Ion selectivity in uncharged tapered nanoslits through heterogeneous water polarization

We employ molecular dynamics simulations to investigate ion and water transport driven by an electric field through quasi-two-dimensional nanoslits with a tapered geometry. Despite the absence of surface charge on the (non-polarizable) channel walls and the associated electric double layer, we do observe robust ion selectivity. This selectivity favors the transport of cations from base to tip when the electric field is directed from base to tip, and anions from base to tip when the field direction is reversed. In addition, we observe a corresponding electro-osmotic water flow from base to tip, regardless of the electric field direction. Intriguingly, ion selectivity and electro-osmotic flow are conventionally associated with surface charge and electric double layers. Here, however, we uncover a novel mechanism for these phenomena in uncharged tapered nanoslits, where ion selectivity arises from the divergence of the heterogeneous water polarization.