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Phosphonium Ionic Liquids in Lubrication

Part 2: CYPHOS® IL 109

In a recent blog [1] we briefly reviewed some literature on the applications of room-temperature ionic liquid (RTIL) trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)amide 1 aka CYPHOS® IL 109 (CAS # 460092-03-9, Strem: 15-6370, 96-6520, Ionic Liquid Kit 3: CYPHOS® IL Phosphonium Salt Kit) in lubrication:

According to a SciFinder search, the largest number of publications (61) on 1 appeared in 2017. The focus of this blog is to briefly review several of the most interesting results from 2017 exploring in greater depth the testing of 1 in lubrication.

Thus, tribological behavior of 1 used as an additive in a diester oil (Priolube1936) for steel-steel contact was studied in [2]. The concentrations were 0.25, 0.5 and 1 wt% which corresponded to 108.5, 217 and 434 ppm, respectively (keeping the phosphorous content in the mixtures below 800 ppm according to ILSAC GF-5 specifications for motor oils at that time).

No corrosion activity for the highest concentration of 1 (1 wt%) was found after a 20-day test. This result was in stark contrast with the previously reported [3] corrosion activity of 1 as a neat sample (the test methods and the materials were the same in both studies). It was concluded that there would be no problem with using 1 as an additive in a steel–steel tribological contact from a corrosion point of view. In terms of friction and wear reduction, all ionic liquid mixtures outperformed the base oil with the best result obtained at 1% concentration.

Atomic force microscopy (AFM) was used to measure the friction of a stainless steel surface lubricated with six different ionic liquids as a function of normal load [4]. Two of the studied phosphonium ionic liquids based on the [P_6,6,6,14]⁺ cation, including 1, were the most effective lubricants, reducing lateral forces and friction coefficients by up to 3 times compared with the imidazolium-based ILs and the shorter-chained P_4,4,4,1 TFSI.

It was concluded that the bulkiness of the [P_6,6,6,14]⁺ cation weakened the electrostatic interaction with the anion, resulting in weaker structure near the surface thus making the ILs with the weakest interfacial structure the best performing lubricants.

The tribological behavior of two phosphonium-based ILs as additives to a bio-oil (BO) was investigated in steel-steel contact for use in wind turbine gearboxes [5]. Adding the halogen-containing 1 to the BO showed a wear reduction at all sliding speeds studied, particularly at the fastest speed where a wear reduction of 74.3% was achieved with respect to the base BO. For comparison, at this speed the addition of the halogen-free ionic liquid trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (aka CYPHOS® IL 104, Strem 15-6374 [6]) to BO, reduced wear volume 68.4% compared to the base BO.

The previous blog [1] and this one have reviewed some interesting results on the application of 1 in tribology, mostly for steel-on-steel and steel-on-aluminum contacts. However, 1 was also shown to be a promising candidate for lubrication of Ni-based alloys, titanium, etc. We will review those publications as well in subsequent blogs. Stay tuned!

References:

Related products

15-6370 Trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)amide, min. 97% CYPHOS® IL 109 (460092-03-9)

15-6374 Trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate, min. 90% CYPHOS® IL 104 (465527-59-7)

96-6520 Ionic Liquid Kit 3: CYPHOS® IL Phosphonium Salt Kit

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