Molecular-level insight into the frictional properties of fluorinated self-assembled monolayers (SAMs) was achieved by combining two recently developed techniques that operate at the subnanometer scale:  control of the interfacial composition through molecular self-assembly and tribological measurements performed with the atomic force microscope. To explore the origin of frictional forces in fluorinated films, the frictional properties of two classes of alkanethiols adsorbed on single crystal gold were measured and compared. In these studies, films of equivalent chain length, packing density and packing energy, but different termination (methyl vs trifluoromethyl), were characterized and investigated. For these films, in which the only detectable difference was the outermost chemical structure/composition, a factor of 3 increase in the frictional response was observed in going from the hydrogenated to the fluorinated film. These results support the conclusion that chemical structure/composition alone plays an integral role in determining the frictional properties of an interface. We propose that the difference in friction arises predominantly from the difference in size of the methyl and trifluoromethyl groups.

No abstract

The origin of frictional forces in self-assembled monolayers (SAMs) was investigated through systematic correlation of the frictional properties with the chemical structure/composition of the films. Atomic force microscopy was used to probe the frictional properties of the SAMs formed by the adsorption of methyl-, isopropyl-, and trifluoromethyl-terminated alkanethiols on Au(111) surfaces. The frictional properties of mixed monolayers composed of varying concentrations of the methyl- and trifluoromethyl-terminated thiols were also studied. Polarization modulation infrared reflection adsorption spectroscopy was used to measure the vibrational spectra of each of these monolayers and in turn to determine that each was characterized by a well-packed backbone structure. For these films, which differed only in the nature of the outermost chemical functionality, a substantial enhancement in the frictional response was observed for films with isopropyl- and trifluoromethyl-terminal groups and for mixed monolayers containing small concentrations of the trifluoromethyl-terminated component. These results strongly support the model that the difference in friction in such systems arises predominantly from the difference in the size of the terminal groups. Larger terminal groups in films of the same lattice spacing give rise to increased steric interactions that provide pathways for energy dissipation during sliding.

Self-assembled monolayers of terminally fluorinated alkanethiols, CF3(CH2) n SH with n = 9−15, and their nonfluorinated analogues, CH3(CH2) n SH with n = 9−15, were prepared by adsorption from solution onto evaporated gold. The monolayers were characterized by contact angle goniometry, ellipsometry, and X-ray photoelectron spectroscopy. The analyses indicate that the CF3-terminated alkanethiols generate terminally fluorinated monolayers that are well-ordered, particularly when the chain lengths consist of 12 or more carbon atoms. Comparison of CF3-terminated films to CH3-terminated films of similar length reveals that terminal fluorination of the surface leads to an overall decrease in the surface tension of the films. This decrease arises from a relatively large decrease in the dispersive component of the surface tension upon the introduction of fluorine. Surprisingly, terminal fluorination also leads to a small but significant increase in the nondispersive component(s) of the surface tension. The origin of these opposing effects is discussed.

No abstract