Effects of Alkyl Chain Length of Sulfate and

Effects of Alkyl Chain Length of Sulfate and Phosphate Anions-Based Ionic Liquids on Tribochemical Reactions

Shouhei Kawada1) *, Seiya Watanabe1), Chiharu Tadokoro2) and Shinya Sasaki3)

1)

Department of Mechanical Engineering, Graduate School of Tokyo University of Science 6-3-1 Niijuku, Katsushika-ku, Tokyo 125-8585, Japan 2)

Department of Mechanical Engineering, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570, Japan

3)

Department of Mechanical Engineering, Tokyo University of Science 6-3-1 Niijuku, Katsushika-ku, Tokyo 125-8585, Japan

*Corresponding author: [email protected]

1

Abstract Ionic liquids are expected to become increasingly popular lubricants as they feature a number of attractive properties. This investigation focused on sulfate and phosphate anion-based ionic liquids and the improvement in lubricating performance with the addition of these anions. However, the detailed lubricating mechanism and effect of alkyl chain length on tribochemical reactions are unclear. This study investigates tribochemical reaction processes using a quadrupole mass spectrometer (Q-MS) and X-ray photoelectron spectroscopy. Seven types of ionic liquids: 1-ethyl-3methylimidazolium hydrogensulfate ([EMIM][HSO4]), 1-ethyl-3-methylimidazolium methylsulfate ([EMIM][MSU]), 1ethyl-3-methylimidazolium ethylsulfate ([EMIM][ESU]), 1-ethyl-3-methylimidazolium n-octylsulfate ([EMIM][OSU]), 1-ethyl-3-methylimidazolium dimethyl phosphate ([EMIM][DMP]), 1-ethyl-3-methylimidazolium diethyl phosphate ([EMIM][DEP]), and 1-ethyl-3-methylimidazolium dibutyl phosphate ([EMIM][DBP]) were selected as lubricants. The friction coefficient of sulfate anion-based ionic liquids increased as their alkyl chain lengthened. However, wear scar diameters in this case showed the opposite tendency. The friction coefficient and wear scar diameter of phosphate anion-based ionic liquids increased with an increase in the alkyl chain length. Q-MS results indicated that the main outgassing components during sliding were the cation components, whereas the anion remained on the sliding surface and formed a tribofilm. The ionic liquids with short alkyl chains reacted with the sliding surface easily and led to very low friction. However, corrosive wear occurred in the case of the sulfate anion. On the other hand, anions with long alkyl chains underwent gradual tribochemical reactions because that led the mitigation of contact with nascent surface. The phosphate-based ionic liquids with long alkyl chains were unable to cause the lubricating effect due to low reactivity. Keywords: Ionic liquids; Tribochemical reaction; Quadrupole mass spectrometer; Sulfate; Phosphate

