Capacitance of the interface between two immiscible electrolyte solutions – A controversial issue

Abstract

Electrochemical impedance spectroscopy is used to evaluate the capacitance of the polarizable interface between a solution of LiCl + HCl in water and a solution of bis(triphenylphospho-ranylidene)ammonium tetrakis(pentafluorophenyl)borate in 1,2-dichloroethane (DCE).A significant effect of both the interfacial potential difference and the electrolyte concentration on the capacitance is observed using two different experimental arrangements. The former effect is supported by an independent evaluation of the capacitance as the second derivative of the interfacial tension vs. the potential difference plot. It is shown that the experimental data can be reproduced relatively well by means of the Gouy-Chapman theory and the modified Verwey-Niessen model of the electric double layer consisting of two back-to-back space charge regions separated by an inner layer of the solvent molecules. However, the effect of the electrolyte concentration points to an easy penetration of ions into the inner layer leading to its negative contribution to the inverse capacitance. These conclusions are at variance with those made recently on the basis of the impedance measurements at the micro-hole supported water/DCE interface. A tentative explanation is proposed, which refers to the possible absence of the direct control of the active area of the liquid/liquid interface shape and position in the micro-hole over a broad range of the potential differences.