Influence of solvent on the interfacial structure of self-assembled alkanethiol monolayers

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) is used to study the structure of octadecanethiol, tetradecanethiol, and decanethiol monolayers in the presence of air, deuterium oxide, and acetonitrile-d(3). It is found that the structure of the monolayer is dependent on both the adjacent environment and the initial structural order of the monolayer. In the D2O solutions, spectral changes are similar for the three monolayers and are characteristic of interaction predominantly between the solvent and the terminal portion of the monolayers. This correlates with electrochemical capacitance measurements that indicate little permeation of the aqueous electrolyte in the monolayer, even with the largely disordered decanethiol monolayer. With acetonitrile solutions, the three monolayers have different behaviors. The octadecanethiol monolayer changes little from the ex situ structure, indicating little interaction between the solvent and the monolayer. Tetradecanethiol and decanethiol, however, have a significant increase in the relative intensity of the asymmetric methyl vibration, compared to the ex situ value, characteristic of a restructuring of the monolayer. These spectral data suggest that solvation of the increasingly defective structure of the shorter alkanethiol monolayers occurs and is responsible for the structural behavior, consistent with electrochemical results.