2

1. Introduction Ionic liquids, which are salts consisting of organic cations and usually inorganic anions, have been reported to exist in the liquid phase at temperatures below 100 °C, although there is no precise definition for these species [1-3]. Ionic liquids feature a number of advantageous properties when used as neat oils and lubricant additives, which include low melting point/poor crystallizability, high thermal stability, refractory properties, nonflammability, and ability to remain in the liquid phase across a wide temperature range [3-9]. In addition, millions of combinations of cations and anions exist in theory; hence, the features of ionic liquids can be easily controlled depending on their make-up. The lubricating performance of ionic liquids has been reported previously, the majority featuring a fluorine element in their chemical structures, such as tetrafluoroborate ([BF4]), hexafluoro phosphate ([PF6]), trifluoromethanesulfonate ([OTF]), bis(trifluoromethane)sulfonimide ([TFSI]), and tris(perfluoroalkyl)trifluorophosphate ([FAP]) [10-14]. Metallic fluoride is produced on the metal surface, which allows these reactants to achieve a low friction coefficient. Unfortunately, hydrolysis of the metallic fluoride leads to corrosive wear [15-17]. It has also been reported that hydrolysis occurs in [BF4] and [PF6] in the presence of water or moisture from the atmosphere [18-20]. As alternatives, fluorine-free anions such as bis(oxalato)borate ([BOB]), dicyanoamide ([DCN]), tricyanomethane ([TCC]), and tetracyanoborate ([TCB]) have recently drawn attention [21-24]. In this investigation, sulfate and phosphate anions were chosen in an attempt to improve the lubricating performance. These are widely used as extreme pressure additives, for example, in zinc dialkyl dithiophosphate (ZDDP) [25-27]. It has been reported that [DMP] and [DPP], which also feature phosphate, exhibited similar or lesser wear than [TFSI] and [FAP] for steel/steel contact [28, 29]. However, the detailed lubricating mechanism for sulfate and phosphate anions is unclear, especially as they have alkyl chains in their structures (although the effects of alkyl chains on lubricating performance have been noticed among all kinds of lubricant additives). The perception of lubricating mechanism of ionic liquids useful information for using ionic liquids as neat oil and additives. It has reported that the alkyl chain of lubricant additives affect the reactivity, degradability, and lubricating performance [30-32]. Thus, the lubricating performance of the sulfate and phosphate anions was evaluated using a ball-on-disk friction tester. The effects of the alkyl length of these anions on tribo-chemical reactions were estimated using a quadrupole mass spectrometer (Q-MS) and X-ray photoelectron spectroscopy (XPS). In addition, the relationship between their alkyl chain lengths and physical properties was evaluated by thermogravimetric analysis (TGA) and viscometer measurements.

3

2. Experimental Details 2.1. Ionic liquids as lubricants Four kinds of sulfate and three kinds of phosphate anions were evaluated. Table 1 lists the chemical names and chemical structures of the tested ionic liquids: 1-ethyl-3-methylimidazolium hydrogen sulfate ([EMIM][HSO4]), 1-ethyl3-methylimidazolium methyl sulfate ([EMIM][MSU]), 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIM][ESU]), 1ethyl-3-methylimidazolium n-octyl sulfate ([EMIM][OSU]), 1-ethyl-3-methylimidazolium dimethyl phosphate ([EMIM][DMP]), 1-ethyl-3-methylimidazolium diethyl phosphate ([EMIM][DEP]), and 1-ethyl-3-methylimidazolium dibutyl phosphate ([EMIM][DBP]). [EMIM][HSO4], [EMIM][ESU] and [EMIM][OSU] were purchased from Merck Chemicals, Germany, as “For Synthesis (S)” grade (halide content < 1000ppm, water content < 10000ppm). [EMIM][MSU], [EMIM][DMP], [EMIM][DEP], and [EMIM][DBP] were purchased from IoLiTec, Germany, as “HP” grade (water content < 500, 1000, 2500, 5000ppm, respectively). All ionic liquids existed in the liquid phase at room temperature under atmospheric pressure.

2.2. Physical properties of the ionic liquids The viscosities of the ionic liquids were measured using a tuning-fork vibration viscometer (SV-1A, A&D Company, Japan). The oscillator was titanium, which had high corrosion resistance. The measurement temperature range was 4070 °C, and pure water was used as a calibrator at room temperature. The thermal stabilities of the ionic liquids were measured using a thermogravimetric unit (TG-DTA2010SA, Bruker, USA). The pan was made of aluminum, the gas environment was N2, the programming rate was 10 °C/min and the measurement range was 50 - 500 °C. The weight of lubricants was approximately 10 mg.

