Journal of Electroanalytical Chemistry 509 (2001) 148 – 154 www.elsevier.com/locate/jelechem Capacitance and ionic association at the electrified oil  water interface: the effect of the oil phase composition Carlos M. Pereira a , Fernando Silva a , Maria J. Sousa a , Kyo ¨ sti Kontturi b , Lasse Murtoma ¨ki b, * a Department of Chemistry, Uniersity of Porto, Rua Campo Alegre, 687, 4169 -007 Porto, Portugal b Laboratory of Physical Chemistry and Electrochemistry, Helsinki Uniersity of Technology, P.O. Box 6100, FIN-02015 HUT, Helsinki, Finland Received 23 February 2001; received in revised form 1 May 2001; accepted 7 May 2001 Abstract The capacitance of the polarisable aqueous  organic interface was measured with the impedance method, varying both the solvent and the electrolyte of the organic phase. The results were analysed using the idea of interfacial ion-pairing presented earlier (J. Chem. Soc. Faraday Trans. 87 (1991) 107). It appeared that this model was capable of explaining the measured capacitances quite well for 2-heptanone, 2-octanone and 1,3-dibromopropane as the organic solvent, while for 1,2-dichloroethane — which is perhaps the most common solvent used in liquid  liquid electrochemistry — the model needed to be modified, assuming a mixed layer between the bulk solvent phases as suggested earlier in the literature. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Liquid  liquid interface; Electrochemistry; Capacitance; Ion-pairing 1. Introduction Electrochemistry of the immiscible aqueous  organic interface (ITIES) has now been practised for almost 30 years, but due to the difficulties in applying spectro- scopic techniques at this interface, the basic question of the molecular structure of the interface is still unre- solved. Quite recently, preliminary results from second harmonic generation [1], quasi-elastic light scattering [2] and neutron reflection [3] have been obtained. Theoret- ical treatments of the interfacial structure have been extended from continuum models, such as the modified Verwey – Niessen theory [4], to discrete models, like the lattice gas model [5] or molecular dynamics simulations [6]. The classical way of studying interfacial properties is to measure the capacitance which also provides the point of comparison for theoretical models. The de- tailed analysis of differential capacitance originates from the late 1940s to the 1960s in the papers of Grahame and Parsons (e.g. Refs. [7,8]) who studied the mercury  aqueous interface in the absence and presence of specific adsorption. In the paper of Cheng et al. [9], their results were transposed to apply to the water  1,2- dichloroethane interface, omitting inner layer contribu- tions but interpreting the marked effect of the alkali ion on the capacitance curve as the result of interfacial association of ions. Later Pereira et al. [10] corrobo- rated the dependence of the capacitance on the nature of the ions in the two adjacent solutions. Further, they attempted to develop a model of interfacial ion associa- tion and the formation of a mixed boundary layer with an average dielectric constant [11,12]. As Girault and Schiffrin showed in their thermodynamic analysis of the ITIES [13], the total charge at the interface must in- clude also the contribution of the ion-pairs at the interface. We continue that analysis in this communica- tion, and report some capacitance measurements for several organic phase compositions. The simple macro- scopic model used here gives some insight of the inter- facial structure, at least on a qualitative level. * Corresponding author. Tel.: +358-9-451-2581; fax: +358-9-451- 2580. E-mail address: lasse.murtomaki@hut.fi (L. Murtoma ¨ki). 0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0022-0728(01)00528-9