Ionic Grease Lubricants: Protic [Triethanolamine][Oleic Acid] and

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Ionic Grease Lubricants: Protic [Triethanolamine][Oleic Acid] and Aprotic [Choline][Oleic Acid] Liwen Mu,† Yijun Shi,*,‡ Tuo Ji,† Long Chen,† Ruixia Yuan,†,§ Huaiyuan Wang,§ and Jiahua Zhu*,† †

Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, Ohio 44325 United States ‡ Division of Machine Elements, Luleå University of Technology, Luleå 97187, Sweden § College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing 163318, PR China ABSTRACT: Ionic liquid lubricants or lubricant additives have been studied intensively over past decades. However, ionic grease serving as lubricant has rarely been investigated so far. In this work, novel protic [triethanolamine][oleic acid] and aprotic [choline][oleic acid] ionic greases are successfully synthesized. These ionic greases can be directly used as lubricants without adding thickeners or other additives. Their distinct thermal and rheological properties are investigated and are well-correlated to their tribological properties. It is revealed that aprotic ionic grease shows superior temperature- and pressure-tolerant lubrication properties over those of protic ionic grease. The lubrication mechanism is studied, and it reveals that strong physical adsorption of ionic grease onto friction surface plays a dominating role for promoted lubrication instead of tribo-chemical film formation. KEYWORDS: ionic grease, lubrication, friction, rheology, interface

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on steel/steel contacts. In a more recent study, Gusain et al.2 developed halogen-free bis(imidazolium)/bis(ammonium)-di[bis(salicylato)borate] ILs as energy-efficient lubricant additives in synthetic PEG-200, and significantly reduced friction and wear were also observed on tested steel balls. To the best of our knowledge, most if not all of the existing lubricating greases require the addition of thickener in the base oil,20,21,33 which increases the manufacturing cost, decreases the biodegradability, and degrades certain lubrication properties. Because extra additives would pose potential contamination to certain applications such as microelectromechanical system (MEMs),24 artificial knee joints,25 and food and pharmaceutical industry,26,27 green and biocompatible grease lubricants and selflubrication composites28−30 are attracting ever increasing interest simply because they can avoid health hazards and product contamination and minimize the environmental impact caused by traditional synthetic and mineral greases. Compared with existing lubricating greases, the major advantage of ionic greases is the extraordinary capability to interact with metal surfaces and form robust tribo-films at the friction interface, attributed to their strong polar nature. Therefore, the film tends to resist being “squeezed out” under surface compression and remains stable under higher pressure conditions.17,31,32 More-

INTRODUCTION Ionic liquids (ILs) are defined as salts with melting temperature less than 100 °C and are frequently liquid at room temperature.1 ILs have been studied as high-performance lubricants or lubricant additive in recent years2−12 because of their unique physicochemical properties including negligible vapor pressure, low melting point, low flammability, excellent thermal stability, and miscibility with a wide range of organic compounds.13 ILs synthesized from renewable bioresources, also named “green ILs”, have attracted tremendous attention because of their biodegradability, nontoxicity, environmentally benign features, and more importantly their comparable physicochemical properties with conventional ILs.14−16 For example, the green [choline][amino acid] ILs have shown excellent anticorrosion properties besides their demonstrated outstanding tribological properties.17 As an important kind of lubricants, lubricating grease is often used because of its inherent sealing, anticorrosion properties, and long life.18,19 ILs have been demonstrated as effective additives to promote lubrication in base oils and greases.2,20−22 Cai et al.20 used benzotriazole group grafted imidazolium IL as additive in poly(ethylene glycol) (PEG) and polyurea grease, which effectively reduced friction/wear of steel pairs that outperforms commercial zincdiakyldithiophosphate-based additive (T204). Wang et al.23 synthesized lubricating grease by using 1-octyl-3-methylimidazolium hexafluorophosphate and 1octyl-3-methylimidazolium tetrafluoroborate as base oil and the polytetrafluoroethylene as thickener to reduce friction and wear © XXXX American Chemical Society

