Double layer on silver single crystal electrodes in contact with electrolytes having anions which are slightly specifically adsorbed: Part II. The (100) face

Abstract

Double-layer measurements for a (100) face of silver in NaF, NaClO 4 and KPF 6 solutions are analysed to obtain quantitative data on the structure of the solid electrode-aqueous electrolyte interface. It is found that the specific adsorption increases as O<PF 6 − <F − <ClO 4 − Surface concentrations of the specifically adsorbed ions are calculated for ClO 4 − and F − in NaClO 4 + KPF 6 and NaF + KPF 6 solutions. These specific absorptions are shown to characterize essentially the property of the (100) face. and not that of the defects of the surface. For the (100) Ag/KPF 6 system, the electrical double-layer model in the absence of specific adsorption is verified; the incomplete dissociation of KPF 6 is taken into account. The (100) Ag pzc is −0.865±0.005 V (SCE). The high inner-layer capacities are interpreted considering the influence of the atomic distribution of the ideal (100) place on the arrangement of water molecules at the interface. Weak metal-solvent interactions at the pzc are deduced from the variation of the inner-layer capacity as a function of the electrode charge. For mixed solutions of NAF + KPF 6, and NaClO 4 + KPF 6, the differential capacity is resolved into its components with the aid of diffuse-layer theory. The two components of the total inner-layer capacity, at constant electrode charge and constant amount adsorbed, are also determined. From this analysis, it is shown that the ionic size is responsible for the lowering of the differential capacity in the region of positive charges when fluoride is replaced by perchlorate although the specific adsorption of perchlorate is larger than that of fluoride.