The Influence of Chemisorbed Organic Monolayers on Electrode Surface Oxidation

The chemisorption at monolayer coverages of various organic molecules from aqueous solutions onto smooth polycrystalline electrodes and the resistance of the resulting monolayers towards electrochemical oxidation have been studied. This survey represents the preliminary stage in a series of investigations aimed at better understanding, at the atomic level, of metal passivation by monolayer organic films. The electrodes studied (Rh, Pd, Ir, Pt, and Au) are those whose anodic dissolution is preceded by surface-oxide formation. For these metals, corrosion resistance can be associated with the retardation of the surface oxidation; hence, the extent of surface-oxide retardation is one measure of the passivation characteristics of an organic monolayer. Thirty-eight organic compounds containing at least one surface-active functional group were investigated: alkenes, alkynes, aromatics, heterocyclic aromatics, alcohols, phenols, sulfides, amines, amides, nitriles, CO, aldehydes, carboxylates, esters, and their selected derivatives. Experimental measurements were based primarily on thin-layer electrochemical methods; selected interfacial systems were investigated by x-ray photoelectron spectroscopy (XPS). The preliminary data suggest the following trends: (1) The more stable the chemisorbed organic molecule towards anodic oxidation, the more efficient its corrosion inhibition. (2) For a given electrode material, the strength of organic chemisorption decreases as: organic mercaptans > aromatic N > aromatic ring > alkyne > alkene > R₂S ≥ arylcyanides > aliphatic N > alcohols > alkylcyanides > carboxylic acids > esters > alkanes. (3) For a given surface-active functional group, the higher the molecular weight of the parent organic compound, the better the corrosion inhibition. (4) The strength of chemisorption of a given functional group varies with electrode material in the order: Pt > Pd ~ Rh ~ Ir > Au. (5) The more reactive the metal towards surface-oxide formation, the greater the extent of organic chemisorption-induced passivation.