Eco-Friendly Protic Ionic Liquids Water-Based Lubricant Additives Contributing to Green Tribology

As global environmental standards grow increasingly strict, developing eco-friendly, high-efficiency water-based lubrication additives has become a pivotal focus in tribology. In this work, three novel heterocycle-, phosphorus-, and sulfur-free protic ionic liquids (PILs), namely EDG-HA, EDG-OA, and EDG-LA, were successfully synthesized and employed as additives in aqueous lubricant systems with the objective of alleviating the environmental contamination and biological toxicity problems related to conventional lubricant additives. A systematic evaluation of the PILs revealed that lubricants blended with the additives demonstrate significantly superior corrosion protection and lubricating performance when compared to pure water. Specifically, the 1% EDG-LA lubricating solution reduced the friction coefficient (COF) and wear volume (WV) by approximately 69.3% and 94.5%, respectively. By combining quartz crystal microbalance (QCM) adsorption tests and X-ray photoelectron spectroscopy (XPS) analysis, the lubrication mechanisms were clarified: PILs form robust physical adsorption and tribo-chemical reaction film on metal friction pairs, effectively preventing direct metal-to-metal contact. The current research promotes the development of water-soluble additives with potential eco-friendly and high-efficiency through a novel perspective. Graphical