Dicationic ionic liquids as lubricants

Abstract

Twelve dicationic ionic liquids have been investigated for their potential inlubricants. The dications selected were synthesized from oligoethylene glycols linking two cationic moieties based on either N-methylimidazolium or N-methylpyrrolidinium at the extremities. As anions, chloride, bis(trifluoromethanesulfonyl)imide (Tf2N), methanesulfonate and butansulfonate were chosen. Thermal stability of the IL has been evaluated by thermogravimetric analysis and differential scanning calorimetry showing good properties up to 150 °C for all IL. SRV tribotests of neat IL using steel–steel contacts with ball-on-disc geometry were performed at 50 °C, 100 °C and 150 °C. Friction and wear were significantly influenced by both anion structure and chain length of the oligoethylene glycol moiety where longer chains yielded better tribological behavior. The tribological behavior improved with a mixture of different types of ILcontaining F, O, S in their anions. A large variation of ionic liquids corrosiveness and corrosion inhibition, respectively, was found attributed to structure variation. A general tendencyof improving the corrosion resistance when increasing the chain length of the neat ILshas been found. In addition to the neat dicationic ionic liquids, mixtures of 1% of the mostinteresting IL in polypropylene glycol base oil have been studied at 100 °C resulting in more favorable tribological properties for ionic liquids with Tf2N anion. X-ray photoelectron spectroscopy analysis showed that the structures with Tf2N anion create a tribofilm of inorganic fluorine that greatly improves the performance of the base oil.