An adsorption model for double-layer capacitance

We propose an adsorption model based on experimental results on the frequency, potential, concentration and temperature behaviour of the double-layer capacitance of several Hg/electrolyte contacts. The model is derived from thermodynamic and chemical relaxation principles and leads to a frequency dependence equivalent to the Debye relations for the complex permittivity of macroscopic dielectrics. The potential dependence of the differential capacitance predicted by the model is similar to that of specific adsorption and displays two humps located symmetrically about the zero-charge potential. The dependence on electrolyte concentration shows a well-defined maximum located at a concentration which is given by the reciprocal adsorption constant. The temperature dependence appears to be almost exponential near the zero-charge potential. Investigation of the abovementioned parameters offers possibilities for evaluating adsorption constant data from which some structural conclusions can be drawn.