2.3. Evaluation of lubricating performance via sliding tests The lubricating performance of each ionic liquid was evaluated using an in-house instrument ball-on-disk friction tester ( 24 mm × t 7.9 mm disk and  4 mm ball made of bearing steel (AISI 52100)) [21]. Number of determinations of each ionic liquid was 10, and the good repeatability was obtained. The surface roughness of both specimens was Ra 0.05 μm, and they were ultrasonically cleaned twice using a mixed solution of petroleum benzine and acetone (1:1 ratio) for 20 min. Table 2 lists the sliding test conditions. After this test, the wear scar diameter of the ball specimen and wear scar images of both specimens were recorded by an optical microscope (OM, VHX-100, Keyence, Japan). The resolution was

4

1μm. Table 1 Molecular structure of used ionic liquids 1-ethyl-3-methylimidazolium hydrogensulfate [EMIM][HSO4]

H3C

N

O +

N

CH3

HO

S

O

-

O 1-ethyl-3-methylimidazolium methylsulfate [EMIM][MSU]

H3C

N

O +

N

CH3

O

O

S

H3C

-

O

1-ethyl-3-methylimidazolium ethylsulfate [EMIM][ESU]

H3C

N

+

N

H3C

CH3

CH2 O O

O

S O

1-ethyl-3-methylimidazolium n-octylsulfate [EMIM][OSU]

H3C

N

O +

N

CH3

O H17C8

S

O

-

O

1-ethyl-3-methylimidazolium dimethylphosphate [EMIM][DMP]

O

H3C

N

+

N

CH3

OCH3

P

O

-

OCH3 1-ethyl-3-methylimidazolium diethylphosphate [EMIM][DEP]

O

H3C

N

+

N

CH3

C2H5O

P

O

-

OH5C2 1-ethyl-3-methylimidazolium dibutylphosphate [EMIM][DBP]

O

H3C

N

+

N

CH3

C4H9O

P

O

-

OH9C4

5

-

Table 2 Frictional test condition Load Rotation speed Lubricants Temperature Pressure Sliding time

3.5 N 100 rpm 30 μL 25°C 2.0 × 10-5 Pa 2hour

2.4. Analysis of tribochemical reactions via Q-MS and XPS The tribochemical reactions of the ionic liquids were estimated using a quadrupole mass spectrometer (Q-MS, MKS Instruments, Inc.). The measurable “mass-to-charge ratio (m/e)” range of Q-MS was 1-200 (m/e), using an ion source sensitivity of 3.8 × 10−7A/Pa with a secondary electron multiplier. The partial pressure of the ions was converted from the ion currents using the conversion rate of N2 by software of Q-MS. The m/e derived from decomposition of ionic liquids in before and after and under the friction tests was measured. The temporal resolution was approximately 1 sec. After the friction tests, the disk specimens were ultrasonically cleaned with a mixed solution of petroleum benzine and acetone (1:1 ratio) for 10 min. The worn disk surfaces were analyzed by XPS (QUANTERAII, ULVAC-PHI, Inc., Japan) with a monochromatic AlKα X-ray source (1486.6 eV). All the spectra were referenced relative to the C1s peak (285.0 eV).

3. Results 3.1. Physical properties Figure 1 shows the drastic change in the viscosity of the ionic liquids upon altering the alkyl chain length of the sulfate anion (Fig. 1 (a)). [EMIM][HSO4], which does not have an alkyl chain, showed the highest viscosity; therefore, it was considered that the ionic radius affected the viscosity, as liquids with a smaller ionic radius showed a stronger electrical charge (Coulombs) between the cations and the anions. [EMIM][MSU] and [EMIM][ESU] showed the lowest viscosity (less than 50mPa·s), which remained almost constant as the temperature was changed. In contrast, [EMIM][OSU], which has the longest alkyl chain, showed the highest viscosity. It was considered that the molecular interactions of the alkyl chain led to high viscosity. The test carried out on the phosphate anion-based ionic liquids (Fig. 1 (b)) confirmed the relationship between the alkyl chain length and viscosity, which what was expected at the beginning of this research.

