Potentiostatic Infrared Titration of 11-Mercaptoundecanoic Acid Monolayers

In situ IR difference spectra of 11-mercaptoundecanoic acid (MUA) monolayers deposited on gold, and in the presence of a NaF solution in deuterated water, have been recorded as a function of both solution pH* and substrate’s potential. IR spectra recorded under voltammetric conditions indicate that the potential-induced ionization of the monolayer is a slow process, involving a simultaneous rearrangement of the hydrocarbon chains, and that a significant population of carboxylic groups are not dissociated at positive potentials, even in the presence of a pH* 9 solution. Steep potentiostatic IR titration curves, consistent with a monolayer pKa = 4.3, are obtained at negative potentials. As the potential is made more positive, titration curves become lower, broader and are shifted towards more basic pHs, as would be expected when only a fraction of the carboxylic groups become ionized in the presence of less favorable interactions. Rather than acting directly on the electrostatic energy of the carboxylate groups, the electrode potential seems to control the ionization of the monolayer by attracting (or repelling) cations from the electrolyte and by reorienting the thiol head groups inside and outside the low permittivity layer of methylene chains.