An anomalous anion transfer order in graphene oxide membranes induced by anion-π interactions

Selective transport of anions across membranes has become an important goal in chemistry and biology. Here, we found an anomalous anion transfer order within the graphene oxide membrane: Cl^- > Br^- > F^- > I^-. This is at odds with the conventional ranking of the transfer order, which usually decreases as the radii of the anions increase, i.e., F^- > Cl^- > Br^- > I^-. The abnormal transportation of F^- can be ascribed to the strong anion-π interactions between F^- and graphene oxide sheets. Such unexpectedly strong anion-π interaction resulted in the lower movement of F^- in the graphene oxide membrane and caused the anomalous anion transfer order. Our findings not only provide experimental evidence of anion-π interactions, but also improve our understanding of anion-π interactions in the selective transport of anions across a two-dimensional membrane.