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 ([Tf2N]) 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 [Tf2N] 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.
PlechkovaNVSeddonKR. Applications of ionic liquids in the chemical industry. Chem Soc Rev2008; 23: 123–150.
2.
TorimotoTTsudaTOkazakiKi. New frontiers in materials science opened by ionic liquids. Adv Mater2010; 22: 1196–1221.
3.
BermúdezMDJiménezAESanesJ. Ionic liquids as advanced lubricant fluids. Molecules2009; 14: 2888–2908.
4.
PalacioMBhushanB. A review of ionic liquids for green molecular lubrication in nanotechnology. Tribol Lett2010; 40(2): 247–268.
5.
MinamiI. Ionic liquids in tribology. Molecules2009; 14: 2286–2305.
6.
ZhouFLiangYLiuW. Ionic liquid lubricants: designed chemistry for engineering applications. Chem Soc Rev2009; 38: 2590–2599.
7.
LuQWangHYeC. Room temperature ionic liquid 1-ethyl-3-hexylimidazoliumbis(trifluoromethylsulfonyl)-imide as lubricant for steel–steel contact. Tribol Int2004; 37: 547–552.
8.
JiménezAEBermúdezMDCarriónFJ. Room temperature ionic liquids as lubricant additives in steel–aluminium contacts: influence of sliding velocity, normal load and temperature. Wear2006; 261: 347–359.
9.
JiménezAEBermúdezMDIglesiasP. 1 -N-alkyl-3-methylimidazolium ionic liquids as neat lubricants and lubricant additives in steel–aluminium contacts. Wear2006; 260: 766–782.
10.
JimenézAEBermúdezMD. Ionic liquids as lubricants of titanium–steel contact. Part 3. Ti6Al4V lubricated with imidazolium ionic liquids with different alkyl chain lengths. Tribol Lett2010; 40: 237–246.
11.
ChenYMZengZXYangSR. The tribological performance of BCN films under ionic liquids lubrication. Diamond Rel Mater2009; 18: 20–26.
12.
GonzálezRHernández BattezABlancoD. Lubrication of TiN, CrN and DLC PVD coatings with 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate. Tribol Lett2010; 40: 269–277.
13.
PalacioMBhushanB. Molecularly thick dicationic ionic liquid films for nanolubrication. J Vac Sci Technol A2009; 27(4): 986–995.
14.
ZhaoWWangYWangL. Influence of heat treatment on the micro/nano-tribological properties of ultra-thin ionic liquid flms on silicon. Coll Surf A2010; 361: 118–125.
15.
SanesJCarriónFJBermúdezMD. Ionic liquids as lubricants of polystyrene and polyamide 6-steel contacts. Preparation and properties of new polymer-ionic liquid dispersions. Tribol Lett2006; 21(2): 121–133.
16.
FoxMFPriestM. Tribological properties of ionic liquids as lubricants and additives. Part 1: Synergistic tribofilm formation between ionic liquids and tricresyl phosphate. Proc IMechE Part J: J Engineering Tribology2008; 222(3): 291–303.
17.
KamimuraHKuboTMinamiI. Effect and mechanism of additives for ionic liquids as new lubricants. Tribol Int2007; 40: 620–625.
18.
MistryKFoxMFPriestM. Lubrication of an electroplated nickel matrix silicon carbide coated eutectic aluminium–silicon alloy automotive cylinder bore with an ionic liquid as a lubricant additive. Proc IMechE Part J: J Engineering Tribology2009; 223: 563–569.
19.
ZhangHXiaYYaoM. The influences of methyl group at C2 position in imidazolium ring on tribological properties. Tribol Lett2009; 36: 105–111.
20.
YaoMLiangYXiaY. Bisimidazolium ionic liquids as the high-performance antiwear additives in poly(ethylene glycol) for steel-steel contacts. ACS Appl Mater Interf2009; 1(2): 467–471.
21.
SchneiderABrennerJTomastikC. Capacity of selected ionic liquids as alternative EP/AW additive. Lubr Sci2010; 22: 215–223.
22.
