Monolayers of a series of terminally substituted alkyl thiols, X(CH2)15SH (X = CH3, CH2OH, CO2H, CO2CH3, and CONH2), have been prepared by adsorption from solution onto evaporated gold substrates. The structures have been characterized by infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), ellipsometry, and temperature-programmed desorption (TPD). The IR data shows the monolayer films to be densely packed, crystalline-like structures with all-trans conformation alkyl chains exhibiting average tilt angles of the chain axis in a range of 28-40° from the surface normal and an approximate 55° twist of the chain axis away from a configuration with the CCC plane perpendicular to the surface plane. TPD results set a lower limit of interaction between the CH2 groups in the films of ∼0.8 kcal/mol. We find that the terminal groups are exposed at the ambient interface, and, based on an analysis of the IR data, an assessment of the conformations and molecular environments of these groups is made. The CO2H terminal groups appear to exist in short hydrogen-bonded (perhaps dimeric) sequences. Detailed examination of the IR data shows some unexplained abnormalities with respect to the quantitative treatment of the terminal functional group spectra. These effects can be traced to the perturbations which occur when a group is placed at an interface relative to the environment experienced in a crystalline solid. The major feature of this study is the finding that the reported series of derivatives form a structurally self-consistent set of well-defined reagent surfaces suitable for use in further studies of the surface properties of organic materials.
ASJC Scopus subject areas
- Colloid and Surface Chemistry