Received: December 15, 2015 Accepted: January 27, 2016

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DOI: 10.1021/acsami.5b12261 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. NMR spectrum of (a) oleic acid, (b) triethanolamine, (c) PIG, and (d) AIG.

over, another benefit of its intrinsic “grease” feature is that the usage of thickener could be avoided. Derived from the concept of IL, ionic grease can be classified as either protic ionic grease (PIG) or aprotic ionic grease (AIG), depending on whether the cationic group contains acidic proton.33 PIG contains acidic proton, whereas AIG does not. The key difference to distinguish PIG from AIG is the proton transfer from acid to the base, leading to the presence of proton-donor and -acceptor sites that can be used to build up a hydrogen-bonded network. These concepts are well-defined in ILs.34 That is to say, PIG will also undergo similar proton transfer from cation to anion to form the neutralized salt species under moderate temperature, whereas AIG will retain the ionic species rather than form neutral species before it decomposes at higher temperatures.35 In this work, novel thickener-free aprotic [choline][oleic acid] and protic [triethanolamine][oleic acid] ionic greases have been synthesized by acid−base neutralization reaction with oleic acid as anion and two bases, choline and triethanolamine, as cations, respectively. The thermal and rheological properties of the AIG and PIG are comparatively investigated. The lubricating property of AIG and PIG is systematically studied with a ball-on-disk friction configuration under different temperature and pressure conditions. The lubrication mechanism is also investigated in this work.

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NMR, Varian Mercury-300) in chloroform-d or D2O at 300 MHz. The following abbreviations are used to designate multiplicities: s = singlet, d = doublet, t = triplet, and m = multiplet. 1 H NMR (chloroform-d, 300 MHz) of PIG. d = 6.29 (s, 4H, OH, NH), 5.24−5.48 (m, 2H, HCCH), 3.58−3.87 (t, 6H, CH2, CH2, CH2), 2.58−2.97 (t, 6H, N−CH2, N−CH2, N−CH2), 2.20−2.30 (t, J = 7.5 Hz, 2H, CH2), 1.80−2.16 (m, 4H, CH2, CH2), 1.44−1.72 (d, 2H,CH2), 1.21−1.37 (d, 20H, CH2, CH2, CH2, CH2, CH2, CH2, CH2, CH2, CH2, CH2), 0.77−0.99 ppm (t, 3H, CH3). Solid evidence from NMR characterization, that is, an obvious peak shift of 0.38 ppm (6H, N−CH2, N−CH2, N−CH2) and 0.33 ppm (6H, CH2, CH2, CH2) shown in Figure 1a−c, clearly indicates that the proton transfer occurred in PIG. 1 H NMR (Deuterium oxide, 300 MHz) of AIG. d = 5.20−5.54 (m, 2H, HCCH), 3.89−4.11 (m, 2H, CH2), 3.43−3.54 (m, 2H, N− CH2), 3.18 (s, 9H, CH3, CH3, CH3), 2.07−2.20 (t, J = 7.5 Hz, 2H, CH2), 1.87−2.07 (d, 4H, CH2, CH2), 1.53 (s, 2H,CH2), 1.15−1.45 (d, 20H, CH2, CH2, CH2, CH2, CH2, CH2, CH2, CH2, CH2, CH2), 0.71− 0.98 ppm (t, 3H, CH3), shown in Figure 1d. MS spectra were obtained on a Bruker HCTultra QIT mass spectrometer (Bruker Daltonics, Billerica, MA): AIG m/z 386.4 (C23H47O3N+) and PIG m/z 414.4 (C24H49O5N+-H2O). These results clearly reveal the complexation between cation and anion in AIG and PIG, respectively. The thermal decomposition temperature (Td) of the greases was determined by thermogravimetric analysis (TGA, TA Instruments Q500) in N2 atmosphere from 20 to 600 °C with a heating rate of 10 °C/min. The decomposition temperature is calculated on the basis of the intersection of the tangent to the maximum change in slope of the weight loss curve with the tangent to the baseline. The glass transition temperature (Tg) was determined with a differential thermal analyzer (DSC, TA Instruments Q2000) from −150 to 100 °C with a heating rate of 10 °C/min after cooling samples to −150 °C in aluminum pan. The viscosity of the greases at 1−1000 s−1 shear rate was reported using a Bohlin CVO 100 rheometer. A cone-on-plate geometry was used with a 1° cone angle and 20 mm cone diameter. The lower plate has a diameter of 60 mm. During the test, the temperature was maintained at 25, 75, and 100 °C, respectively. X-ray photoelectron spectroscopy (XPS) scanning on friction steel disk was accomplished using a PHI VersaProbe II Scanning XPS Microprobe with Al Kα line excitation source. An Optimol SRV-III oscillating friction and wear tester was used to evaluate tribological properties of the greases under lubrication conditions based on ASTM D 6425 protocol. During the test, the upper steel ball (52100 bearing steel, diameter 10 mm, surface roughness (Ra) 20 nm) slides under reciprocating motion against a stationary steel disc (100CR6 ESU hardened, Ø24 mm × 7.9 mm, and surface roughness (Ra) 120 nm). The disc was supplied by Optimol Instruments Prüftechnik GmbH, Germany. The ball was provided by SKF, Sweden. Before each test, the device and specimens were cleaned with acetone and ethanol. Then, 0.5 mL of grease or triethanolamine were applied on the steel disc uniformly using a glass rod. All tests were conducted under the loads of 77 N (2.0 GPa Maxium Hertzian