Hernández BattezAGonzálezRViescaJL. Tribological behaviour of two imidazolium ionic liquids as lubricant additives for steel/steel contacts. Wear2009; 266: 1224–1228.
23.
ChooJHSpikesHARatoiM. Friction reduction in low-load hydrodynamic lubrication with a hydrophobic surface. Tribol Int2007; 40: 154–159.
24.
NagendrammaPKaulSBishtRPS. Study of synthesised ecofriendly and biodegradable esters: fire resistance and lubricating properties. Lubr Sci2010; 22: 103–110.
25.
van VoorstRAlamF. Polyglycols as base fluids for environmentally-friendly lubricants. J Synth Lubr2000; 16: 313–322.
26.
Hernandez BattezAFernandez RicoJERodriguezC. Rolling fatigue tests of three polyglycol lubricants. Wear2005; 258: 1467–1470.
27.
MattaCJoly-PottuzLDe Barros BouchetMI. Superlubricity and tribochemistry of polyhydric alcohols. Phys Rev B2008; 78: 085436(8pp)–085436(8pp).
28.
Joly-PottuzLMartinJMDe Barros BouchetMI. Anomalous low friction under boundary lubrication of steel surfaces by polyols. Tribol Lett2009; 34: 21–29.
29.
HabchiWMattaCJoly-PottuzL. Full film, boundary lubrication and tribochemistry in steel circular contacts lubricated with glycerol. Tribol Lett2011; 42: 351–358.
30.
MattaCDe Barros BouchetMILe-MogneT. Tribochemistry of tetrahedral hydrogen-free amorphous carbon coatings in the presence of OH-containing lubricants. Lubr Sci2008; 20: 137–149.
31.
StolteSArningJBottin-WeberU. Anion effects on the cytotoxicity of ionic liquids. Green Chem2006; 8: 621–629.
32.
StolteSMatzkeMArningJ. Effects of different head groups and functionalised side chains on the aquatic toxicity of ionic liquids. Green Chem2007; 9: 1170–1179.
33.
MatzkeMStolteSThieleK. The influence of anion species on the toxicity of 1-alkyl-3-methylimidazolium ionic liquids observed in an (eco) toxicological test battery. Green Chem2007; 9: 1198–1207.
34.
PrettiCChiappeCBaldettiI. Acute toxicity of ionic liquids for three freshwater organisms: Pseudokirchneriella subcapitata, Daphnia Magna and Danio Rerio. Ecotoxicol Environ Saf2009; 72: 1170–1176.
35.
ItohTWatanabeNInadaK. Design of alkyl sulfate ionic liquids for lubricants. Chem Lett2009; 38(1): 64–65.
36.
BermúdezMDJiménezAE. Surface interactions and tribochemical processes in ionic liquid lubrication of aluminium-steel contacts. Int J Surf Sci Eng2007; 1(1): 100–110.
37.
ZhaoDLiaoYZhangZ. Toxicity of ionic liquids. Clean2007; 35(1): 42–48.
38.
GonzálezBGómezEDomínguezA. Physicochemical characterization of new sulfate ionic liquids. J Chem Eng Data2011; 56: 14–20.
39.
DzyubaSVBartschRA. Influence of structural variations in 1-alkyl(aralkyl)-3-methylimidazolium hexafluorophosphates and bis(trifluoromethylsulfonyl) imides on physical properties of the ionic liquids. ChemPhysChem2002; 3: 161–166.
40.
PringleJMGoldingJBaranyaiK. The effect of anion fluorination in ionic liquids—physical properties of a range of bis(methanesulfonyl)amide salts. New J Chem2003; 27: 1504–1510.
41.
ChiricoRDDikyVMageeJW. Thermodynamic and thermophysical properties of the reference ionic liquid: 1-hexyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide (including mixtures). Part 2. Critical evaluation and recommended property values (IUPAC Technical Report). Pure Appl Chem2009; 81(5): 791–828.
42.
JinHO'HareBDongJ. Physical properties of ionic liquids consisting of the 1-butyl-3-methylimidazolium cation with various anions and the bis(trifluoromethylsulfonyl)imide anion with various cations. J Phys Chem B2008; 112: 81–92.
