Effects of Surface Chemical Properties on the Frictional Properties of Self-Assembled Monolayers Lubricated with Oleic Acid

Abstract

Self-assembled monolayers (SAMs) of hexadecanethiol (HDT) and 16-mercaptohexadecanoic acid (MHDA) were used to vary the surface chemical properties, and oleic acid (OA) was used as a lubricant. A pin-on-disk tribometer equipped with Fourier transform IR (FT-IR) spectroscopy was used to evaluate the results of lubricants under various lubricating condition, and sum frequency generation (SFG) spectroscopy was used to obtain static information on the molecular structure of the boundary layer. The FT-IR results for the frictional properties of the SAMs lubricated with OA indicated that a transition from the hydrodynamic to mixed lubrication regime for the HDT-SAM occurred, however, the MHDA-SAM remained in the hydrodynamic lubrication regime under our experimental conditions. The FT-IR results indicate that in the hydrodynamic lubrication regime, the gauche/trans conformer ratio in the OA molecules decreased with the thinning of the oil film thickness for both HDTand MHDA-SAMs. Upon the transition from the hydrodynamic to mixed lubrication regime, the gauche/trans conformer ratio increased. The SFG results for OA/HDT interface indicated that the HDT layer was only slightly influenced by the adsorption of OA, however, the adsorbed layer of the OA contained significant gauche defects. For the MHDA-SAM, OA molecules were adsorbed on the surface through hydrogen-bonding between their carboxyl groups. From the SFG and FT-IR results, the difference in the interaction between the types of SAMs and the OA is considered to affect the frictional properties of SAMs lubricated with OA.