EXPERIMENTAL SECTION

Materials. Choline hydroxide aqueous solution (48−50 wt % in water) was purchased from Tokyo Chemical Industry. Oleic acid (≥99%), triethanolamine (≥99%), deuteroxide (D2O, 99.9 atom % D), and acetone (≥99.8%) were purchased from Sigma-Aldrich. Chloroform-d (99.8 atom % D) was purchased from Cambridge Isotope Laboratories, Inc. The commercial multipurpose grease (MolyWay Li 712, lithium-soap-thickened mineral base grease, 1% molybdenum disulfide) was purchased from Statoil Lubricants. All chemicals were used as received without further treatment. Preparation of Protic Ionic Grease and Aprotic Ionic Grease. PIG. Equimolar triethanolamine was added dropwise to oleic acid with cooling. The mixture was magnetically stirred at 70 °C for 2 h. Then, PIG was dried overnight in vacuum oven at 60 °C before use. AIG. Oleic acid was first dissolved in 20 mL of acetone. Equimolar choline hydroxide aqueous solution was added dropwise to oleic acid aqueous solution with cooling. The mixture was magnetically stirred at room temperature for 48 h. Water and acetone were then removed under reduced pressure at 50 °C in rotary evaporator. The product was dried in a vacuum oven at 70 °C for 48 h. Characterization. The molecular structures of protic and aprotic ionic greases were analyzed by proton nuclear magnetic resonance (1H B


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Figure 2. (a) DSC and (b) TGA/DTG curves of AIG and PIG. Synthesis routes and molecular structures of (c) AIG and (d) PIG.

Figure 3. Log−log plots of (a) shear stress and (b) viscosity as a function of shear rate; pictures of PIG at (c) 25 °C and (d) 75 °C and AIG at (e) 25 °C and (f) 75 °C.

and then melting at −10.5 °C. The crystallization peak appears at −29.4 °C during cooling. Different from AIG, PIG shows two pairs of peaks during heating and cooling. One pair of peaks with larger intensity at relatively lower temperature is ascribed to the melting (17.5 °C) and crystallization (2.1 °C) behavior of PIG. The other pair of smaller peaks at higher temperature of ∼41.7 °C is attributed to the liquid crystal mesophase transition, which is often observed in IL and some neutral organic compounds involving intermolecular van der Waals interaction, dipole−dipole interaction, and π−π stacking.36,37 Below 41.7 °C, PIG remains anisotropic even after the melting peak because of the intense hydrogen bonding attributed to the dense hydroxyl groups in triethanolamine. Further heating decreases the hydrogen bonding intensity; therefore, an anisotropic to isotropic mesophase transition could be expected thereafter.

