Adsorption of pyridine at the Au (100)-solution interface

Chronocoulometry has been used in order to characterize quantitatively the energetics of pyridine adsorption onto a gold (100) single crystal electrode surface. Over the potential region investigated (−0.8 to+0.6 V), three orientations of the pyridine molecules on the gold surface have been observed. The pyridine orientation is influenced strongly by the electrode potential. At a positively charged surface, the pyridine assumes a vertical orientation with the nitrogen atom facing the gold surface. A limiting surface concentration corresponding to 6 × 10−10 mol cm−2 was determined for the vertical orientation. At a negatively charged surface and at low surface concentrations (Γ Γ > 1 × 10−10 mol cm−2) and for potentials close to zero charge, a third orientation, presumably intermediate between the flat and the vertical orientations, was observed. Reorientation from the intermediate to the vertical orientation takes the form of a phase transition. The potential of the phase transition coincides approximately with the potential of zero charge. The Gibbs energies of adsorption, electrosorption valencies, potentials and charges of the maximum adsorption were determined for the flat and the vertical orientation of pyridine. Adsorption of pyridine on the Au (100) surface is compared with the adsorption at polycrystalline Au and mercury electrodes.