Surface Barrier Properties of Self-Assembled Monolayers as Deduced by Sum Frequency Generation Spectroscopy and Electrochemistry

This paper presents a model on the electrochemical behaviors of n -alkanethiol self-assembled monolayers (SAMs) on gold electrodes with different chain length (C10, C12, C16, and C18) using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and sum frequency generation spectroscopy (SFG). The SAMs studied reached their maximum density with a deposition time of ≈72 h according to the kinetics results shown by EIS Bode phase plots. For shorter chain SAMs, C10 and C12 monolayers, the results from CV, EIS, and SFG were consistent with each other. Significant amounts of thiols were removed by reductive desorption of the films for C10 and C12 SAMs, as indicated by decreases in the electrochemical waves and SFG peaks. Surprising differences were observed for longer-chain SAMs made from C16 and C18 monolayers. The CV results suggested that a large amount of the C16 and C18 monolayers chemically desorbed from the gold surface, while the EIS and SFG results illustrated that C16 and C18 layers still remained in the vicinity of the double-layer region near the gold surface and retained their two-dimensional, densely packed ordered structure, respectively. Only small decreases in phase angle were observed from EIS, and all the CH vibrational modes were nearly unchanged in SFG spectra for C16 and C18 SAMs. Furthermore, SFG orientation analysis of the C16 and C18 SAMs revealed a molecular structure for C16 and C18 SAMs after 30 CV cycles similar to that of their original films. Cyclic voltammograms also showed no observable oxidative readsorption waves for C10 and C12 SAMs, while for C16 and C18 monolayers detectable reoxidation peaks appeared. The results from EIS, SFG, and CV strongly suggest that the commonly accepted notion, the formation of aggregates and micelles for alkanethiol SAMs after reductive desorption, does not apply to the long-chain SAMs that were studied. After applying the potential to the long-chain SAMs, both the chemically desorbed and intact SAM molecules maintained molecular structures similar to that of the freshly prepared monolayers. The persistent crystallinity of longer-chain SAMs against negative potentials could be explained by dense packing and much lower solubility of the molecules in aqueous solutions. Implication for corrosion inhibited by the monolayer is discussed.