On the structure of charged interfaces

Abstract Former models of the electric double layer, and several alternative possibilities, lack consistency with six well-known facts. The paper attempts to adduce models which allow interpretation of these. The potential, not the charge, is the dependent variable in the double layer. The distortion dielectric constant of the water adsorbed on an electrode is six, not n∞2. The constant minimum capacity on the negative branch is interpreted in terms of a model in which Gouy adsorption begins outside the first layer of water molecules. Super-equivalent adsorption is purely electrostatic in nature and becomes appreciable if the ionic radius is > 1.4 Å. The isotherm for this type of adsorption is treated in terms of lateral repulsion. The model is shown to give rise to a correct dependence of amount adsorbed on charge. The ‘capacitance hump’ arises from the slowing down of the rate of increase of the charge on the anions in the Helmholtz layer with increasingly positive charge on the metal due to repulsion among the anions. Some properties of the oriented dipole layer on electrodes are treated. The capacitance contributions due to change in the potential contribution of such dipoles to the Galvani p.d. with change in electrode charge is negligible. There is no unique value of Xdipole, for this is potential dependent. The maximum value, calculated for extreme values of charge on the electrode, is < 100 mV. The adsorption of aliphatic substances in terms of numerous models is discussed. Some newly available facts are inconsistent with theories previously discussed. They are consistent with a ‘competition with water’ model in which the essential feature is the turning over of water molecules on the negative side of the e.c.m.