Field-enhanced selectivity in nanoconfined ionic transport

Abstract

Fluid transport confined in nanochannels shows ultrafast permeation and highly efficient separation performance. However, the size-controlled selectivity of hydrated ions with a similar valence and size, such as alkali ions, is well below 5. We propose in this work to boost ion selectivity through the interaction with the wall of flow channels, which can be enhanced by applying an external electric field across the channel. Molecular simulations show that for ions diffusing near the walls of a graphene nanochannel, the hydration shells are perturbed, endowing the contrast in ion–wall interactions to modify the ion-specific free energy landscape. The trapping/hopping nature of ion diffusion near the wall leads to the conclusion that the diffusivity depends on the free energy barriers rather than the hydration size. This effect can be magnified by elevating the field strength, yielding more than ∼10-fold enhancement in the diffusivity-specific selectivity. With recent experimental advances in external electric field control and local electric field modulation near the surface, this work demonstrates a possible route to achieve high selectivity of alkali ions in nanofluidics, and explore the molecular structures and dynamics of hydrated ions near a surface.