The study of hexanoate-based protic ionic liquids used as lubricants in steel-steel contact

Abstract

Three novel hexanoate-based protic ionic liquids with different ammonium cations (2-hydroxyethylammonium 2-ethylhexanoate, 2-hydroxymethylammonium 2-ethylhexancate, and 2-hydroxydimethylammonium 2-ethylhexanoate) were synthesized, characterized, and studied as neat lubricants under a steel-steel sliding contact mode. The ionic liquid with the 2-hydroxyethylammonium cation presented the highest values of density, viscosity, contact angle and thermal stability. The high value of viscosity and contact angle in the 2-hydroxyethylammonium salt can be attributed to hydrogen bonding. The three protic ionic liquids, especially 2-hydroxyethylammonium 2-ethylhexanocate and 2-hydroxymethylammonium 2-ethylhexanocate, outperformed other studied lubricants showing lower and stable friction coefficient. The improved lubrication performance is attributed to the strong physical adsorption of the ionic liquids on the steel-steel surfaces to form ordered lubrication layers due to the electrostatic interaction. In addition, the formation of carbon- and oxygen-enriched layers on worn steel surfaces helped to reduce friction. The formation of this tribolayer may be due to the high contact temperature and pressure conditions that promote the reaction of the active elements of steel and functional groups ionic liquids.