Electrodesorption Kinetics and Molecular Interactions at Negatively Charged Self-Assembled Thiol Monolayers in Electrolyte Solutions

The electrodesorption of self-assembled monolayers of methyl-terminated alkanethiols, 11-mercaptoundecanonic acid, and 3-mercaptopropionic acid adsorbed on Au(111) has been investigated by scanning tunneling microscopy and electrochemical methods. Results show that 11-mercaptoundecanonic acid and 3-mercaptopropionic acid form √3×√3 R30° and c(4×2) lattices similar to those found for methyl-terminated thiols. For both methyl- and COO--terminated alkanethiols, the electrodesorption process follows a combined adsorption−desorption nucleation and growth-of-holes model under charge-transfer control. The nucleation rate of holes at COO--terminated alkanethiol adlayers is smaller than at methyl-terminated alkanethiol adlayers, which explains the slower electrodesorption kinetics. The activation energy to remove a COO--terminated alkanethiol molecule from the edge of a growing hole is smaller than that of a methyl-terminated molecule because of the repulsive forces introduced by the negatively charged groups. The magnitude of these repulsive forces, which is partially compensated for by ion binding between the COO- groups and the counterions in the electrolyte, depends on both the nature of the cation and the pH.