In Situ Infrared Study of the Adsorption and Surface Acid−Base Properties of the Anions of Dicarboxylic Acids at Gold Single Crystal and Thin-Film Electrodes

The adsorption of malonic and succinic acids and their anions at gold electrodes has been studied by in situ infrared spectroscopy by combining external and internal reflection experiments performed, respectively, with gold single crystal and sputtered gold thin-film electrodes deposited on silicon substrates. The in situ infrared spectra obtained in solutions with pH = 1 confirmed the potential-dependent specific adsorption of bimalonate anions in a bidentate configuration irrespective of the crystallographic orientation of the gold electrodes. The high signal-to-noise ratio associated with the surface-enhanced infrared absorption (SEIRA effect) in the case of the gold thin-film electrodes allows the observation of the carbonyl band for adsorbed bimalonate together with additional bands between 2000 and 3000 cm^-1 that can be tentatively assigned to the formation of hydrogen bonds between neighbor bimalonate anions. The intensities of all these bands characteristic of adsorbed bimalonate decrease with increasing solution pH for which adsorbed malonate anions predominate. The analysis of the intensities of the ν_as(OCO) and ν(CO) bands for the uncoordinated carboxylate/carboxylic group of the adsorbed malonate/bimalonate and succinate/bisuccinate systems allows the estimation of the pK_a value for their surface acid−base equilibria. The obtained values, around 4.8 for adsorbed malonic acid anions and around 5.3 for adsorbed succinic acid anions, are below those of the corresponding solution equilibria. The decrease of pK_a2 upon adsorption of bimalonate/malonate anions is lower than for adsorbed bioxalate/oxalate anions and higher than for the bisuccinate/succinate system. This behavior is related to the effect of the distance between the uncoordinated carboxylic group and the electrode surface on the electrostatic interaction of the former with the positively charged surface.