pressure), 150 N (2.5 GPa Maxium Hertzian pressure), and 258 N (3 GPa Maxium Hertzian pressure) at 25, 75, or 100 °C, a sliding frequency of 50 Hz, and an amplitude of 1 mm. The temperature was measured and controlled by a cooling/heating device under the steel disk that served as the sample holder for the disc. The temperature variation range is ±1 °C. The friction coefficient curves were recorded automatically with a data-acquiring system linked to the SRV-III tester. After the tests, the wear volumes of the lower discs and wear diameter of the balls were determined using an optical profiling system (Zygo 7300). Three duplicate friction and wear tests were carried out to minimize experimental error.

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RESULTS AND DISCUSSION Thermal property is one of the most important characteristics for ionic grease. Both AIG and PIG show melting and crystallization behavior, as seen from DSC curves in Figure 2a. Upon heating, AIG first exhibits its glass transition at −97.2 °C C


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Figure 4. Wear volume of disc and wear diameter of ball lubricated by AIG and PIG at (a and d) 2.0 GPa, (b and e) 2.5 GPa, and (c and f) 3.0 GPa. Test duration: 1 h.

continuously, which is typical “shear-thinning” phenomena.37 Shear thinning in protic ILs has been attributed to the weakened hydrogen bonding at higher shear rate,39 which explains well the observed shear thinning behavior of both PIG and AIG with increasing shear rate. A previous study also reported the weakened hydrogen bonding at elevated temperatures,40 which explains the reduced viscosity of both PIG and AIG as temperature goes up. It is worth mentioning that AIG remains its Bingham plastic fluid character at all testing temperatures from 25 to 100 °C (Figure 3b). However, PIG switches from Bingham plastic fluid at 25 °C to Newtonian fluid at 75/100 °C. This switching behavior is in good agreement with the mesophase transition from anisotropic to isotropic as evidenced by DSC results in Figure 2a and phase change from grease to liquid observed experimentally in Figure 3. Massive machinery equipment often run under lubrication conditions with typical operation conditions of high pressure, low velocity, and frequent start/stop switching.41 In this work, the wear and friction properties of the ionic greases were investigated using a steel ball on steel disc configuration because steel is the most widely used material in industry. Figure 4 shows the wear volume of the disc and wear scar diameter of the ball after 1 h friction test with the presence of AIG and PIG ionic greases. Both AIG and PIG show relatively low wear volume at 25 °C. However, the disc wear volume lubricated by PIG increases significantly at elevated temperature of 75/100 °C, whereas slight increase was observed in AIG (Figure 4a−c). Specifically, the wear volume of the disc lubricated by AIG is only 4.8% of the one lubricated by PIG at 75 °C and 2.0 GPa. With increasing pressure from 2.0 to 3.0 GPa, the wear volume continuously increases as expected with the highest wear volume observed in PIG under 3.0 GPa. Figure 5 presents the 3D surface profiles of the corresponding wear tracks on discs after friction tests under different temperature and pressure conditions. It is obvious that the wear tracks become wider and deeper with increasing testing pressure and temperature. Comparing the 3D images from first/second, third/fourth, or fifth/sixth rows of Figure 5,