43.
DomańskaU. Physico-chemical properties and phase behaviour of pyrrolidinium-based ionic liquids. Int J Mol Sci2010; 11: 1825–1841.
44.
UerdingenMTreberCBalserM. Corrosion behaviour of ionic liquids. Green Chem2005; 7: 321–325.
45.
TallianTE. On competing failure modes in rolling contact. ASLE Trans1967; 10(4): 418–439.
46.
DowsonDHigginsonGR. Elasto-hydrodynamic lubrication: the fundamentals of roller and gear lubrication. Oxford: Pergamon Press. 1966; 1–235.
47.
LuRMoriSKobayashiK. Study of tribochemical decomposition of ionic liquids on a nascent steel surface. Appl Surf Sci2009; 255: 8965–8971.
48.
SahooRRBiwasSK. Frictional response of fatty acids on steel. J Coll Int Sci2009; 333: 707–718.
49.
ToniazzoVMustinCPortalJM. Elemental sulfur at the pyrite surfaces: speciation and quantification. Appl Surf Sci1999; 143: 229–237.
50.
AlonsoFGarciaAUgarteJJ. Changes in tribological properties of an AISI 440 C martensitic stainless steel after ion implantation of carbon at very high doses. Surf Coat Technol1996; 86: 263–268.
51.
XPS, AES, UPS and ESCA. Online Referencing. Available from: www.lasurface.com (accessed 23 February 2011).
52.
SmartRStCJasienakMPrinceKE. SIMS studies of oxidation mechanisms and polysulfide formation in reacted sulfide surfaces. Miner Eng2000; 13(8-9): 857–870.
53.
KronbergerMKalinMVižintinJ. Quaternary alkyl ammonium methyl sulphate as green additive in steel-steel contacts. Slotrib '10, Ljubljana. 25 November 2010: Slovenska društvo za tribologijo. p. 119 (16 pp).
54.
StachowiackGWBatchelorAW. Engineering tribology, 3rd ed. New York: Elsevier Butterworth-Heinemann, 2005.
55.
RosenblumBBräunlichPHimmelL. Spontaneous emission of charged particles and photons during tensile deformation of oxide-covered metals under ultra-high conditions. J Appl Phys1977; 48: 5262–5273.
56.
KajdasC. About an anionic-radical concept of the lubrication mechanisms of alcohols. Wear1987; 116: 167–180.
57.
NakayamaKSuzukiNHashimotoH. Triboemission of charged particles and photons from solid surfaces during frictional damage. J Phys D: Appl Phys1992; 25: 303–308.
58.
WernerEA. The isomeric transformation of ammonium methyl sulphate, and of substituted ammounium methyl sulphates; the interactions of amines and methyl sulphate. J Chem Soc Trans1914; 105: 2762–2769.
59.
StummWMorganJJ. Aquatic chemistry, 3rd ed. New York: Wiley Interscience, 1996.
60.
WolfendenRYuanY. Monoalkyl sulfates as alkylating agents in water, alkylsulfatase rate enhancements, and the ‘‘energy-rich’’ nature of sulfate half-esters. PNAS2007; 104(1): 83–86.
61.
WuHLiJMaH. The tribological behaviours of dithiocarbamate-triazine derivatives as additives in mineral oil. Surf Interf Anal2009; 41: 151–156.
62.
NajmanMKasraiMBancroftGM. Combination of ashless antiwear additives with metallic detergents: interactions with neutral and overbased calcium sulfonates. Tribol Int2006; 39: 342–355.
63.
De BarrosMIBouchetJRaoultI. Friction reduction by metal sulfides in boundary lubrication studied by XPS and XANES analyses. Wear2003; 254: 863–870.
64.
MinamiIKuboTShimamotoD. Investigation of wear mechanism by organic sulphides in vegetable oils. Lubr Sci2007; 19: 113–126.
65.
BühlerWDinjusEEdererHJ. Ionic reactions and pyrolysis of glycerol as competing reaction pathways in near- and supercritical water. J Supercrit Fluids2002; 22: 37–53.
