Impedance spectroscopy and video-image pendant-drop method are used to evaluate the capacity and interfacial tension of the polarised interface between NaCl in water and tetrabutylammonium tetraphenylborate or tetraheptylammonium tetraphenylborate in 1,2-dichloroethane. It is shown that the interfacial tension and capacity data coincide in a limited range of potentials close to the potential of zero charge (PZC). In this potential range, an effect of the organic cation is observed, which can be ascribed to a change of the inner layer thickness associated with a change of the radius of the excess ion, rather than to the specific adsorption of the tetrabutylammonium cation. This conclusion is supported by the density-functional theory calculations for three common cations and two common anions, which allowed us to estimate the maximum and minimum distance of the plane of the closest approach of the ion to the liquid-liquid boundary. The observed difference between the potential of the capacity minimum and PZC is discussed. (c) 2005 Elsevier B.V. All rights reserved.