Interactions between Keggin Anions in Water: The Higher Their Charge, the Higher Their Condensation? A Simulation Study. Issue 10 (29th October 2012)

• Record Type: Journal Article
• Authors: Chaumont, Alain; Wipff, Georges
• Other Names: Kortz Ulrich sponsoringEditor.; Liu Tianbo sponsoringEditor.
• Is Part Of: European journal of inorganic chemistry. Issue 10/11(2013)
• Page Start: 1835
• Page End: 1853
• Publication Date: 2012-10-29
• Subjects: Molecular dynamics -- Solvation -- Ionic interactions -- Polyoxometalates -- Counterion effects -- Interfaces; Chemistry, Inorganic -- Periodicals; Organometallic chemistry -- Periodicals; Bioinorganic chemistry -- Periodicals; Solid state chemistry -- Periodicals; 546
• DOI: 10.1002/ejic.201200883
• Languages: English
• ISSNs: 1434-1948

Abstract

Abstract: We report molecular dynamics (MD) simulations of aqueous solutions of Keggin heteropolyacids XW n – n H3 O + (XW n – = [PW12 O40 ] 3–, [SiW12 O40 ] 4–, and [AlW12 O40 ] 5– ), the aim of which was to investigate whether the anions increasingly repulse each other in water when their charge increases from –3 to –5. The MD and PMF (potential of mean force) results indicate that this is not the case. All three anions are found to display short XW n – ··· XW n – contacts ("dimers"), the amount of which increases at increasing concentrations. However, the proportion of dimers follows the sequence SiW 4– < PW 3– < AlW 5–, which is unexpected from interanionic Coulomb forces. The least‐charged anions are mainly attracted by hydrophobic forces, whereas the most charged AlW 5– anions afford the highest proportion of dimers through specific interactions with H3 O + counterions. Similar features are observed in two series of fictitious analogues of XW n – ( n = 3 to 5): the S n – spherical anions, and the XW n + cations (with all atomic charges of XW n – inverted and "H3 O – " as counterions). The key role of counterion condensation on the stabilization of the most negatively charged dimers is further demonstrated by comparing Li + salts to Na + and Cs + salts of the different XW n – anions: anion–anion dimerization is found to be highest between AlW 5– anions and in the presence of the "hardest" counterions (Li + ). Likewise, with Eu 3+ multivalent AlW 5– counterions condense into a concentrated ionic domain in water. Finally, all three types of XW n – anions are found to adsorb at the aqueous interface with graphite. These findings are important for understanding processes like dimerization, condensation, third phase formation, and the assembly of polyoxometalates and macroions in water or at aqueous interfaces. Abstract : In water, [AlW12 O40 ] 5– anions are shown to form dimers and oligomers by a higher amount than their less charged [PW12 O40 ] 3– or [SiW12 O40 ] 4– analogues. This counterintuitive feature is due to specific counterion condensation onto the most charged dimers. Microscopic insights into the effects of anion granularity, sign of the charge, concentration, counterions, and solvation are presented.