State of counter-cations in polyoxometalate solutions

The interaction between water-soluble Keplerate Polyoxometalate (POM) macroions and small counter-cations is explored by laser light scattering, anomalous small-angle X-ray scattering (SAXS), and isothermal titration calorimetry (ITC) techniques. Small Angle X-ray scattering (SAXS) is used to study the distribution of monovalent cations around 2.5-nm-diameter "Keplerate" polyoxomolybdate {Mo72V30} macroanions. Small angle X-ray scattering data indicates that most of the counter-ions are "free" in solution at low macroion concentrations. The association of counter-ions was found to increase with increasing macroion concentration and with decreasing solvent polarity. A close connection between the counter-ion association and the self-assembly of the {Mo72V30} into the "blackberry" supramolecular structures is observed. Polyoxometalates macroions exhibit a preference for the type of associated counter-cations based upon the counter-ions' valence and hydration, when multiple types of additional cations are present in solution. Additionally a critical salt concentration (CSC) for each type of cation exists for the blackberry formation of {Mo72Fe30 } macroions, above which the blackberry size increases significantly with the increasing total ionic strength in solution. The CSC values are much smaller for cations with higher valences, and also decrease with the cations' hydrated size for various monovalent cations. The solution stability of hydrophilic Keplerate polyoxometalate {Mo72Fe30} macroions was investigated at various ionic strengths and was found to vary with the type of monovalent cations and the macroion concentration.