Ionic association and Wien effect in 2D confined electrolytes

TL;DR

This paper investigates ionic association and non-linear transport in 2D nanochannels, revealing a Kosterlitz-Thouless-like transition, Wien effect modifications, and ionic Coulomb blockade, with implications for nanofluidic device design.

Contribution

It introduces a combined molecular dynamics and analytical theory approach to understand ionic pairing and non-linear conduction in 2D confined electrolytes, highlighting new phenomena.

Findings

Ions form pairs or clusters analogous to a Kosterlitz-Thouless transition.

Electric field induces non-linear ionic conduction via pair breaking.

Conductivity exhibits non-universal, temperature-dependent scaling with electric field.

Abstract

Recent experimental advances in nanofluidics have allowed to explore ion transport across molecular-scale pores, in particular for iontronic applications. Two dimensional nanochannels -- in which a single molecular layer of electrolyte is confined between solid walls -- constitute a unique platform to investigate fluid and ion transport in extreme confinement, highlighting unconventional transport properties. In this work, we study ionic association in 2D nanochannels, and its consequences on non-linear ionic transport, using both molecular dynamics simulations and analytical theory. We show that under sufficient confinement, ions assemble into pairs or larger clusters in a process analogous to a Kosterlitz-Thouless transition, here modified by the dielectric confinement. We further show that the breaking of pairs results in an electric-field dependent conduction, a mechanism usually known as the second Wien effect. However the 2D nature of the system results in non-universal, temperature-dependent, scaling of the conductivity with electric field, leading to ionic coulomb blockade in some regimes. A 2D generalization of the Onsager theory fully accounts for the non-linear transport. These results suggest ways to exploit electrostatic interactions between ions to build new nanofluidic devices.