How anion and cation species influence the tribology of a green lubricant based on ionic liquids

Abstract

A group of halide-free ionic liquids involving two different anions (methyl sulfate and methyl sulfonate) and four types of cations (short-chain tetraalkylammonium, dialkylpyrrolidinium, choline, and methoxycholine) were investigated as 2.5 wt% additives in glycerol as a model base fluid, yielding highly biodegradable polar lubricants for study of ionic liquid interaction with a substrate. The results were compared to the behavior of conventional bis(trifluoromethylsulfonyl)imide ([Tf 2 N]) ionic liquids with identical counter-ions. The neat ionic liquids (100 wt%) were tested in identical manner and compared to the behavior when they operate as additives. Tribotests were performed in a ball-on-disc configuration under boundary conditions, by lubricating steel-steel couples at room temperature and at 100 C. Wear reduction was achieved for all temperatures, and the results were strongly anion-dominated, with good results for methyl sulfates and the [Tf 2 N] references. Particularly for higher temperatures, ionic liquids were also able to reduce friction by a substantial amount, with a clear order between the individual anions, and the lowest values were again obtained for methyl sulfates. Cationic influence on the test results was found to be subordinate for both temperatures. It could be recognized that at elevated temperatures, the newly formulated lubricants containing an ionic liquid as an additive behaved similarly to neat ionic liquids in terms of friction and wear reduction. Using X-ray photoelectron spectroscopy analyses, the formation of a beneficial iron sulfide film was detected, with the sulfur originating from the sulfate of the ionic liquid, presumably as a result of a redox reaction with metallic iron. For this mechanism, a hypothesis for possible reaction pathways was developed.

Pyridinium based dicationic ionic liquids as base lubricants or lubricant additives

“…Due to the harsh testing conditions, the methylsulfate anion could hydrolize giving origin to sulfuric acid. Similar explanations have already been offered in literature [21].…”

“…For all these reasons, we have synthesized and studied the tribological properties of a series of DILs that incorporate polyethylene glycol chains linking two alkylpyridinium moieties. Alkylsulfate [RSO 4 ] and bis(trifluoromethane) sulfonimide [NTf 2 ] were selected as anions taking into account the reduction on wear and friction observed when these anions were used in previous works on monocationic ILs [20,21].…”

A Review of Ionic Liquid Lubricants

“…Table 4 gives a) b) a summary of the ionic liquids and base oils used and their performance in wear tests for various systems. Ionic liquids have been added to a number of base oils, such as hydrocarbons, polyethylene glycol (PEG), polyalphaolefins (PAO) propylene glycol dioleate (PGDO) and glycerol [7,16,17,27,[29][30][31][43][44][45][46]52,53,[68][69][70]. The amount of IL added to base oils has varied from 0.3 wt% to 10 wt%, with the proportion depending on the solubility and the amount required to give the best performance [16].…”

Ionic Liquids in Surface Protection

“…In the first place, there exists a strong tendency to avoid the presence of halides, so the new so-called halogen-free ILs are attracting a growing attention [62][63][64].…”