Stability of Keplerate polyoxometalate macroanionic assemblies in salt-containing aqueous solutions

The solution stability of hydrophilic Keplerate polyoxometalate {Mo72Fe30} macroions was investigated at various ionic strengths. The solution stability varies with the type of monovalent cations with the lowest stability with large, weakly hydrated monovalent cations. Even lower stability was observed when the counter-cations were multivalent cations. Interestingly, the stability of {Mo72Fe30} solutions was found to increase with increasing POM concentration. The close association of monovalent counter-ions around macroions exists in aqueous solution and is likely enhanced at higher macroion concentrations. This behavior can further decrease the net charge on the macroions leading to a decreased electrostatic repulsion, which decreases the screening length of the macroions and causes them to irreversibly precipitate.