Pyrylium- and Pyridinium-Based Ionic Liquids as Friction Modifiers for Greases

The use of ionic liquids (ILs) as lubricants or additives has been studied extensively over the past few decades. However, the ILs considered for lubricant applications have been part of a limited structural class of phosphonium- or imidazolium-type compounds. Here, new pyrylium- and pyridinium-based ILs bearing long alkyl chains were prepared and evaluated as friction- and wear-reducing additives in naphthenic greases. The physical properties of the synthetic ILs and additized naphthenic grease were measured. The tribological performance of the greases was measured by using standard benchtop tests. The addition of ILs was detrimental to wear, causing an increase in the amount of material removed by sliding relative to the base greases in most cases. In contrast, the friction performance improved under nearly all conditions tested due to the IL additives. The compatibility of the synthetic ILs with the naphthenic greases and its potential influence upon miscibility and tribological performance are tentatively proposed to be a result of the molecular structure.


Synthetic procedures for ILs a) Synthesis of 1a
This synthetic procedure was modified from the original report by Balaban et al. 1992.Reaction conditions were optimized to afford higher isolated yield of this compound.NMR data is also provided for the first time.Anhydrous SnCl 4 (6.4 g, 24.6 mmol) was added under stirring and with external cooling (ice and water bath) to lauroyl chloride (26.8 g, 123.3 mmol).Then mesityl oxide (10 g, 100.0 mmol) was added so that the temperature was maintained between 22 and 25 °C (the ice was removed from the cooling bath).Stirring was continued after complete addition (seven hours), then the mixture was left to stir overnight.The evolution of hydrogen chloride ceased after final heating for five hours at 40 °C under stirring.The reaction mixture was quenched by pouring over 40 g crushed ice, 4 ml of concentrated aqueous hydrochloric acid and 35 ml diethyl ether.The aqueous layer was separated and extracted twice with 20 ml diethyl ether.The combined organic layers were washed four times with small portions of dilute aqueous hydrogen chloride, after which concentrated aqueous perchloric acid was added.This led to the separation of an upper oily layer which was collected, washed with distilled water, and extracted three times with methylene chloride.These extracts were then dried and concentrated.The resulting oil is transferred to a short, thick, silica gel chromatography column.Excess lauric acid was eluted first flashing the column with hexane after which the pyrylium perchlorate was eluted employing ethyl acetate.From this fraction, after concentration, the pyrylium salt crystallized as colorless micro-crystals (22.1 g), in 50% yield (based on mesityl oxide) having m.p. of 66-67 °C.

c) Synthesis of 2a
This synthetic procedure was modified from the original report by Balaban et al. 1992.Reaction conditions were optimized to afford higher isolated yield of this compound.NMR data is also provided for the first time.

Figure S15 .
Figure S15.Stacked FT-IR spectra of base and blended grease B with ILs.

Figure S16 .
Figure S16.Stacked TGA profiles of base grease B and blended derivatives with ILs

Figure S18 :
Figure S18: Representative results from a ball-on-disk test.(a) Friction as a function of test time with a schematic of the test set up inset.Optical microscope images of (b) a small section of the wear track on the disk and (c) the circular worn patch on the ball.

Figure S19 :
Figure S19: Representative results from a four-ball test.(a) Schematic of the test set up.(b) Optical microscope images of the circular worn patch on the three lower balls that were fixed in place during the test.

Figure S20 :
Figure S20: Friction and wear data for the neat and IL-additized greases.When error bars are present, they represent the standard deviation from two separate tests.Data reported with no error bars indicates only one test was run for that grease or condition.

Figure S21 .
Figure S21.Change in wear diameter from the (a) 4-ball EP test, (b) ball-on-disk test at 40°C, and (c) ballon-disk test at 100°C resulting from adding the various ILs to grease A and grease B. Negative change indicates performance improvement.

Figure S22 :
Figure S22: Blending procedure of ILs and base greases.