Simulating the Favorable Aggregation of Monolacunary Keggin Anions

We here present a series of classical molecular dynamics simulations (MD) on aqueous solutions of the salts Li₅AlW₁₂O₄₀ and Li₉AlW₁₁O₃₉, providing us with valuable insight on their aggregative behavior. Analysis of relative dipole moment orientation in pairs of aggregated [AlW₁₁O₃₉]^9- excludes that their large dipole moment is behind their greater propensity to aggregate. On the other hand, MD simulations of the aqueous Li⁺ salt of the fictitious [AlW₁₂O₄₀]^9--as high in charge as [AlW₁₁O₃₉]^9-, but lacking dipole moment and tetrahedral in shape like [AlW₁₂O₄₀]^5--reveal that it is in fact the higher negative charge itself that promotes aggregation, by allowing to recruit a higher number of Li⁺ countercations, which then act as an electrostatic glue. The lower charge on [AlW₁₂O₄₀]^5-, on the other hand, is not able to muster enough Li⁺ countercations for it to aggregate favorably.