Enhanced transport of ions by tuning surface properties of the nanochannel

TL;DR

This paper develops a theoretical framework to understand how surface properties like wettability and charge mobility influence ion transport in nanochannels, revealing regimes where conductivity can be significantly enhanced or limited.

Contribution

It introduces a unified theory linking surface wettability, charge mobility, and electrostatic conditions to ion transport regimes and conductivity scaling in nanochannels.

Findings

High conductivity in hydrophilic channels explained by the theory.

Conductivity amplification possible with hydrophobic slip in thick channels.

Weakly charged surfaces show limited conductivity enhancement.

Abstract

We revisit the theory of ion transport in parallel-plate channels and also discuss how the wettability of a solid and the mobility of adsorbed surface charges impact the transport of ions. It is shown that depending on the ratio of the electrostatic disjoining pressure to the excess osmotic pressure at the walls two different regimes occur. In the thick channel regime this ratio is small and the channel effectively behaves as thick, even when the diffuse layers strongly overlap. The latter is possible for highly charged channels only. In the thin channel regime the disjoining pressure is comparable to the excess osmotic pressure at the wall, which implies relatively weakly charged walls. We derive simple expressions for the mean conductivity of the channel in these two regimes, highlighting the role of electrostatic and electro-hydrodynamic boundary conditions. Our theory provides a simple explanation of the high conductivity observed experimentally in hydrophilic channels, and allows one to obtain rigorous bounds on its attainable value and scaling with salt concentration. Our results also show that further dramatic amplification of conductivity is possible if hydrophobic slip is involved, but only in the thick channel regime provided the walls are sufficiently highly charged and the most of adsorbed charges are immobile. However, for weakly charged surfaces the massive conductivity amplification due to hydrodynamic slip is impossible in both regimes. Interestingly, in this case the moderate slip-driven contribution to conductivity can monotonously decrease with the fraction of immobile adsorbed charges. These results provide a framework for tuning the conductivity of nanochannels by adjusting their surface properties and bulk electrolyte concentrations.