  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.
