Role of charge regulation and flow slip on the ionic conductance of nanopores: an analytical approach

TL;DR

This paper presents an analytical model for ionic conductance in nanopores that incorporates charge regulation, flow slip, and electro-osmotic effects, helping to explain diverse experimental conductance behaviors.

Contribution

It introduces a comprehensive analytical approach that accounts for multiple mechanisms affecting nanopore conductance, including charge regulation and flow slip, which were not simultaneously considered before.

Findings

The model fits experimental data for carbon nanotubes.

Different transport regimes are identified based on salt concentration.

The approach explains variability in conductance versus salt concentration.

Abstract

The number of precise conductance measurements in nanopores is quickly growing. In order to clarify the dominant mechanisms at play and facilitate the characterization of such systems for which there is still no clear consensus, we propose an analytical approach to the ionic conductance in nanopores that takes into account (i) electro-osmotic effects, (ii) flow slip at the pore surface for hydrophobic nanopores, (iii) a component of the surface charge density that is modulated by the reservoir $p$H and salt concentration $c_s$ using a simple charge regulation model, and (iv) a fixed surface charge density that is unaffected by $p$H and $c_s$. Limiting cases are explored for various ranges of salt concentration and our formula is used to fit conductance experiments found in the literature for carbon nanotubes. This approach permits us to catalog the different possible transport regimes and propose an explanation for the wide variety of currently known experimental behavior for the conductance versus $c_s$.