Joshua BUZOLIC1, Hua LI1, Zachary AMAN1, Gregory WARR2, Rob ATKIN1
1The University of Western Australia, Perth, Australia
2The University of Sydney, Sydney, Australia
Ionic liquids (ILs) have numerous applications including lubrication, batteries, and biomass processing, mainly because typical ILs have pronounced nanostructure. The nanostructure of these ILs originates from the solvophobic effect, where interactions between charged moieties lead to the formation of polar domains, which solvophobically exclude alkyl chains into apolar domains. Surface active ionic liquids (SAILs) are an emerging type of ILs that feature a surface active ion, and the more amphiphilic nature of these SAILs enables a more pronounced nanostructure compared to typical ILs. Pure SAILs have shown good lubrication on various surfaces [1,2], but their relatively high cost restricts their further application. To address this knowledge gap, we investigated SAILs as lubricant additives in both polar and apolar base oils.
The interfacial structure and properties of catanionic SAILs formed from trihexyltetradecylphosphonium [P6,6,6,14] mixed with either dioctyl sulfosuccinate [AOT] or dodecyl sulfate [DS] on stainless steel were studied using atomic force microscopy (AFM) for nano?friction measurements and force curves, and using rheometry and tribology for macro?friction measurements. Both nano?friction and macro?friction reveal that even small amounts of either SAIL as an additive in both base oils significantly reduce friction, and that [P6,6,6,14] [AOT] in diethyl succinate shows the lowest friction of the mixtures, due to differences in miscibility caused by anion structure and polarity. AFM force curves show that interfacial structure weakens as SAIL concentration decreases, consistent with nano?friction and macro?friction. These results provide a new pathway for SAILs as additives in various applications, such as for lubricants in engines and brakes used in industry.