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Thermochemical Measurements of Alkali Cation Association to Hexatantalate

Department of Chemistry, Oregon State University, Corvallis, OR 97331-4003, USA
Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA
Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California Davis, Davis, CA 95616, USA
Authors to whom correspondence should be addressed.
Academic Editor: Santiago Reinoso
Molecules 2018, 23(10), 2441;
Received: 6 September 2018 / Revised: 18 September 2018 / Accepted: 19 September 2018 / Published: 24 September 2018
(This article belongs to the Special Issue Polyoxometalates and Polyoxometalate-Based Systems)
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. View Full-Text
Keywords: polyoxometalates; hexatantalate; tantalum; countercations; ion-pairing; calorimetry; solubility polyoxometalates; hexatantalate; tantalum; countercations; ion-pairing; calorimetry; solubility
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MDPI and ACS Style

Sures, D.J.; Nagabhushana, G.P.; Navrotsky, A.; Nyman, M. Thermochemical Measurements of Alkali Cation Association to Hexatantalate. Molecules 2018, 23, 2441.

AMA Style

Sures DJ, Nagabhushana GP, Navrotsky A, Nyman M. Thermochemical Measurements of Alkali Cation Association to Hexatantalate. Molecules. 2018; 23(10):2441.

Chicago/Turabian Style

Sures, Dylan J., G. P. Nagabhushana, Alexandra Navrotsky, and May Nyman. 2018. "Thermochemical Measurements of Alkali Cation Association to Hexatantalate" Molecules 23, no. 10: 2441.

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