Aggregation Patterns in Low- and High-Charge Anions Define Opposite Solubility Trends

Molecular dynamics simulations in aqueous solution reveal the existence of two distinct patterns of aggregation in low and high charge density Lindqvist-type polyoxometalates (POMs). Our results indicate the presence of contact and solvent-shared ion pairs and specific and preferential interactions of alkalis with POMs. Highly charged POMs are capable of breaking apart the Li⁺ and Cs⁺ solvation shell, thus enhancing the formation of long-lived alkali–POM contact ion pairs, where alkalis act as an electrostatic “glue” forming large oligomers. Stronger ion pair interactions for Li⁺ than for Cs⁺ promote lower solubility for Li⁺ than for Cs⁺, evoking anomalous solubility trends. Lower charge density POMs are not capable of disrupting the Li⁺ solvation shell and only solvent-shared ion pairs are formed, whereas for Cs⁺, contact ion pairs exist. The large number of oxygen atoms in the POM surface enhances the hydrogen bonds between POM and water, thus promoting aggregation. In this case, aggregation follows normal solubility trends. Thus, aggregation depends on the strength of ion pair interactions, the capacity of POM to disrupt alkali’s solvation shell, and the contact surface area between the solvent and POM.