Polyoxometalate Keggin Anions at Aqueous Interfaces with Organic Solvents, Ionic Liquids, and Graphite: a Molecular Dynamics Study

A recent computational study (Chaumont, A; Wipff, G. Phys. Chem, Chem. Phys. 2008, 10, 6940) showed that in spite of their high charge and spherical shape, polyoxometalate alpha-PW12O40 Keggin anions PW3 can "attract each other" and form oligomers in water. This led us to speculate that these anions should also display surface-active properties, a feature, supported by molecular dynamics results presented here at aqueous interfaces with an organic solvent (chloroform), with three ionic liquids (ILs) and a solid (graphite), Simulations have been performed with 20 PW3- per box and, in sonic cases, with different Mn+ counterions (Cs+, H3O+, H5O2+, NBu4+, Eu3+), At the chloroform interface, the highest PW3- activity (90-100%) is found with NBu4+ counterions, while with all other counterions, it is ca. 50%, The results, obtained in "standard conditions" (juxtaposed solvent boxes simulated with additive potentials and TIP3P water) are with. tests using either bigger boxes, SPC/E water, or a polarizable force field and with mixing/demixing experiments. PWs are also attracted at aqueous interfaces with [XMI][Y] ionic liquids, with marked effects of their constitutive XMI+ (butymethylimidazolium BMI+ or octylmethylimidazolium OMI+) and Y- (PF6- or Tf2N-) ions. The most spectacular result is found with the [OM1[PF6] IL where nearly all PWs adsorb at the interface, attracted by interfacial OMI+ cations, while PF6- anions solubilize in the bulk water ("anion exchange mechanism"). PWs also adsorb onto the neutral graphite surface, without forming a saturated monolayer, though. As expected, when the graphite surface gets positively charged, PW adsorption is enhanced. These results have bearing oil the supramolecular organization and reactivity of polyoxometalate anions at surfaces, as well as oil the mechanism of ion exchange between water and ILs.