Semi-Empirical Molecular Modeling of Ionic Liquid Tribology: Ionic Liquid–Aluminum Oxide Surface Interactions

The interactions between the selected ionic liquids (ILs) and aluminum oxide surfaces are modeled in this report using theoretical methods. A wide range of ILs and their interactions with an aluminum oxide surface are modeled using the PM5 semi-empirical method. The ILs modeled in this study contain imidazolium (C3, 4, 6, 8 or 10mim) or ammonium cations including (C6H13)3NH+, (C8H17)3NH+, C8H17NH3+, (C2H5)3NH+, and (C8H17)NH3+. The anions include Cl−, Br−, PF6−, (CF3SO2)2N−, and (C2F5SO2)2N−. The interactions of these ILs with an Al–O surface are modeled in a stepwise manner. The lowest energy forms of the individual ILs are determined, and these ILs are allowed to form a complex with the Al–O surface. The resulting reaction enthalpies of ionic liquid-surface complex formation are seen to correlate with the tribological properties of the ILs. The strongest correlations occur within those ILs containing similar cations.