A Two-Step Intermolecular Interaction Of Molecular Macroions With Multivalent Counterions

Macroion-counterion interaction is essential for regulating the solution behaviors of hydrophilic macroions, as simple models for polyelectrolytes. Here, we explore the interaction between uranyl peroxide molecular cluster Li₆₈K₁₂(OH)₂₀[UO₂(O₂)OH]₆₀ (U₆₀) and multivalent counterions. Different from interaction with monovalent counterions that shows a simple one-step process, isothermal titration calorimetry, combined with light/X-ray scattering measurements and electron microscopy, confirm a two-step process for their interaction with multivalent counterions: an ion-pairing between U₆₀ and the counterion with partial breakage of hydration shells followed by strong U₆₀-U₆₀ attraction, leading to the formation of large nanosheets with severe breakage and reconstruction of hydration shells. The detailed studies on macroion-counterion interaction can be nicely correlated to the microscopic (self-assembly) and macroscopic (gelation or phase separation) phase transitions in the dilute U₆₀ aqueous solutions induced by multivalent counterions.