6

[EMIM][HSO 4] [EMIM][MSU] [EMIM][ESU] [EMIM][OSU]

Viscosity [mPa・s]

300

200

100

0 40

50 60 Temperature[℃]

70

Fig. 1 (a) The viscosity of the sulfate anions based ionic liquids

Viscosity [mPa・s]

300

[EMIM][DMP] [EMIM][DEP] [EMIM][DBP]

200

100

0 40


70

Fig. 1 (b) The viscosity of the phosphate anions based ionic liquids

Figure 2 shows the thermal stabilities of each ionic liquid. The test on the sulfate anion-based ionic liquids (Fig. 2 (a)) revealed very little difference in the thermal stability as the alkyl chain length was altered. [EMIM][OSU] underwent thermal decomposition more quickly in comparison to the other tested ionic liquids. Contrary to the case of the sulfate anion-based liquids, the phosphate anion-based ionic liquids (Fig. 2 (b)) showed a significant difference in thermal stability as the alkyl chain length was changed; in particular, [EMIM][DBP] which showed very high thermal stability. 7

However, the relationship of alkyl chain length and thermal stability was not confirmed. It was considered that the effect of molecular interactions of the alkyl chain and coulombs were entwined.

mass[％]

100

50

[EMIM][HSO 4] [EMIM][MSU] [EMIM][ESU] [EMIM][OSU] 0

100


400

500

Fig. 2 (a) The thermal stability of the sulfate anions based ionic liquids

mass[％]

100

50

[EMIM][DMP] [EMIM][DEP] [EMIM][DBP] 0

100


400

500

Fig. 2 (b) The thermal stability of the phosphate anions based ionic liquids

3.2. Lubricating performance Figure 3 shows the mean friction coefficient over a period of 5 min, as well as the wear scar diameter of the ball specimens. The results for the sulfate anion-based ionic liquids (Fig. 3(a)) confirmed the relationship between the alkyl chain length of the anions and the lubricating performance, as the friction coefficients increased as the alkyl chain became longer. However, the wear scar diameters of the ball specimens tended to decrease as the friction coefficient decreased. 8

The friction coefficient and wear scar diameter of [EMIM][HSO4], which did not have an alkyl chain, were approximately 0.04 and 190 μm, respectively. On the other hand, [EMIM][OSU], which had the longest alkyl chain, had a friction coefficient and wear scar diameter of approximately 0.06 and 110 μm, respectively. For the phosphate anion-based ionic liquids (Fig. 3 (b)), the friction coefficient and wear scar diameter showed an increasing tendency as the alkyl chain became longer. Figure 4 shows the worn surface images of both specimens. This figure shows that corrosive wear only

Friction coefficient

0.1

Friction coefficient Wear

200

0.05 100

0

[EMIM] [HSO4]

[EMIM] [MSU]

[EMIM] [ESU]

[EMIM] [OSU]

Wear scar diameter [μm]

occurred in [EMIM][HSO4] and [EMIM][MSU], most prominently on the exterior of the wear scar.

0


0.1

Friction coefficient Wear

200

0.05 100

0

[EMIM] [DMP]

[EMIM] [DEP]

[EMIM] [DBP]

Wear scar diameter [μm]

Fig. 3 (a) The friction coefficient and wear scar diameter of the sulfate anions based ionic liquids

0

Fig. 3 (b) The friction coefficient and wear scar diameter of the phosphate anions based ionic liquids

9

Fig. 4 The worn surface images (a) [EMIM][HSO4], (b) [EMIM][MSU], (c) [EMIM][ESU], (d) [EMIM][OSU], (e) [EMIM][DMP], (f) [EMIM][DEP], and (g) [EMIM][DBP]

