Interfacial activity of the diprotonated 222 cryptand at the water/"oil" interface revealed by molecular dynamics simulations

According to molecular dynamics (MD) simulations the diprotonated bicyclic 222(2+) cryptand molecules concentrate on the water side of a water-chloroform interface. This is found for its endo-endo and exo-exo isomers and with Cl(-), I(-) or Pic(-) as counterions. The positively charged interfacial cryptands attract the halide anions which also concentrate at the interface, in spite of their hydrophilic character. The Pic(-) anions, often used in extraction or transport experiments, display spectacular stacking arrangements in bulk water, as well as on the aqueous side of the interface. The results are important for the mechanism of assisted cation extraction from water, as well as for electrical properties of the interface. They demonstrate how solvation at the inherently asymmetrical interface induces bidimensional concentration of charged "hydrophobic" solutes.