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.