Ion Selectivity in Uncharged Tapered Nanoslits through Heterogeneous Water Polarization

TL;DR

This study uses molecular dynamics to reveal how uncharged tapered nanoslits exhibit ion selectivity and electro-osmotic flow driven by water polarization heterogeneity, independent of surface charge.

Contribution

It uncovers a novel mechanism where ion selectivity and flow are caused by water polarization divergence in uncharged nanoslits, challenging traditional surface charge theories.

Findings

Ion selectivity depends on electric field direction.

Electro-osmotic flow occurs regardless of field direction.

Water polarization heterogeneity drives ion transport.

Abstract

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 channel walls and the associated electric double layer, we do observe a 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. Additionally, 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. However, in uncharged tapered nanoslits, we uncover a novel mechanism for these phenomena, where ion selectivity arises from the divergence of the heterogeneous water polarization.