ADSORPTION OF BENZOIC-ACID ON A POLYCRYSTALLINE GOLD ELECTRODE

Adsorption of benzoic acid on a polycrystalline Au electrode, obtained by electroplating of gold, has been studied in 0.1 M HClO4 using cycl ic voltammetry and radiotracer technique. Adsorption has been found to take place in the entire range of studied potentials, from 0.05 to 1. 75 V (rhe), with the surface concentration of the adsorbate exceeding 5 x 10(14) molecules cm-2 at saturation. Desorption into a clean suppo rting electrolyte is small and extremely slow. On the other hand, surf ace/bulk exchange of benzoic acid is much faster, attesting to the dyn amic equilibrium between the adsorbed and solution species. Adsorption data and model calculations strongly indicate that two different orie ntations of the adsorbed molecules are present on the surface. Flat (p arallel to the surface) orientation dominates at less positive potenti als while the vertical (perpendicular to the surface) orientation domi nates at more positive potentials. Regardless of orientation, benzoic acid adsorption on gold falls into the chemisorption category. General behavior of the system bears close resemblance to the adsorption of b enzoic acid on platinum that was reported earlier in Zelenay and Sobko wski, Electrochim. Acta 29, 1715 (1984).