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

O. AZZARONI; M.E. VELA; H. MARTIN; A. HERNÁNDEZ-CREUS; , G.ANDREASEN; R.C. SALVAREZZA

LANGMUIR

Año: 2001 p. 6647 - 6654

0743-7463

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 sqrt3 x sqrt 3 R30° and c(4x2) lattices similar to those found for methyl-terminated thiols. For both methyl- andCOO--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.