Thermogravimetric analysis of AIG and PIG reveals their significantly different degradation behavior (Figure 2b). Obviously, AIG shows a relatively higher onset decomposition temperature (189.5 °C) than that of PIG (177.3 °C), indicating its better thermal stability in high-temperature lubrication. Besides the degradation at 177.3 °C, it is also worth mentioning that PIG shows the other major thermal decomposition at around 400 °C, which is attributed to the degradation of the esterification reaction products between triethanolamine and oleic acid during thermal heating (Figure 2d). The characteristic features of grease are the linear shear stress (resistance to flow)−shear rate relationship and a finite yield stress before it begins to flow, which is well-defined as Bingham plastic fluid behavior.38 Figure 3 shows the rheological behavior of both AIG and PIG at 25, 75, and 100 °C, which are denoted as AIG/PIG-25, AIG/PIG-75, and AIG/PIG-100, respectively. The extended dashed lines of the shear stress for AIG-25/75/ 100 and PIG-25 do not pass through the point (0, 0) (Figure 3a), which clearly reveals the characteristic Bingham plastic behavior, whereas the extended dashed line of PIG-75/100 passes through point (0, 0), implying the Newtonian fluid character of PIG at 75 and 100 °C. The pictures of corresponding ionic greases in tilted glass vials show their gel characteristics at 25 °C (Figure 3c,e). PIG loses its gel character and flows freely when heated up to 75 °C, as seen in Figure 3d, whereas AIG remains its high viscosity even at 75 °C. The gel to liquid transformation of PIG from 25 to 75 °C is wellcorrelated to the melting and mesophase transition as evidenced by DSC results in Figure 2a. First, the sharp melting peak of PIG in Figure 2a indicates its intrinsic fast phase change property with temperature. After melting, the intermolecular hydrogen-bonding interaction becomes weak but still remains bonded to maintain its grease character. Once the weak interaction breaks down after mesophase transition at ∼41.7 °C, the PIG loses its structural support and turns into liquid phase. The viscosity of AIG-25/75/100 and PIG-25 starts from a much higher value than that of PIG-75/100 at low shear rate (Figure 3b). With increasing shear rate, the viscosity decreases D


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Figure 5. 3D microscopic images of wear tracks: (a−c) AIG at 2.0, 2.5, and 3.0 GPa, 25 °C, (d−f) PIG at 2.0, 2.5, and 3.0 GPa, 25 °C, (g−i) AIG at 2.0, 2.5, and 3.0 GPa, 75 °C, (j−l) PIG at 2.0, 2.5, and 3.0 GPa, 75 °C, (m−o) AIG at 2.0, 2.5, and 3.0 GPa, 100 °C, and (p−r) PIG at 2.0, 2.5, and 3.0 GPa, 100 °C.

it is apparent that wear tracks with PIG are relatively wider and deeper than the ones lubricated by AIG, which further confirms the superior lubrication properties of AIG. All above results indicate that (1) AIG offers better lubrication properties than PIG in the steel/steel contact, (2) AIG is less sensitive to operational temperature and pressure than PIG in maintaining excellent lubrication and antiwear properties, and (3) AIG could serve as superior lubricant without adding thickener or other additives. The 3D microscopic images of the corresponding wear scars on the steel balls are provided in Figure 6. Circular and smooth wear surfaces were observed at relative low wear loss. Deep furrows along the motion direction were observed on the wear surface when wear became severe. The development of deep furrows may be attributed to the direct contact of the asperities at friction interface where lubricants cannot provide a good protection film for the steel at large pressure and high

temperature. The diameter of the wear scar is measured and summarized in Figure 4d−f as an indicator to dictate the lubrication property of AIG and PIG. Consistent with the results obtained from wear disk, the diameter of the wear scar on the ball is relatively smaller with AIG lubricant than with PIG under all testing conditions. Specifically, the ball wear scar diameter with AIG is only 75% of the one lubricated by PIG at 100 °C and 3.0 GPa. Considering both the steel disc wear volume and ball wear scar diameter, it is concluded that AIG outperforms PIG especially under higher temperature and pressure conditions. To understand the lubrication property relating to their specific molecular structure of AIG and PIG, tribological testing is also planned to perform on their cation molecules, i.e, pure choline and triethanolamine. However, lubrication by pure choline is not achievable because it is in solid form. A 1 order of magnitude higher wear volume than that of either AIG or PIG E


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Figure 6. 3D microscopic images of ball wear tracks: (a−c) AIG at 2.0, 2.5, and 3.0 GPa, 25 °C, (d−f) PIG at 2.0, 2.5, and 3.0 GPa, 25 °C, (g−i) AIG at 2.0, 2.5, and 3.0 GPa, 75 °C, (j−l) PIG at 2.0, 2.5, and 3.0 GPa, 75 °C, (m−o) AIG at 2.0, 2.5, and 3.0 GPa, 100 °C, and (p−r) PIG at 2.0, 2.5, and 3.0 GPa, 100 °C.

