Thermochemical Measurements of Alkali Cation Association to Hexatantalate

Abstract

Ion association is an important process in aqueous dissolution, precipitation, and crystallization of ionic inorganic, organic, and biological materials. Polyoxometalates (POMs) are good model compounds for understanding the complex relationships between lattice energy, ion-pairing in solution, and salt solubility. Here we perform calorimetric measurements to elucidate trends in cluster stability, lattice energy, and ion-pairing behavior studies of simple hexatantalate salts in neat water, parent hydroxide solutions, and molybdate melts, extending previous studies on the isostructural hexaniobates. High temperature calorimetry of alkali salts of hexatantalate reveals that the enthalpies of formation from oxides of the K, Rb, and Cs salts are more similar to each other than they are for their niobate analogues and that the tantalate cluster is energetically less stable than hexaniobate. Aqueous dissolution calorimetry reveals that the cesium salt of hexatantalate has a similar concentration dependence on its dissolution enthalpy to that of hexaniobate. However, unlike rubidium hexaniobate, rubidium hexatantalate also exhibits increased concentration dependence, indicating that hextantalate can undergo increased ion-pairing with alkali salts other than cesium, despite the dilute environments studied. Dissolution enthalpies of POM salts in the parent alkali hydroxides shows that protonation of clusters stabilizes lattices even more than the strongly associating heavy alkali cations do. Additionally, neither weak nor strong lattice ion associations necessarily correlates with respectively high or low aqueous solubility. These studies illuminate the importance of considering ion-pairing among the interrelated processes in the aqueous dissolution of ionic salts that can be extended to serving as a model of cation association to metal oxide surfaces.

Computational Prediction of Speciation Diagrams and Nucleation Mechanisms: Molecular Vanadium, Niobium, and Tantalum Oxide Nanoclusters in Solution

“…This evidence is in good agreement with the enthalpy data reported experimentally. 52 However, DFT formation constants cannot be directly used to predict the aqueous speciation because theoretical constants are overestimated with respect to the experimental values. 48,49 For molybdenum and tungsten oxides, we have recently found a strong linear dependence between DFT and empirical formation constants that allows the application of a systematic linear scaling.…”

Aggregation Patterns in Low- and High-Charge Anions Define Opposite Solubility Trends

“…Many open-ended questions about ion solvation and formation of ion pairs still remain . Although the overall energetics of dissolution can be measured and quantified, it is difficult to isolate the individual energetic contributions of ion pairing, solvation, and protonation/deprotonation that occur upon dissolution of a POM salt. , Ion pairing and ion aggregation occur in many solution types, intuitively in highly concentrated solutions but also in low concentrations . The behavior of POMs in aqueous solutions and at interfaces is guided by numerous complex and interconnected phenomena; for instance, countercation type, concentration, and distribution influence the stability and self-assembly of POMs.…”