ELSEVIER Journal of Electroanalytical Chemistry 385 (1995) 257-263 30URNAL OF The surface potential shift upon adsorption at the air [solution and the uncharged Hg [solution interfaces P. Nikitas, A. Pappa-Louisi Laboratory of Physical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, 54006 Thessaloniki, Greece Received 13 June 1994; in revised form 20 September 1994 Abstract A generalized expression for the potential drop across an adsorbed film developed in earlier work is used to interpret the adsorption potential shift at the free water surface and at the uncharged mercury Isolution interface when they are composed of adsorbate and solvent molecules. It is shown that according to the theory the inverse 1/A x of the adsorption potential shift should vary linearly with the inverse 1/0 of the surface coverage by the adsorbate. This prediction is verified in all the experimental systems analysed. The ratio p = eA/eS, where e A and e s are the "distortional" dielectric constants of the adsorbed layer when 0 = 1 and 0 = 0 respectively, is calculated from the intercept on the ordinate and the slope of the linear 1/A X vs. 1/0 plot. It is found that p < 1 at the uncharged Hg Isolution interface and p > 1 at the free water surface. In order to explain this difference various models of the adsorbed layer are analysed and discussed. Keywords: Adsorption potential shift; Interfaces; Mathematical models 1. Introduction The surface potential shift A X due to adsorption is an important parameter manifesting the orientation of adsorbed molecules in the interracial region [1-4]. The systematic study of the dependence of AX on the surface coverage 0 by a neutral organic adsorbate at the free water surface and at the uncharged Hg ]aque- ous solution interface has revealed a striking differ- ence. At the uncharged Hg ]solution interface the AX vs. 0 curves are concave upwards, whereas these curves are either linear or concave downwards at the air Isolu- tion interface [1,3,5-14]. Up to now several attempts have been made to explain the physical meaning of the different variation patterns of the adsorption potential shift at the two interfaces. The Russian school of Frumkin, Damaskin and coworkers [1,7,8] attributed this difference to the different models valid at the two interfaces. In particu- lar, the lack of equipotential surfaces which define the boundaries of the adsorbed layer at the free water surface causes the appearance of a non-uniform elec- tric field perpendicular to these surfaces and devia- tions from Frumkin's model of two parallel condensers. However, this explanation is purely qualitative. More- 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0022-0728(94)03782-5 over, the fact that the two-parallel-condensers model predicts AX vs. 0 curves concave upwards does not necessarily mean that deviations from that model lead to AX vs. 0 curves that are concave downwards. A qualitative interpretation of the differences in the surface potential shift at the two interfaces has also been attempted by Trasatti and coworkers [3]. Accord- ing to this group, the different behaviour of the two interfaces should be attributed to differences in the intermolecular interactions which lead to a depen- dence of water orientation and structure on the inter- face. We believe that reorientation phenomena of the water molecules may explain the different magnitude of A x at the free surface and the Hg Isolution inter- face, but not the concavity of the AX vs. 0 curves. Schuhmann [15,16] was the first to attempt to use electrostatic theory to give a quantitative description of the A X vs. 0 plots at the free water surface. However, his first treatment [15] is based on an erroneous appli- cation of Coulomb's law to a system of two dielectrics. In particular, Eq. (15) of ref. [15] is valid only when a sheet of point dipoles is located within a dielectrically uniform medium. In contrast, in a system of two semi- infinite dielectrics, where a sheet of dipoles is located in one of them and is parallel to their boundary, the