Electrophoresis of ions and electrolyte conductivity: from bulk to nanochannels

TL;DR

This paper revisits the theory of electrolyte conductivity, accounting for ion mobility reduction with salt concentration, and extends it to confined nanochannels, providing a comprehensive framework for interpreting conductivity measurements.

Contribution

It introduces a generalized theoretical framework for electrolyte conductivity that includes ion mobility reduction and applies it to both bulk and nanochannel environments.

Findings

Derived a more accurate expression for bulk conductivity up to several molars.

Extended the formalism to electrolyte solutions in charged nanochannels.

Provided a framework for interpreting conductivity measurements in confined geometries.

Abstract

When electrolyte solutions are confined in micro- and nanochannels their conductivity is significantly different from those in a bulk phase. Here we revisit the theory of this phenomenon by focusing attention on the reduction in the ion mobility with the concentration of salt and a consequent impact to the conductivity of a monovalent solution, from bulk to confined in a narrow slit. We first give a systematic treatment of electrophoresis of ions and obtain equations for their zeta potentials and mobilities. The latter are then used to obtain a simple expression for a bulk conductivity, which is valid in a concentration range up to a few molars and more accurate than prior analytic theories. By extending the formalism to the electrolyte solution in the charged channel the equations describing the conductivity in different modes are presented. They can be regarded as a generalization of prior work on the channel conductivity to a more realistic case of a nonzero reduction of the zeta potential and electrophoretic mobility of ions with salt concentration. Our analysis provides a framework for interpreting measurements on the conductivity of electrolyte solutions in the bulk and in narrow channels.