Crystal-face specificity of electrical double-layer parameters at metal/solution interfaces

The structure of the boundary region between a metal and a solution (the so-called electrical double layer) depends on the chemical nature of the metal and of the solvent, the type and amount of solute, and the structure of the metal surface. This article is focussed on the effect of varying the crystallographic orientation of the metal surface of an electrode on 'classical' double-layer parameters such as (i) potential of zero charge, (ii) its temperature coefficient, (iii) the interfacial permittivity (capacitance), (iv) and interfacial intermolecular interactions (adsorption). Similarities and differences between UHV experiments and related electrochemical situations are first scrutinized. The components of the electrode potential are analysed and their relevance to gas-phase experiments is discussed. Discrepancies between UHV and electrochemical data related to the same situation are explained in terms of temperature of experiment, lack of potential control and deviation of the structure of the surface solvent layer from bulk properties. A few experimental problems related to the preparation of single crystals, their use in an electrochemical cell, and the instrumental technique are then described, pointing to some ambiguous aspects. An interfacial parameter, ΔX, measuring the extent of the modifications occurring in the surface potential at the surface regions of the two phases as they are brought into contact, is derived from potential of zero charge–work function plots. For metal electrodes in aqueous solutions, ΔX is proportional to the metal–water interaction strength, although it is not only related to the surface water dipole orientation. ΔX is shown to correlate quite well with other double-layer parameters for different metals. The crystal-face specificity of the double-layer parameters is also correlated to ΔX, which thus turns out to be a unifying parameter for predictive analysis. The crystal-face specificity of capacitance and adsorption is discussed on the basis of several examples.