Non-equilibrium electric double layers at the interface between two electrolytes

TL;DR

This paper extends the Verwey-Niessen theory to analyze non-equilibrium electric double layers at electrolyte interfaces with ion fluxes, revealing their surprising stability and the formation of opposing space charge regions under strong electric fields.

Contribution

It introduces an analytical framework for non-equilibrium electric double layers using matched asymptotic expansions, highlighting new phenomena like net interfacial charge and stability.

Findings

Electric double layers remain stable even under strong electric fields.

Opposing space charge regions form when the field cancels charge cloud fields.

The interface can acquire a net charge affecting flow stability.

Abstract

The charge distribution at the interface between two electrolytes is studied for the case of non-vanishing ion fluxes. The analysis is an extension of the established Verwey-Niessen theory to non-equilibrium situations. Applying matched asymptotic expansions for the region of the electric double layer and the bulk electrolytes, analytical expressions for the ion concentrations and the electrostatic potential are derived. It is found that the electric double layer is surprisingly stable. Even in cases where the applied electric field is so strong that it completely cancels the field of the charge clouds at the interface, two opposing space charge regions are formed. This phenomenon is qualitatively explained using a simple model incorporating ion partitioning and diffusive ion transport. Another notable phenomenon is that the interface acquires a net charge in the presence of an applied electric field, being of relevance for the hydrodynamic stability of interfacial flows.