In Situ Investigation of Molecular Adsorption on Au Surface by Surface-Enhanced Infrared Absorption Spectroscopy

The kinetics of the self-assembled monolayer (SAM) formation of 3-mercaptopropionic acid (MPA) on Au surface and of the adsorption of hexadecyltrimethylammonium chloride (C16TAC) on the SAM was investigated by surface-enhanced infrared absorption spectroscopy (SEIRAS) of attenuated total reflection (ATR) mode. The SAM formation of MPA depended on solvents. At the first stage of MPA adsorption in a chloroform-d solution, hydrogen-bonded MPA coexisted with free (non-hydrogen-bonded) MPA which was partly protonated. During the process of the adsorption, the free nonprotonated MPA was protonated or hydrogen-bonded. After the abundant SAM formation, alkyl backbone was rearranged. With adsorption in an ethanol solution, the hydrogen-bonded MPA SAM increased in content without any structural rearrangement. No free species and no protonated species were observed. The molecular orientation was also different between SAMs in chloroform-d and in ethanol. The dependence of the CH2 antisymmetric stretching vibration band intensity on adsorption time was examined on the basis of the adsorption kinetics. It was confirmed that the MPA SAM formation on Au surface proceeds according to the simple Langmuir (monolayer) adsorption theory. On the other hand, the adsorption of C16TAC on the MPA SAM obeyed the kinetics where the fast adsorption at an early stage is followed by the slow adsorption. ATR-SEIRAS indicated that there are the adsorption of C16TAC on the MPA SAM and the transition of carboxylic acid to carboxylate. Finally, ion pairs connected by the electrostatic interaction are formed.