Reductive desorption of carboxylic-acid-terminated alkanethiol monolayers from Au(111) surfaces

The peak potential E_p of the reductive desorption for COOH-terminated alkanethiols at the gold(111)|0.1 M KOH solution interface has been studied at several different alkyl chain lengths and compared with those for n-alkanethiols. The desorption of COOH-terminated alkanethiols is also confirmed by the increase in the capacitance of gold|solution interface and the reflectivity of gold electrode surface. The E_p for both CH₃- and COOH-terminated alkanethiols shift by 15 mV to the negative direction per methylene group as the alkyl chain length n increases. The E_p vs. n plots for the two series of alkanethiols are parallel with each other, suggesting that the chemical interaction of thiol head group with gold, the chain-chain attractive interaction, and the electrostatic repulsion between the carboxylate groups make an additive contribution to the determination of E_p. The E_p values for COOH-terminated alkanethiols are 110mV more positive than those for n-alkanethiols having the same alkyl chains, owing in part to the repulsive interaction between the negatively-charged carboxylate groups in a COOH-terminated alkanethiol monolayer. The positive shift of E_p for mercaptopropionic acid with decreasing concentration of KOH solution also suggests the primary role of the electrostatic repulsive interaction between carboxylate groups in the shift of E_p.