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

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.


Syntheses of the PILs:
1-Methyl-3-hexylimidazolium mesylate: [C6mim][MeSO3] 1.2 mL of 1-methyl-3-hexylimidazolium bromide and 1 equivalent of anionic resin Amberlyst A-26 (OH) (~1.5 g) were added to a 50 mL round-bottom flask, with 20 mL of ethanol.The mixture was stirred for 30 minutes at room temperature and, after that, 0.4 mL of methanesulfonic acid in ethanol were added.The mixture was stirred for 24h at room temperature.The solvent was evaporated and the final product was dried in vacuum and obtained as a viscous yellow liquid (1.61 g, ƞ = 78%).

1-Methyl-3-Hexylimidazolium hydrogen sulfate: [C6mim][HSO4]
0.65 mL of 1-methyl-3-hexylimidazolium bromide and 0.48g of potassium hydrogen sulfate were dissolved in water and added to a 50 mL round-bottom flask.The mixture was stirred for 24h at room temperature.The solvent was evaporated and the final product was dried in vacuum and obtained as a viscous yellow liquid (0.84 g, ƞ = 66%).
Scheme S1.Snapshots of the MD simulations at 300 K for the IL-glass interface of (a) [4-

Figure S10 .Figure S11 .
Figure S10.Ring orientational ordering parameter S as a function of the distance to the glass

Figure S12 .Figure S13 .Figure S14 .Figure S15 .Figure S16 .
Figure S12.Number density profiles along the direction normal to glass surface for OH groups

Table S1 .
Force field parameters used in MD simulations of 4-picolinium cation.

Table S2 .
Force field parameters used in MD simulations of methylimidazolium cation.

Table S4 .
Force field parameters used in MD simulations of hydrogen sulfate anion.

Table S5 .
Force field parameters used in MD simulations of methylsulfonate anion.

Table S6 .
Force field parameters used in MD simulations of polyethylene glycol (PEG200).

Table S6 .
Force field parameters used in MD simulations of amorphous silica.

Table S7 .
Final box length (lb, in nm)and calculated densities (MD, in g.cm -3 ) for the bulk systems simulated at 300 K.

Table S8 .
Surface area per ion pair (or per PEG200 molecule) at the glass interface (in nm 2 ) for the different systems studied.The number of ions and molecules were determined by integration of the numerical density profiles between z = 0 nm (glass surface) and z = 0.5 nm (first minima in