Coacervate Formation in Dilute Aqueous Solution of 2.5-nm Size Molecular Macroanions with Simple Di- or Trivalent Cations*

Complex coacervates are commonly thought to be formed from the liquid-liquid phase separation of two oppositely charged polyelectrolytes in aqueous solutions, due to mainly the electrostatic attraction followed by entropy-driven release of small counterions near the polyelectrolytes. Here, we report a completely unique coacervate formed by a water soluble uranyl peroxide molecular cluster (Li₆₈K₁₂(OH)₂₀)[UO₂(O₂)-OH]₆₀, U₆₀) in the presence of only di- or trivalent countercations. Three distinct phases: homogenous solution, biphasic coacervate and gel, are observed in dilute aqueous solution with varied salt-to-U₆₀ molar ratios. To the best of our knowledge, this is the first example of coacervation between only macroion clusters and small simple counterions without any large-sized polyelectrolytes involved. The formation of the coacervate by the U₆₀/di/trivalent cations is confirmed by combined techniques, including turbidity, rheology and transmission electron microscopy (TEM). Unlike the conventional polyelectrolyte complexation, the U₆₀ coacervate complexes can be formed over a much broader range of salt-to-U₆₀ molar ratio. The dependence of coacervate formation on the salt-to-U₆₀ molar ratio is attributed to the balance of the non-covalent interactions. Moreover, TEM characterization reveals large, thin-film-like nanosheet structures of the coacervates. The imbalance between macroions and small multivalent counterions leads to counterion association around U₆₀^60- macroions and consequently the counterion-mediated attraction among macroions, resulting in their assembly into 2-D nanosheet structures. In the presence of stronger di-or trivalent counterions, the rigid nanosheets do not close to form well-studied hollow, spherical, single-layered blackberry structures, by stay open and stack closely forming gel and coacervates in dilute aqueous solutions.