3.3. Tribochemical reaction The tribochemical reactions of sulfate and phosphate anion-based ionic liquids were evaluated using a Q-MS, which can measure the residual gas in the vacuum chamber. Outgassing of the ionic liquids was investigated as the index for the tribochemical reaction. Furthermore, the kind of gas generated from the ionic liquids was predicted prior to the analysis. The outgassing of CH3 (m/e = 15) and C2H6 (m/e = 30) was derived from the cations and H2S (m/e = 34), CH3O (m/e = 31), C2H5O (m/e = 45), SO2 (m/e = 64), and PO3 (m/e = 79) was derived from the anions [10]. However, the partial pressures of the cation and anion themselves could not be detected without the noise signal or beyond the measurement range. Figure 5 and 6 show the outgassing and friction behavior of the sulfate and phosphate anion-based ionic liquids, respectively. As can be seen in Fig. 5 (a) outgassing derived from the cation in [EMIM][HSO4] was only slightly detected, whereas outgassing of H2S derived from the anion was significant in the initial sliding period. Following this, the outgassing showed a stable behavior and the outgassing of SO2, derived from the anion, was detected over the entire sliding period. The friction also showed stable behavior when this liquid was tested.

10

C2H6


Partial pressure [Pa]

CH3

H2S

SO2

−8

10

CH3 H 2S

−9

10

C2H6 −10

10

SO2 Sliding period

−11

10


0.1 0.05 0

0

30

60 Sliding time [min.]

90

120

Fig. 5 (a) The outgassing and friction behavior of [EMIM][HSO4]

In the case of [EMIM][MSU] (Fig. 5 (b)), outgassing derived from both ions was detected and the outgassing of CH3, C2H6, and CH3O showed unstable behavior. Moreover, the friction coefficient showed unstable behavior. The outgassing of H2S and SO2 was high just after the sliding started and then became stable. In the case of [EMIM][ESU] (Fig. 5 (c)), the outgassing of CH3 and C2H6 was detected, which also showed unstable behavior. The outgassing of CH3O and C2H5O was high in the initial sliding period and gradually decreased with time. However, outgassing of H2S and SO2 was hardly



detected. The friction coefficient showed nearly similar behavior to the outgassing of CH3O and C2H5O.

CH3

−8

CH3 H2S

CH3O

10

C2H6 SO2

CH3O −9

SO2

C2H6

10

H2S

−10

10

Sliding period

−11

10


0.1 0.05 0

0

30


90

120

Fig. 5 (b) The outgassing and friction behavior of [EMIM][MSU] 11

C2H6



CH3

CH3O

C2H5O

H2S

SO2

−8

10

CH3

CH3O −9

10

C2H6

C2H5O

H2S −10

10

SO2

−11

10

Sliding period

0.1


0.05 0

0

30


90

120

Fig. 5 (c) The outgassing and friction behavior of [EMIM][ESU]

In the case of [EMIM][OSU] (Fig. 5 (d)), the outgassing of CH3, C2H6, CH3O, and C2H5O was only slightly detected and stable behavior was observed, but outgassing of SO2 was not detected. As can be seen from this figure, the friction



coefficient showed similar behavior as the outgassing of CH3O.

CH3O

−8

10

C2H5O

C2H6 SO2

CH3 H2S

CH3 C2H6

−9

10

CH3O −10

10

C2H5O

H2S

SO2

−11

10

Sliding period

0.1


0.05 0

0

30


90

120

Fig. 5 (d) The outgassing and friction behavior of [EMIM][OSU]

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For [EMIM][DMP] (Fig. 6 (a)), the outgassing of CH3, C2H6, CH3O, and PO3 was large in the initial sliding period. However, a peak was found in CH3, C2H6, and CH3O at 20 min, 55 min, and 115 min. The friction coefficient showed


similar behavior to the outgassing of CH3, C2H6, and CH3O.

−8

CH3

10

C2H6 PO3

C2H6

−10

10


CH3 CH3O

PO3

CH3O

−12

10

Sliding period

0.1


0.05 0

00

30

3600 60 Sliding time [min.]

