Halogen-Free Bis(imidazolium)/Bis(ammonium)-Di[bis(salicylato)borate] Ionic Liquids As Energy-Efficient and Environmentally Friendly Lubricant Additives

Bis­(imidazolium)- and bis­(ammonium)-di­[bis­(salicylato)­borate] ionic liquids with variable alkyl chain and cyclic ring structures, were synthesized and then evaluated them as potential lubricant additives. The copper strip test results revealed noncorrosive properties of these ionic liquids. Introduction of halogen content in bis­(imidazolium) ionic liquid by replacement of bis­(salicylato)­borate (BScB) anion with hexafluorophosphate (PF₆^–), severely corroded the copper strip. Thermogravimetric results showed that bis­(imidazolium) ionic liquids exhibited higher thermal stability than bis­(ammonium) ionic liquids owing to compact structure provided by imidazolium rings, higher intermolecular interactions, smaller free volume and low steric hindrance. The lubrication properties of these ionic liquids as additives to synthetic lubricant poly­(ethylene) glycol (PEG 200) were evaluated for steel balls. Results showed that bis­(ammonium)- and bis­(imidazolium)-(BScB)₂ ionic liquids as additives significantly reduced both friction coefficient and wear of PEG 200. The structure of cations, particularly the variation in substituted alkyl chain length monitored the degree of reduction in friction and wear. The excellent lubrication properties were attributed to the formation of adsorbed tribo-thin film and tribochemical product during the tribo-contact. Being halogen-, phosphorus-, and sulfur-free, these ionic liquids (a) protects contact surfaces from tribo-corrosive events, (b) reduces the friction and wear, and (c) keep environment green and clean