Probing the different phases of self-assembled monolayers on metal surfaces: Temperature dependence of the C–H stretching modes

Reflection-absorption infrared spectroscopy at near grazing incidence is used to investigate the thermal stability of self-assembled monolayers of alkyl thiols, HS–(CH2)n−1-R, adsorbed on polycrystalline metal surfaces. The spectral features of the C–H stretching region have been carefully analyzed in the temperature range from 150 to 450 K. These features are highly sensitive to chain orientation and chain conformation. On gold, the chains are found to gradually untilt but remain largely all-trans up to about 350 K. Above this temperature, a broad, irreversible transition to a liquidlike phase is observed, and which is characterized by a large number of gauche conformational defects. This overall behavior did not depend upon the chain length or terminal group for the systems studied. In contrast, significant differences were observed on a silver substrate.