Adsorption of 2-thiobarbituric acid at the electrochemical interface: Contrasted behaviours on mercury and gold

Abstract

The adsorption of 2-thiobarbituric acid (TBA) from acid solutions (pH ~1) has been investigated at the Au(111) single crystal electrode and at the dropping mercury electrode. In contrast to the gold electrode where the adsorption is driven by the strong interaction between Au and the S atom, the TBA adsorption at the mercury electrode is more contrasted and distinct features such as capacitance pits are observed in the capacitance–potential curves. The kinetics of phase transformation were investigated by the potential step technique. The formation of films B and C prevailing in two distinct capacity pit regions is controlled by a nucleation and growth mechanism. Ordering of film A observed at the positive end of the potential region is also evidenced on the basis of the charge transients associated with its formation from film B or C. The two successive sigmoidal parts in the charge transient obtained at small overpotentials for the transformation of C to A is explained by the presence of an intermediate metastable film.