Phosphonium-Organophosphate Ionic Liquids as Lubricant Additives: Effects of Cation Structure on Physicochemical and Tribological Characteristics

Abstract

Our previous work suggested great potential for a phosphonium-organophosphate ionic liquid (IL) as an antiwear lubricant additive. In this study, a set of five ILs were carefully designed and synthesized, with identical organophosphate anions but dissimilar phosphonium cations, to allow systematic investigation of the effects of cation alkyl chain length and symmetry on physicochemical and tribological properties. Symmetric cations with shorter alkyl chains seem to increase the density and thermal stability due to closer packing. On the other hand, either higher cation symmetry or longer alkyl moieties induce a higher viscosity, though the viscosity index is dependent more on molecular mass than on symmetry. While a larger cation size generally increases an IL's solubility in nonpolar hydrocarbon oils, six-carbon seems to be the critical minimum alkyl chain length for high oil miscibility. Both the two ILs, that are mutually oil miscible, have demonstrated promising lubricating performance at 1.04% treat rate, though the symmetric-cation IL moderately outperformed the asymmetric-cation IL. Characterizations on the tribofilm formed by the best-performing symmetric-cation IL revealed the film thickness, nanostructure, and chemical composition. Results here provide fundamental insights for future molecular design in developing oil-soluble ILs as lubricant additives.

The Lubrication Ability of Ionic Liquids as Additives for Wind Turbine Gearboxes Oils

“…(Figure 5b). It has been reported [12][13][14][15][16][17][18][19][20][21] that ILs have the ability to form highly ordered absorbed layers on metal surfaces. This layer prevents direct contact between mating surfaces, reducing friction and wear.…”

“…However, halogen-containing anions are very sensitive to moisture and, in the presence of water, will react, causing: (1) the corrosion of metallic surfaces by formation of metallic halides and (2) the evolution of toxic species (HF) from hydrolysis of the anions [17,18]. Recent literature has suggested the potential of using ILs with halogen-free anions [11,19,20]; however, studies on the tribological performance of these ILs as additives of lubricants for wind turbine applications are still very scarce.…”

Isoconversional kinetic analysis applied to five phosphonium cation-based ionic liquids

“…This fact provoked the introduction of new and more stable ILs based on [FAP] and [NTf 2 ] anions [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53], which present enhanced tribological properties due to the formation of fluoride tribofilms, especially at high loads [54]. Recently, phosphonium cation-based ILs have been introduced in this field because of their growing commercial availability and good tribological performance [26,[55][56][57][58][59][60][61][62]. Finally, taking into account the importance of thermal stability in the lifetime of lubricants and additives, the determination of thermal models has become an interesting research topic from scientific and technical points of view.…”

Time-Resolved Characterization of Dynamic Tribochemical Processes for Dicationic Imidazolium Ionic Liquid

“…During the last decade, imidazolium-base ionic liquids with fluorinated anions have been specially addressed in a number of studies [1,[15][16][17]. The application range of the imidazolium ILs was further expanded by rational designing of the molecular structure and the side chains offering the possibility of obtaining versatile ILs with tailored polarity, hydrophobicity, and corrosive and antioxidant properties [7,[18][19][20][21][22][23][24][25]. Generally, the increase of the length of the alkyl side chains attached to a cation head group leads to the improvement of wear resistance, but this effect is not straightforward [2,22,26].…”