90

7200 120

Fig. 6 (a) The outgassing and friction behavior of [EMIM][DMP]

In the case of [EMIM][DEP] (Fig. 6 (b)), the outgassing and friction coefficient showed almost the same behavior as

Partial Pressure [Pa]

[EMIM][DMP], but the outgassing of PO3 was not detected. CH3 CH3O

−8

10

C2H6 C2H5O

PO3

CH3 C2H6

−10

10

CH3O

C2H5O


PO3

−12

10

Sliding period

0.1


0.05 0

00

30


90

7200 120

Fig. 6 (b) The outgassing and friction behavior of [EMIM][DEP]

13

With regard to [EMIM][DBP] (Fig. 6 (c)), the outgassing of C2H6 and CH3O was large in the initial sliding period and then showed stable behavior. The outgassing of PO3 was not detected, and the friction coefficient showed stable behavior


during the entire sliding period. CH3 CH3O

−8

10

PO3

CH3 C2H6 CH3O C2H5O

−10

10


C2H6 C2H5O

PO3

−12

10

Sliding period

0.1


0.05 0

00

30


90

7200 120

Fig. 6 (c) The outgassing and friction behavior of [EMIM][DBP]

3.4. Surface analysis Information on the chemical structure of the sliding surface was obtained using XPS analysis. Figure 7 shows the S2p and P2p spectra of the sulfate and phosphate anions, respectively. For the sulfur anion-based ionic liquids, the FeSO4 peak (168-169 eV) was observed on the worn surface and on the outside of the worn surface; however, FeS and FeS2 (162-163 eV) peaks were not observed [33-35]. It was considered that the FeSO4 was produced by sliding, as well as in the static state. In the case of the phosphate anion-based ionic liquids, the peak due to FePO4 (133-134 eV) was observed on the worn surface [36-38]. XPS results also confirmed that the tribofilm was generated, and these films dominate the lubricating performance.

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(b) S2p

165 160 155 Binding energy [eV]

165 160 155 Binding energy [eV] 133.3 eV

(f) P2p

Intensity [a. u.] 135 130 125 Binding energy [eV]

140

(c) S2p

168.5 eV

(d) S2p

Intensity [a. u.]

Intensity [a. u.] 170

(e) P2p

Intensity [a. u.]

133.4 eV

169.1 eV

170


133.3 eV

170


(g) P2p

Intensity [a. u.]

170

140

168.5 eV Intensity [a. u.]

(a) S2p

Intensity [a. u.]

168.1eV

135 130 125 140 Binding energy [eV]


Fig. 7 The spectra of S2p, and P2p of sulfate and phosphate anion (a) [EMIM][HSO4], (b) [EMIM][MSU], (c) [EMIM][ESU], (d) [EMIM][OSU], (e) [EMIM][DMP], (f) [EMIM][DEP], and (g) [EMIM][DBP]

4. Discussion As per the results of lubricating performance tests (Fig. 3), the sulfate anion-based ionic liquids exhibited low friction and the phosphate anion-based ionic liquids exhibited low wear volume. The results of Q-MS analysis indicated that the outgassing of anion components was small in comparison to that of the cation components. Thus, the anions remain on the worn surface and react with the worn surface. It has often been reported that sulfur compounds have a friction reduction effect and that phosphorous compounds have a wear reducing effect [10-14, 25-32]. In the case of this experiment, the peaks due to these reactants were confirmed and it was considered that they worked as the tribofilm, as mentioned in previous reports. From the results of Q-MS, during the tribochemical reactions of the sulfur anion-based ionic liquids, high reactivity on the steel sliding surface was confirmed when anions with short or no alkyl chains, such as [EMIM][HSO4] and [EMIM][MSU], were used. In comparison, the tribochemical reactions of [EMIM][ESU] and [EMIM][OSU] showed mild behavior, but the thermal stability was almost the same. Thus, this tribochemical reaction was not dominated by friction heat. It is well known that the sliding surface has a high activities because of the appearance of the nascent surface, and its surface work as a catalytic substance [39-41]. These results illustrated that short-alkyl-chain ionic liquids were

