Protic Ionic Liquids as Lubricant Additives

Protic ionic liquids (PILs) are an emerging class of environmentally friendly lubricants and lubricant additives PILs can be easily synthesized through proton transfer from a Brønsted acid to a Brønsted base and exhibit significant potential as lubricant additives on account of their high thermal stability, non-flammability, low melting points, lack of vapor pressure, low cost, and biodegradability. This research covers the tribological performance of three halogen-free protic ionic liquids, 2-hydroxyethylammonium 2-ethylhexanoate (Eet), 2-hydroxyethylammonium p-toluenesulfonate (Ets), and 2-hydroxyethylammonium 4-dodecylbenzenesulfonate (Eds) which are designed and synthesized using the same base (ethanolamine) but different acids of varying acidity and alkyl chain length. The relationship among their ionicity, molecular structures, and tribological properties are investigated when these protic ionic liquids are used as lubricant additives to water. Each PIL was evaluated as a lubricant additive in a 1 wt.% aqueous solution. The solutions were tested using a ball-on-flat reciprocating tribometer under room temperature for steel-ceramic contact using distilled water was used as a base line lubricant. PIL solutions yielded significant reductions in frictional coefficients and wear volumes. The wear mechanisms and surface interactions were also discussed in terms of PIL-steel surface interactions from SEM and EDX data.