The electronic capacitor at a metal–electrolyte interface

Abstract

A model for the dense part of electrical double layer is proposed using the concept that conduction electrons penetrate into a solution to form an electronic capacitor on the metal surface. The potential drop between the metal and solution, the charge of the metal, its surface tension, and the electron work function for a metal–solution interface are calculated within the framework of the model, and the formulae derived are compared with experiment. It is shown that for a metal–vacuum interface of 38 metals in the left-hand subgroups of the Mendeleev table the discrepancy between the theoretical and experimental values of surface tension and work function does not exceed the experimental error (i.e. it is less than 10%); for six metals in the right-hand subgroups and especially for semimetals the theoretical error is two to three times higher. The density of free electrons in a metal determined in terms of the concepts of the model is shown to vary monotonously with the element number for all 54 metals with available experimental data.A relationship, previously unknown, between surface tension and zero-charge potential is established, which enabled one to calculate the electronic capacitor charge for mercury (the theoretical value is 33 C/cm2, and the experimental value is 36–38 C/cm2). This paper also reports the calculated values of the integral capacity of a mercury electrode in water: the experimental value is 29 F/cm2 (at the zero-charge point), and the theoretical value is 28 F/cm2. The model predicts an increase of the capacity in the anodic region and a decrease in the cathodic region, in a good agreement with experiment. It should be stressed that although the theory includes only one fitting parameter, the density of free electrons in the metal, it correctly explains a wide range of phenomena.