to form hollow, spherical blackberry-type structures with mono-valent counterions; while when introducing di/tri-valent counterions, the rigid nanosheets stand along in dilute aqueous solution, leading gelation with significant excluded volume. Coacervates and precipitates phenomena can be detected at higher ionic strengths as the result of 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.