Molecular Interactions between Ionic Liquid Lubricants and Silica Surfaces: An MD Simulation Study

Abstract

The unique physicochemical properties of ionic liquids (ILs) attracted interest in their application as lubricants of micro/nano-electromechanical systems. This work evaluates the feasibility of using the protic ionic liquids [4-picH][HSO₄], [4-picH][CH₃SO₃], [MIMH][HSO₄], and [MIMH][CH₃SO₃] and the aprotic ILs [C₆mim][HSO₄] and [C₆mim][CH₃SO₃] as additives to model lubricant poly(ethylene glycol) (PEG200) to lubricate silicon surfaces. Additives based on the cation [4-picH]⁺ exhibited the best tribological performance, with the optimal value for 2% [4-picH][HSO₄] in PEG200 (w/w). Molecular dynamics (MD) simulations of the first stages of adsorption of the ILs at the glass surface were performed to portray the molecular behavior of the ILs added to PEG200 and their interaction with the silica substrate. For the pure ILs at the solid substrates, the MD results indicated that weak specific interactions of the cation with the glass interface are lost to accommodate the larger anion in the first contact layer. For the PEG200 + 2% [4-picH][HSO₄] system, the formation of a more compact protective film adsorbed at the glass surface is revealed by a larger trans population of the dihedral angle -O(R)-C-C-O(R)- in PEG200, in comparison to the same distribution for the pure model lubricant. Our findings suggest that the enhanced lubrication performance of PEG200 with [4-picH][HSO₄] arises from synergistic interactions between the protic IL and PEG200 at the adsorbed layer.