Figure 7. (a) Wear volume of disc and ball wear diameter and (b) friction coefficient lubricated by triethanolamine at 25 °C and 3.0 GPa for 1 h. 3D microscopic images of (c) disc and (d) ball wear tracks lubricated by triethanolamine at 25 °C and 3.0 GPa for 1 h.

Figure 8. Friction coefficient lubricated by AIG and PIG at (a) 2.0 GPa, (b) 2.5 GPa, and (c) 3.0 GPa. Test duration: 1 h.

then stabilized at ∼0.15 afterward (Figure 7b). Both the wear track on disk and wear scar on ball surface are provided in Figure 7c,d for visual comparison. Figure 8 shows friction coefficient evolution during a 1 h friction test lubricated by ionic greases. AIG is capable of

under corresponding testing conditions is observed by using pure triethanolamine as lubricant (Figure 7a). The wear scar diameter is measured ∼0.8 mm, which is 157 and 129% larger than the ones lubricated by AIG and PIG, respectively. The friction coefficient experienced a fluctuation at early stage and F


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ACS Applied Materials & Interfaces stabilizing the friction coefficient at 3.0 GPa at 25 °C, whereas severe fluctuation is observed at later stage by using PIG lubricant. The unstable friction coefficient at 3.0 GPa is caused by the failure of lubrication film, which leads to a significantly increased disc wear volume and ball wear diameter as summarized in Figure 4c,f. At 75 °C, lower friction coefficient was obtained by AIG (0.082−0.111), which is about 73−85% of the ones lubricated by PIG (0.113−0.137). At 100 °C, PIG exhibits obvious fluctuation in friction coefficient, whereas AIG maintains stable friction all through the end. These results further demonstrate the superior lubrication property of AIG. To compare the lubrication property of AIG/PIG with commercial greases, tribological testing of commercial multipurpose grease (MPG, lithium-soap-thickened mineral base grease with 1% molybdenum disulfide) is also carried out at 2.0 GPa and 25 °C. The tested average friction coefficient of 0.131 is relatively larger than that of PIG and AIG (0.092−0.096) under the same testing conditions, whereas the wear volume of the disc lubricated by MPG (1.07 × 10−4 mm3) is similar to the ones lubricated by PIG and AIG. Relatively higher friction coefficient of 0.110 and 0.106 was also reported in other commercial lithium and calcium greases.42 Without incorporating any additives, the lubricating performance of AIG/PIG is comparable or even better than commercial lubricating greases. XPS analysis of the wear surfaces was carried out to investigate the tribological mechanism of the ionic greases (Figure 9). Similar binding energies of C 1s, O 1s, and Fe 2p3

steel ball and steel disc to reduce friction coefficient and wear loss. Besides, the hydrodynamic lubrication or the formation of transient fluid patterns may happen in viscoelastic fluids, which facilitates the lubrication at interface. In the case of PIG, it undergoes reverse reaction from ionic grease to their neutral oleic acid and triethanolamine species at elevated temperature (Figure 2d) and thus loses its lubrication function gradually as the reverse reaction proceeds.

