Molecular orientation and bonding of monolayer stearic acid on a copper surface prepared in air

The molecular orientation of a monolayer stearic acid, adsorbed on a copper surface in air, was studied by means of Xray absorption spectroscopy above the carbon and oxygen K edges. Carbon-K-edge X-ray absorption measurements were used to quantify the molecular tilt of the alkyl chain from the surface normal, which was found to be 49 and 39 respectively before and after rubbing. Tribochemical bonding chemistry was observed using oxygen-K-edge X-ray absorption measurements. Chemisorbed stearic acid was found to adsorb initially in both monodentate and bidentate sites. The monodentate chemisorbed stearic acid was then transformed to a bidentate surface configuration after tribochemical reaction. Furthermore, spectra from the chemisorbed stearic acid monolayer were similar to the spectrum from a bulk stearic acid powder standard, and the data from the tribochemically modified stearic acid monolayer surface appeared to be identical to that from a bulk cupric stearate.