15

easily susceptible to catalytic degradation on nascent steel surfaces because the ionic liquids easily contact on the active site, and then led to very low friction coefficients. However, corrosive wear occurred due to excessive reactions on the nascent surface. On the contrary, due to the steric hindrance of the anion, it was considered that anions with long alkyl chains led the mitigation of contact with the nascent surface and slowed down the tribochemical reaction [41]. In the case of the phosphate anion-based ionic liquids, the same trend was confirmed. However, from the results of Q-MS long-alkylchain ionic liquids could not achieve the same lubricating effect due to gradual tribochemical reactions.

5. Conclusions In this study, the tribochemical reactions of sulfate and phosphate anion-based ionic liquids on steel surfaces were investigated using Q-MS and XPS analyses. The conclusions drawn from this research were the following:

1.

The friction coefficient of the sulfate anion-based ionic liquids increased as the alkyl chain lengthened, but the wear scar diameter of the ball specimens decreased as the alkyl chain lengthened. On the other hand, the friction coefficient and wear scar diameter of the phosphate anion-based ionic liquids showed an increasing tendency.

2.

The tribochemical reaction of all ionic liquids was confirmed using Q-MS analysis. The main outgassing species were the cation components and alkoxide of the anions. It was considered that the sulfate and phosphate remained on the steel surface and formed the tribofilm. Additionally, reactants derived from the ionic liquids were confirmed using XPS analysis.

3.

It was considered that sulfate anion-based ionic liquids with short alkyl chain lengths reacted easily with the sliding surface and led to very low friction; however, corrosive wear occurred in this case. On the contrary, longalkyl-chain ionic liquids showed gradual tribochemical reactions on the sliding surface due to the mitigation of contact with the nascent surface; hence, corrosive wear was prevented by the long alkyl chain. Phosphate anionbased ionic liquids with short alkyl chains showed low friction coefficients and wear; however, when these ionic liquids had long alkyl chains, higher friction coefficients and wear were observed due to the low reactivity.

Acknowledgements This work was supported by a Grant-in-Aid for JSPS Fellows No. 15J05958 and JSPS KAKENHI Grant Numbers, JP16H02310, JP26630041

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Figure caption Fig. 1 (a) The viscosity of the sulfate anions based ionic liquids Fig. 1 (b) The viscosity of the phosphate anions based ionic liquids Fig. 2 (a) The thermal stability of the sulfate anions based ionic liquids Fig. 2 (b) The thermal stability of the phosphate anions based ionic liquids Fig. 3 (a) The friction coefficient and wear scar diameter of the sulfate anions based ionic liquids Fig. 3 (b) The friction coefficient and wear scar diameter of the phosphate anions based ionic liquids Fig. 4 The worn surface images (a) [EMIM][HSO4], (b) [EMIM][MSU], (c) [EMIM][ESU], (d) [EMIM][OSU], (e) [EMIM][DMP], (f) [EMIM][DEP], and (g) [EMIM][DBP] Fig. 5 (a) The outgassing and friction behavior of [EMIM][HSO4] Fig. 5 (b) The outgassing and friction behavior of [EMIM][MSU] Fig. 5 (c) The outgassing and friction behavior of [EMIM][ESU] Fig. 5 (d) The outgassing and friction behavior of [EMIM][OSU] Fig. 6 (a) The outgassing and friction behavior of [EMIM][DMP] Fig. 6 (b) The outgassing and friction behavior of [EMIM][DEP] Fig. 6 (c) The outgassing and friction behavior of [EMIM][DBP] Fig. 7 The spectra of S2p, and P2p of sulfate and phosphate anion (a) [EMIM][HSO4], (b) [EMIM][MSU], (c) [EMIM][ESU], (d) [EMIM][OSU], (e) [EMIM][DMP], (f) [EMIM][DEP], and (g) [EMIM][DBP]

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