SERS INVESTIGATIONS ON THE ADSORPTION OF PYRIDINE CARBOXYLIC-ACIDS ON SILVER - INFLUENCE OF PH AND SUPPORTING ELECTROLYTE

The surface geometries of the three isomeric pyridinecarboxylic acids (PCA) (picolinic acid (2-PCA), nicotinic acid (3-PCA) and isonicotinic acid (4-PCA)) on polycrystalline silver electrodes were deduced from their SER spectra. Special emphasis was thereby placed on the variation of the adsorption behaviour with the change of coadsorbed electrolyte anion. In a non specifically adsorbing sulphate electrolyte one detects the three PCA anions as the surface active species in the entire pH and potential range investigated. Nicotinic and isonicotinic acid adopt a planar alignment, while picolinic acid adsorbs perpendicularly with both functional units (N-atom of the heteroaromatic ring and carboxylate group) coordinated to the metal surface. In the presence of specifically adsorbing chloride ions only the interaction of isonicotinate in a perpendicular alignment was observed. In the case of 2- and 3-PCA no stable surface signals could be obtained. The repulsion between the negatively charged surface and the negative charge on the carboxylate substituent permits an interaction between substrate and investigated molecules merely in the case of maximum charge separation. The identification of the adsorbed molecular species was performed by comparison to the pH dependent solution Raman data. The surface geometries were determined by application of the SERS surface selection rule and chemical interpretation of the pH dependence of the SER spectra.