Ionic Liquid Lubrication of Stainless Steel: Friction is Inversely Correlated with Interfacial Liquid Nanostructure

Atomic force microscopy (AFM) is used to measure the friction of a stainless steel surface lubricated with six different ionic liquids (ILs). The measurements reveal that, contrary to expectations, friction and the degree of interfacial ionic liquid nanostructure are inversely correlated. Six ILs were confined between a sharp Si tip and a stainless steel surface, and the friction force was measured as a function of normal load. Trihexyl­(tetradecyl)­phosphonium bis­(trifluoro­methane)­sulfonamide (P_6,6,6,14 TFSI) and trihexyl­(tetradecyl)­phosphonium bis­(2,4,4-trimethyl­pentyl)­phosphinate (P_6,6,6,14 (^<i>i</i>C₈)₂PO₂) are the most effective lubricants, reducing lateral forces and friction coefficients by up to 3 times compared with the imidazolium-based ILs and the shorter-chained P_4,4,4,1 TFSI. Force–separation profiles reveal no obvious interfacial layers for P_6,6,6,14 (^<i>i</i>C₈)₂PO₂ and P_6,6,6,14 TFSI, in contrast to extensive layering for the other ILs. The bulkiness of the [P_6,6,6,14]⁺ cation weakens the electrostatic interaction with the anion, resulting in weaker structure near the surface. The sliding AFM tip expels the weak near surface structure with minimal energy loss (low friction), whereas for the strongly structured ILs, more energy is dissipated by breaking the stronger interlayer bonds. The inverse correlation between friction and IL structure can be used to design weakly structured ILs which are excellent lubricants