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 (C 3, 4, 6, 8 or 10 mim) or ammonium cations including (C 6 H 13 ) 3 NH + , (C 8 H 17 ) 3 NH + , C 8 H 17 NH 3 + , (C 2 H 5 ) 3 NH + , and (C 8 H 17 )NH 3 + . The anions include Cl − , Br − , PF 6 − , (CF 3 SO 2 ) 2 N − , and (C 2 F 5 SO 2 ) 2 N − . 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.