Ionic liquids from amino acids: fully green fluid lubricants for various surface contacts

Abstract

Five tetrabutylammonium-based amino acid ionic liquids (AAILs), tetrabutylammonium serine ([N₄₄₄₄][Ser]), tetrabutylammonium threonine ([N₄₄₄₄][Thr]), tetrabutylammonium valine ([N₄₄₄₄][Val]), tetrabutylammonium leucine ([N₄₄₄₄][Leu]) and tetrabutylammonium lysine ([N₄₄₄₄][Lys]) have been easily prepared by the neutralization of tetrabutylammonium hydroxide ([N₄₄₄₄][OH]) with the five corresponding amino acids. These AAILs only contain the elements C, N, O and H in their structures, so they are more environmentally friendly than conventional phosphorus-, halogen- or sulfur-containing lubricants. The preparation process of these AAILs is simple and also fully green, because no other by-products except water were produced during the whole process. The AAILs were evaluated as green lubricants for various surface contacts, such as steel/steel, steel/copper and steel/aluminum, using an Optimol SRV-IV oscillating reciprocating friction and wear tester at room temperature (RT). Due to their halogen-free character, these AAILs have a higher hydrolysis stability and a far lower corrosion level than the halogen-containing lubricants imidazolium ILs, 1-butyl-3-methylimidazolium tetrafluoroborate ([C₄mim][BF₄]) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₆mim][NTf₂]), which were also tested. The tribotest results revealed that these AAILs could act as lubricants to effectively reduce friction and wear of all sliding pairs, and they even showed superior tribological performances compared to poly-alpha-olefin (PAO40) and [C₆mim][NTf₂] under the experimental conditions. The worn surfaces of the lower discs were analyzed by X-ray photoelectron spectrometry (XPS). Based on the XPS results, it can be concluded that the good lubricating properties of the AAILs are attributed to the formation of physically adsorbed layers on the metal surfaces during the rubbing process.