Self-assembly of terminally aryl-substituted long-chain alkanethiols on silver

Self-assembled monolayers (SAM) from para-substituted benzyl-16-mercaptohexadecanoates (R = -Cl; -OCH3; -NO2; -CN) were prepared using standard deposition procedures on silver substrates. The substituents were attached to the terminal benzyl group of the SAM precursor molecules in order to modify the properties of the outer film surface and to probe their influence on the organizational order of the monolayers. The SAMs were characterized by infrared spectroscopic ellipsometry and contact angle measurements. The carbonyl stretching as well as the methylene symmetric and antisymmetric stretching vibrations were employed to determine the tilt angle of the methylene chains. The water advancing contact angles implied that the wetting of the film's outermost surface is substantially different from that expected for more polar groups like NO2, CN and OCH3 and is much closer to that reported for aryl terminated SAMs. The flexible methylene linkage can facilitate the free rotation around C-Ph bond that leads to a specific orientation of the terminal phenyl rings in which they are closely exposed to the interface. An attempt was made to qualitatively characterize the phenyl ring orientations of the different SAMs using the IR spectroscopic ellipsometry data and IR absorption spectra of the same molecules.