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

Self-assembled monolayers (SAMs) were prepared by the adsorption of a serie s of 2,2-diallcylpropane-1,3-dithiols (1-5) and 2-pentadecylpropane-1,3-dit hiol (6) onto the surface of gold. These SAMs were characterized by ellipso metry, contact angle goniometry, X-ray photoelectron spectroscopy (XPS), po larization modulation infrared reflection absorption spectroscopy (PM-IRRAS ), and atomic force microscopy (NFM). The studies demonstrate that a system atic 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 t o provide control over the conformational order and interchain packing of t he hydrocarbon tail group assembly. The tail group conformation and packing were found to influence profoundly both the wettability and the tribologic al properties of the SAMs. Assemblies of well-ordered, well-packed hydrocar bon tail groups yielded interfaces that exhibited low wettabilities and low frictional responses when compared to assemblies of disordered, loosely pa cked hydrocarbon tail groups. The trends in wettability and friction were r ationalized by considering the magnitude of the van der Waals interactions between the hydrocarbon film and the contacting probe liquid and AFM tip, r espectively.