Standalone 2-D Nanosheets and the Consequent Hydrogel and Coacervate Phases Formed by 2.5 nm Spherical U Molecular Clusters in Dilute Aqueous Solution.

Unexpected hydrogel and coacervate are observed for dilute (1 mM) uranyl peroxide molecular cluster (LiK(OH)[UO(O)(OH)], U) solution in the presence of di- or trivalent salts. We report the mechanism as the formation of anisotropic two-dimensional (2-D) single-layer nanosheets, driven by counterion-mediated attraction due to the size disparity between U and small counterions. With weak monovalent cations, the nanosheets are bendable, resulting in hollow, spherical blackberry-type supramolecular assemblies in a homogeneous solution. With extra strong divalent or trivalent cations, the tough, free-standing sheets lead to gelation at ∼1 mM U. These stiff nanosheets are difficult to bend into spherical blackberry-type structures; instead, they stay in solution and form hydrogel based on their significant excluded volumes. At higher ionic strength, the large, thin filmlike nanosheet structures stack together more compactly and consequently lead to the transition from gel phase to a coacervate phase, another surprise since it was formed without the presence of bulky polycations.