Ion aggregation in concentrated aqueous and methanol solutions of polyoxometallates Keggin anions: the effect of counterions investigated by molecular dynamics simulations

Abstract

Aqueous solutions of the polyoxometallate α-PW₁₂O₄₀³⁻ Keggin anion “PW³⁻” have been simulated by molecular dynamics, comparing two anion concentrations (0.06 and ca. 0.15 mol l⁻¹) and Cs⁺, NBu₄⁺, UO₂²⁺, Eu³⁺, H₃O⁺ and H₅O₂⁺ as neutralizing Mⁿ⁺ counterions. They reveal marked counterion effects of the degree of salt dilution, cation–anion and anion–anion relationships. The hydrophobic NBu₄⁺ cations tend to surround PW³⁻’s via loose contacts, leading to “phase separation” between water and a humid salty domain, overall neutral, where all ions are concentrated. The other studied cations are more hydrophilic and generally separated from the PW³⁻ anions. The most important finding concerns the aggregation of PW’s, mostly as dimers with short contacts (P⋯P < 12 Å), but also as higher (PW³⁻)_n oligomers (n = 3 to 5) in concentrated solutions where the proportion of the aggregates ranges from ca. 9 to 46%, depending on the counterion. While Eu³⁺ and UO₂²⁺ are fully hydrated and interact at short distances with PWs as solvent-separated ion pairs, Cs⁺ can form contact ion pairs, as well as solvent-separated ions. Among the mono-charged counterions, H₅O₂⁺ gives highest aggregation (ca. 47%, involving 32% of dimers, 11% of trimers and 3% of tetramers), pointing to the influence of the proton state (H₅O₂⁺vs. H₃O⁺) on PW’s aggregation and condensation. The dynamic properties are also dependent on Mⁿ⁺: the PW’s diffusion coefficients are lowest with NBu₄⁺, and highest for Cs⁺, thus reflecting the degree of ion condensation in water. The role of water on the solution state of the PW salts is further demonstrated by simulating the most concentrated systems in methanol solution. Because MeOH solvates less well the Mⁿ⁺ cation than does H₂O and cannot afford bridging relays between PW’s, one finds a higher proportion of PW³⁻⋯Mⁿ⁺ contacts, and no (PW³⁻)_n oligomers with short contacts in methanol.