Thermodynamic, Kinetic, Surface pKa, and Structural Aspects of Self-Assembled Monolayers of Thio Compounds on Gold

The thermodynamic and kinetic aspects of the formation of self-assembled monolayers (SAMs) of thio compounds on gold have been studied via electrochemical and quartz crystal microbalance (QCM) techniques. The data indicate that the adsorption process involves a significant free energy of adsorption (Delta G degrees = 36.43 kJ/mol) and that there are slight repulsive interactions between adjacent molecules on the surface. A method for the calculation of surface pK(a) values of molecules containing more than one protonation site is proposed and used for the determination of the pK(a) values of SAMs derived from thioisonicotinamide, thionicotinamide, 5-(4-pyridyl)-1,2,4-oxadiazole-2-thiol, and 4-mercaptopyridine (pyS) on gold. Structural aspects of the SAMs were studied by using impedance with [Fe(CN)(6)](4-/3-) as redox probe. Evidence of faster kinetics for an oxidative decomposition of pyS SAM in the presence of [Fe(CN)(6)](3-) is discussed based on electrochemical and impedance data.