Structure, wettability, and frictional properties of phenyl-terminated self-assembled monolayers on gold

Alkanethiols possessing terminal phenyl groups (C6H5(CH2)(n)SH, n = 12-15) were adsorbed onto the surface of gold to afford phenyl-terminated self-assembled monolayers (SAMs). The SAMs were characterized by optical ellipsometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), atomic force microscopy (AFM), and contact angle goniometry. The films generated from the phenyl-terminated alkanethiols exhibited greater thicknesses but similar crystallinities and well-ordered lattice structures when compared to analogous SAMs generated by the adsorption of normal alkanethiols onto gold. Advancing contact angle measurements using water as the test liquid supported the presence of interfacial phenyl moieties. Furthermore, contact angle measurements using the test liquids methylene iodide (MI), dimethyl formamide (DMF), and nitrobenzene (NB) revealed an odd-even effect as a function of the number of methylene units underneath the terminal phenyl groups. The tribological properties of the phenyl-terminated films were characterized by AFM and compared to those of films derived from normal alkanethiols and other materials presenting aromatic hydrocarbon moieties at the interface (i.e., graphite and C-60). The phenyl-terminated SAMs exhibited a substantially higher frictional response than graphite, a slightly higher frictional response than normal alkanethiol SAMs, but a much lower frictional response than C-60-terminated SAMs.