Conformations of an amino–amido–thiolate self-assembled layer on gold in air and in electrolytes

The amino–amido–thiol AT (HS(CH2)10CONHCH2CH2NH2), adsorbed on Au(111), was studied by sum frequency generation (SFG) and infrared reflection absorption spectroscopy in the CH2 stretching vibration region. The SFG and infrared spectra are qualitatively similar; both show the symmetric stretching band and an intense and unusually broad antisymmetric stretching band. The infrared spectra show a reversible difference between a sample dried after rinsing in electrolyte and dried after rinsing in ethanol, thus exhibiting a ‘solvent memory’ effect. SFG spectra demonstrate that the dried samples have a high density of conformational defects. The defects are located in the C10 methylene chain. We ascribe them to intermolecular hydrogen-bonding. When the layer is studied by SFG spectroscopy in aqueous solution, even in the presence of coordinating Pd2+ ions no vibrational bands are observed in the CH2 stretching vibration region. This indicates a strongly reduced defect density with a straightened molecular backbone. We ascribe this to the solvation of the amine group; in this case intermolecular bonds play a minor role.