Ionic Liquids Composed of Phosphonium Cations and Organophosphate, Carboxylate, and Sulfonate Anions as Lubricant Antiwear Additives

Abstract

Oil-soluble phosphonium-based ionic liquids (ILs) have recently been reported as potential ashless lubricant additives. This study is to expand the IL chemistry envelope and to achieve fundamental correlations between the ion structures and ILs' physiochemical and tribological properties. Here we present eight ILs containing two different phosphonium cations and seven different anions from three groups: organophosphate, carboxylate, and sulfonate. The oil solubility of ILs seems largely governed by the IL molecule size and structure complexity. When used as oil additives, the ranking of effectiveness in wear protection for the anions are organophosphate > carboxylate > sulfonate. All selected ILs outperformed a commercial ashless antiwear additive. Surface characterization from the top and the cross-section revealed the nanostructures and compositions of the tribo-films formed by the ILs. Some fundamental insights were achieved: branched and long alkyls improve the IL's oil solubility, anions of a phosphonium-phosphate IL contribute most phosphorus in the tribo-film, and carboxylate anions, though free of P, S, N, or halogen, can promote the formation of an antiwear tribo-film.

Dissolution of Trihexyltetradecylphosphonium Chloride in Supercritical CO2

“…RTILs are also designer solvents in the sense that a wide variety of cations and anions can be combined to tailor ILs for task-specific processes or reactions [1,2]. RTILs have unique characteristics, such as low volatility, non-flammability and water miscibility, that make them candidates for use in many applications, and has given rise to their use as cosolvents [3][4][5][6] in numerous applications [1,[3][4][5][6][7][8][9][10][11][12][13] that include electrochemistry [14][15][16], organic synthesis [17,18], gas separation/SLM's [19,20] and catalysis [17,21]. The inherent polar and nonpolar nanodomains that form in ILs allow them to readily solvate polar and non-polar solutes, and this has given rise to a significant increase in their study over the last decade [8,14,15,17,[19][20][21][22].…”

Towards Phosphorus Free Ionic Liquid Anti-Wear Lubricant Additives

“…Since this discovery, similar structured ILs, such as phosphonium carboxylates and a tetraoctyl phosphonium phosphate have been produced that have also proven to be miscible. The wear performance of these has proven to be mixed, with the tetraoctyl phosphonium showing a reduction in wear, while the carboxylates showed an increase in wear [13,14]. More recently Westerholt et al found an oil miscible ammonium phosphate, trioctylmethylammonium dibutylphosphate, that performed well in a three-ball-on-disk wear test in comparison to the previously reported trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate [15].…”

“…One of the major advantages of ILs as a class of chemicals is their flexibility in design. Using what is known about the structural requirements for an effective anti-wear additive that is oil miscible, researchers have started to design ILs specifically for this application [13][14][15]. Using this structure knowledge, the research detailed here reports on steps towards progressively eliminating phosphorus from anti-wear additives for steel-on-steel systems.…”

Behavior of Ionic Liquids Under Nanoconfinement Greatly Affects Actual Friction

“…The formation of tribochemical layers on metal surfaces from ILs containing a halogen such as fluorine under sliding conditions has been studied by X-ray photoelectron spectroscopy (XPS) [9][10][11][12], scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDS) [9][10][11], and time-of-flight secondary ion mass spectrometry (TOF-SIMS) [12]. Therefore, ILs have also been used as additives for the formation of a tribochemical layer under high loads of several GPa [14][15][16]. When used, these layers have been considered to contribute to the reduction of friction in the system.…”