A New Strategy for Highly Efficient Separation between Monovalent Cations by Applying Opposite-Oriented Pressure and Electric Fields.

Biological ion channels exhibit excellent ion selectivity, but it has been challenging to design their artificial counterparts, especially for highly efficient separation of similar ions. Here, a new strategy to achieve high selectivity between alkali metal ions with artificial nanostructures is reported. Molecular dynamics (MD) simulations and experiments are combined to study the transportation of monovalent cations through graphene oxide (GO) nanoslits by applying pressure or/and electric fields. It is found that the ionic transport selectivity under the pressure driving reverses compared with that under the electric field driving. Moreover, MD simulations show that different monovalent cations can be separated with unprecedentedly high selectivity by applying opposite-oriented pressure and electric fields. This highly efficient separation originates from two distinctive ionic transporting modes, that is, hydration shells drive ions under pressure, but drag ions under the electric field. Hence, ions with different hydration strengths can be efficiently separated by tuning the net mobility induced by the two types of driving forces when the selected ions are kept moving while the other ones are immobilized. And nanoconfinement is confirmed to enhance the separation efficacy. This discovery paves a new avenue for separating similar ions without elaborately designing biomimetic nanostructures.