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-mercaptop ropionic acid (MPA) on Au surface and of the adsorption of hexadecyltrimeth ylammonium chloride (C(16)TAC) on the SAM was investigated by surface-enhan ced infrared absorption spectroscopy (SEIRAS) of attenuated total reflectio n (ATR) mode. The SAM formation of MPA depended on solvents. At the first s tage of MPA adsorption in a chloroform-d solution, hydrogen-bonded MPA coex isted with free (non-hydrogen-bonded) MPA which was partly protonated. Duri ng the process of the adsorption, the free nonprotonated MPA was protonated or hydrogen-bonded. After the abundant SAM formation, alkyl backbone was r earranged. With adsorption in an ethanol solution, the hydrogen-bonded MPA SAM increased in content without any structural rearrangement. No free spec ies 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 o f C(16)TAC on the MPA SAM obeyed the kinetics where the first adsorption at an early stage is followed by the slow adsorption. ATR-SEIRAS indicated th at there are the adsorption of C(16)TAC on the MPA SAM and the transition o f carboxylic acid to carboxylate. Finally, ion pairs connected by the elect rostatic interaction are formed.