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 (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.