  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.
