INFLUENCE OF SOLVENT ON THE INTERFACIAL STRUCTURE OF SELF-ASSEMBLED ALKANETHIOL MONOLAYERS

Polarization modulation infrared reflection absorption spectroscopy (P M-IRRAS) is used to study the structure of octadecanethiol, tetradecan ethiol, and decanethiol monolayers in the presence of air, deuterium o xide, and acetonitrile-d(3). It is found that the structure of the mon olayer is dependent on both the adjacent environment and the initial s tructural order of the monolayer. In the D2O solutions, spectral chang es are similar for the three monolayers and are characteristic of inte raction predominantly between the solvent and the terminal portion of the monolayers. This correlates with electrochemical capacitance measu rements 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 beha viors. The octadecanethiol monolayer changes little from the ex situ s tructure, indicating little interaction between the solvent and the mo nolayer. 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 incre asingly defective structure of the shorter alkanethiol monolayers occu rs and is responsible for the structural behavior, consistent with ele ctrochemical results.