Spiroalkanedithiol-based SAMs reveal unique insight into the wettabilities and frictional properties of organic thin films

Self-assembled monolayers (SAMs) were prepared by the adsorption of a series of 2,2-diallcylpropane-1,3-dithiols (1-5) and 2-pentadecylpropane-1,3-dithiol (6) onto the surface of gold. These SAMs were characterized by ellipsometry, contact angle goniometry, X-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and atomic force microscopy (NFM). The studies demonstrate that a systematic variation in the structure of R'RC(CH2SH)(2) from symmetrical (1; R' = R) to progressively more unsymmetrical (2-6; R' not equal R) can be used to provide control over the conformational order and interchain packing of the hydrocarbon tail group assembly. The tail group conformation and packing were found to influence profoundly both the wettability and the tribological properties of the SAMs. Assemblies of well-ordered, well-packed hydrocarbon tail groups yielded interfaces that exhibited low wettabilities and low frictional responses when compared to assemblies of disordered, loosely packed hydrocarbon tail groups. The trends in wettability and friction were rationalized by considering the magnitude of the van der Waals interactions between the hydrocarbon film and the contacting probe liquid and AFM tip, respectively.