Influence of Cation Structure on Physicochemical and Antiwear Properties of Hydroxyl-Functionalized Imidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquids

A series of hydroxyl-functionalized imidazolium bis (trifluoromethylsulfonyl)imide ionic liquids (ILs) with alkyl chain lengths ranging from C1 to C10 were successfully synthesized and systematically investigated as lubricants for steel–steel contacts at room temperature and three different lubricating conditions. The effect of either hydroxyethyl group or alkyl chain length on physicochemical and tribological properties of the ILs was discussed at length, and the optimum chemical structure was also obtained. It was found that incorporation of a hydroxyethyl group to the imidazolin cation had a great impact on thermal stability, frictional coefficient, and wear but less impact on viscosity and corrosivity. It is also found that increasing alkyl chain length in the imidazolin cation had a significant influence on corrosion, the frictional coefficient, and wear but the effect dropped rapidly from C1 to C6 and progressively recovered as it increased from C6 to C10. To a much lesser extent, a similar effect was seen on thermal stability. As expected, increasing alkyl chain length monotonically increased viscosity. Furthermore, the lubrication mechanism in the friction process was explored in depth by means of scanning electron microscopy (SEM) and X-ray photoelectron spectrometry (XPS), and a proposed interaction model between the ILs and the friction surfaces was provided.