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CONCLUSIONS We report the synthesis of novel aprotic and protic ionic greases via facile neutralization reactions. Aprotic ionic grease retains Bingham plastic fluid behavior and shows stable lubrication performance at temperature up to 100 °C. Protic ionic grease experiences mesophase transition and switches fluid behavior from that of Bingham plastic at 25 °C to that of Newtonian fluid at 75/100 °C, which consequently causes the poor tribological performances. The excellent lubrication performance of the thermally stable aprotic ionic grease allows its promising practical lubrication applications under wider temperature and pressure conditions.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: 0046920492064. *E-mail: [email protected]. Tel.: 1-330-972-6859. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Acknowledgement is made to the Donors of the American Chemical Society Petroleum Research Fund for support of this research (#55570-DNI10). Partial support from start-up funds, Faculty Research Committee, and Firestone Faculty Research Fellowship from The University of Akron are also acknowledged.

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REFERENCES

(1) Pinkert, A.; Marsh, K. N.; Pang, S.; Staiger, M. P. Ionic Liquids and Their Interaction with Cellulose. Chem. Rev. 2009, 109 (12), 6712−6728. (2) Gusain, R.; Gupta, P.; Saran, S.; Khatri, O. P. Halogen-Free Bis(imidazolium)/Bis(ammonium)-Di[bis(salicylato)borate] Ionic Liquids As Energy-Efficient and Environmentally Friendly Lubricant Additives. ACS Appl. Mater. Interfaces 2014, 6 (17), 15318−15328. (3) Ye, C. F.; Liu, W. M.; Chen, Y. X.; Yu, L. G. Room-Temperature Ionic Liquids: a Novel Versatile Lubricant. Chem. Commun. 2001, 21, 2244−2245. (4) Li, H.; Wood, R. J.; Rutland, M. W.; Atkin, R. An Ionic Liquid Lubricant Enables Superlubricity to Be ″Switched On″ In Situ Using an Electrical Potential. Chem. Commun. 2014, 50 (33), 4368−4370. (5) Abbott, A. P.; Ahmed, E. I.; Harris, R. C.; Ryder, K. S. Evaluating Water Miscible Deep Eutectic Solvents (DESs) and Ionic Liquids as Potential Lubricants. Green Chem. 2014, 16 (9), 4156−4161. (6) Otero, I.; López, E. R.; Reichelt, M.; Villanueva, M.; Salgado, J.; Fernández, J. Ionic Liquids Based on Phosphonium Cations As Neat Lubricants or Lubricant Additives for a Steel/Steel Contact. ACS Appl. Mater. Interfaces 2014, 6 (15), 13115−13128. (7) Zhou, Y.; Dyck, J.; Graham, T. W.; Luo, H.; Leonard, D. N.; Qu, J. Ionic Liquids Composed of Phosphonium Cations and Organophosphate, Carboxylate, and Sulfonate Anions as Lubricant Antiwear Additives. Langmuir 2014, 30 (44), 13301−13311. (8) Elbourne, A.; Sweeney, J.; Webber, G. B.; Wanless, E. J.; Warr, G. G.; Rutland, M. W.; Atkin, R. Adsorbed and Near-Surface Structure of

Figure 9. XPS-scanned spectra of disc wear scar lubricated by the different ionic greases at 75 °C and 3.0 GPa.

were observed on the disc worn surfaces with and without lubrication. Because neither additional peaks such as N 1s peak nor peak shifts were detected on the worn surfaces lubricated by AIG and PIG, the tribochemical reaction does not likely to occur on metal surface during friction test. Hence, the outstanding tribological properties of ionic greases could be attributed to the formation of strong ionic grease films by physical adsorption. Our previous study reveals that outstanding tribological properties of [choline][glycine] and [choline][L-proline] ILs are attributed to the formation of IL films by physical adsorption.17 Similarly, in this work, the lowenergy electrons on metal surface are released from contact convex sites during friction; thus, the negatively charged carboxylic acid group in oleic acid exhibits strong affinity to the positively charged steel surface. AIG acquires stable chemical structure and strong affinity to metal surface, which positively contributes to the formation of mechanically strong lubrication film and thus effectively prevents the direct contact between G


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Ionic Grease Lubricants: Protic [Triethanolamine][Oleic Acid] and

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