Ibuprofen-Based Ionic Liquids as Additives for Enhancing the Lubricity and Antiwear of Water–Ethylene Glycol Liquid

Ibuprofen-based (L-Ibu) halogen-free ionic liquids (ILs) were synthesized and evaluated as water-based lubricant additive. In contrast to the conventional ILs with fluoro-containing anions, the present L-Ibu ILs have no corrosive attack to steel and cast iron even in the water environment and exhibit extremely stable hydrolytic stability in water–ethylene glycol (W–EG) system. In addition, the Ibuprofen ILs as the water-based lubricant additives have better friction-reducing, extreme-pressure and antiwear properties than a commercial antiwear water-based lubricant additive of Hostagliss L4. The mechanism for excellent lubricating characteristics is investigated by combining electrical contact resistance measurement, X-ray photoelectron spectroscopy and interfacial adsorption behaviour by quartz crystal microbalance. The results indicate that a formed protective film on the contact surface prevents the direct contact of sliding pairs and contributes to the friction reduction and antiwear properties. More importantly, the prepared ILs provide a comparatively green alternative to the traditional halogenated ILs for the water-based lubrication application owing to halogen-, sulphur-, and phosphorus-free. So, these ILs are expected to develop into a highly efficient lubricant additive applied in water-based hydraulic fluids.