Adsorption properties of polycrystalline and single crystal electrodes and the influence of surface inhomogeneity

It is shown that the adsorption of some neutral organic compounds on individual faces of a zinc single crystal can be described by the Frumkin-Damaskin theory with the use of the two parallel capacitors model. For a polycrystalline electrode a departure from the two parallel capacitors model is observed which is due to the energetic inhomogeneity of the surface. A method is suggested for calculation of the adsorption isotherm, and of the dependence of the surface coverage θ on the potential of a polycrystalline electrode, from capacity measurements data under the condition that for all n crystallographic faces (grains) exposed on the surface, the difference (C_on ? C′_n) is the same (C_on and C′_n are the double layer capacities in the supporting electrolyte solution and at θ = 1 of the n-th face, respectively). An adsorption isotherm is proposed for a uniformly inhomogeneous surface which takes account of the attractive interaction between adsorbate particles. Some methods are suggested for evaluation of the surface inhomogeneity factor (f). This factor has been determined from adsorption data for cyclohexanol and n-amyl alcohol. The differential capacity curves measured on polycrystalline zinc in the presence of cyclohexanol are in good agreement with the calculated curve for a uniformly inhomogeneous surface.