Anion Structures Govern Interfacial and Tribological Properties of Ionic Liquid Additives for Water Glycol Lubricants: From Adsorption to Corrosion and Antibacterial Functions

Abstract Achieving high performance and multifunctionality at minimal concentration is a critical demand for water-based lubricant additives yet remains challenging in harsh environments, which is closely regulated to molecular configuration and interfacial behavior. Herein, three protic ionic liquids (PILs) with tailored polar groups and steric hindrance were designed as additives for water glycol base fluids. The significantly enhanced corrosion resistance, adsorption, and lubricity can be obtained with the specific structure-activity relationship, resulting from the strong interaction between PIL molecules and metal sliding asperity. For anions, linear short-chain alkoxy PIL delivers the fastest adsorption and optimal friction reduction, while the highly branched alkyl phosphorus PIL provides maximal steric hindrance, providing superior antiwear performance. The moderately branched long-chain alkoxy PIL achieves an optimal balance between rapid film formation and tribo-chemical reactivity, endowing the highest load-bearing capacity under variable loads. Beyond lubrication, all three PILs exhibit broad-spectrum antibacterial activity (99.99% inhibition against both Escherichia coli and Staphylococcus aureus). Collectively, this work elucidates the governing role of anion architecture in determining the comprehensive performance of water-based multifunctional lubricants, offering a theoretical foundation for developing advanced water-based additives.