Distinctive Interactions of Cesium and Hexaniobate in Water

The Cs-effect states Cs has more covalent character in bonding interactions than the lighter alkalis. It is exploited in organic synthesis and influences behavior in water, most notably radioactive Cs in nuclear wastes or the environment. Niobium polyoxometalates (Nb-POMs) provide a unique opportunity to probe aqueous phase ion-pairing responsible for cesium’s solution behavior, because Nb-POMs are most soluble in conditions of maximum ion-association. Moreover, POMs broadly resemble metal-oxide surfaces representative of interfaces found in the environment and industrial processes. Aqueous dissolution calorimetry reveals that Cs Nb-POM exhibits greater concentration dependence in its endothermic dissolution, compared to the lighter alkali analogues. This phenomenon is attributed to persistent ion-pairs upon dissolution, even in very dilute and otherwise ion-free solutions. While dissociation of these cation-anion interactions in the crystalline lattice is the dominant endothermic step of dissolution, deprotonation of the Nb-POM is the most exothermic. These studies highlight the importance of the competing effects of aqueous ion association and acid-base chemistry that control solubility of compounds from simple oxoanions to metal-oxo clusters to supramolecular assemblies to solid metal oxides.