The peak potential Ep 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 Ep 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 Ep 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 Ep. The Ep 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 Ep 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 Ep.
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