Ionic transport through sub-10 nm diameter hydrophobic high-aspect ratio nanopores: experiment, theory and simulation

TL;DR

This study combines experiment, theory, and simulation to understand ionic transport in hydrophobic nanopores with diameters from 1 to 10 nm, revealing the influence of surface charge and ion solvation on conductance.

Contribution

It introduces a hybrid mesoscopic model for electrolyte conductivity in nanopores that accounts for electro-osmotic flow and flow slip, validated by experimental data and molecular dynamics simulations.

Findings

Conductance plateaus at low salt concentrations are explained by a weak surface charge.

Higher surface charge densities are observed in smaller nanopores due to ion solvation effects.

The model accurately fits experimental conductance data across a range of nanopore sizes.

Abstract

Fundamental understanding of ionic transport at the nanoscale is essential for developing biosensors based on nanopore technology and new generation high-performance nanofiltration membranes for separation and purification applications. We study here ionic transport through single putatively neutral hydrophobic nanopores with high aspect ratio (of length L=6 \mu m with diameters ranging from 1 to 10 nm) and with a well controlled cylindrical geometry. We develop a detailed hybrid mesoscopic theoretical approach for the electrolyte conductivity inside nanopores, which considers explicitly ion advection by electro-osmotic flow and possible flow slip at the pore surface. By fitting the experimental conductance data we show that for nanopore diameters greater than 4 nm a constant weak surface charge density of about 10$^{-2}$ C m$^{-2}$ needs to be incorporated in the model to account for conductance plateaus of a few pico-Siemens at low salt concentrations. For tighter nanopores, our analysis leads to a higher surface charge density, which can be attributed to a modification of ion solvation structure close to the pore surface, as observed in the molecular dynamics simulations we performed.