Impedance spectroscopy of ions at liquid-liquid interfaces

TL;DR

This paper presents a new two-step fitting method for impedance spectroscopy that accurately separates bulk and interfacial properties of liquid-liquid interfaces, improving analysis precision and robustness.

Contribution

A novel two-step fitting procedure for impedance spectra that enhances the accuracy of interfacial property extraction in liquid-liquid systems.

Findings

The method effectively separates bulk and interfacial elements.

It avoids overfitting of bulk properties and underfitting of interfacial properties.

The approach is robust and applicable to various liquid-liquid interface systems.

Abstract

The possibility to extract properties of an interface between two immiscible liquids, e.g., electrolyte solutions or polyelectrolyte multilayers, by means of impedance spectroscopy is investigated theoretically within a dynamic density functional theory which is equivalent to the Nernst-Planck-Poisson theory. A novel approach based on a two-step fitting procedure of an equivalent circuit to impedance spectra is proposed which allows to uniquely separate bulk and interfacial elements. Moreover, the proposed method avoids overfitting of the bulk properties of the two liquids in contact and underfitting of the interfacial properties, as they might occur for standard one-step procedures. The key idea is to determine the bulk elements of the equivalent circuit in a first step by fitting corresponding sub-circuits to the spectra of uniform electrolyte solutions, and afterwards fitting the full equivalent circuit with fixed bulk elements to the impedance spectrum containing the interface. This approach is exemplified for an equivalent circuit which leads to a physically intuitive qualitative behavior as well as to quantitively realistic values of the interfacial elements. The proposed method is robust such that it can be expected to be applicable to a wide class of systems with liquid-liquid interfaces.