Fatty-Acid-Constituted Halogen-Free Ionic Liquids as Renewable

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Fatty Acids Constituted Halogen-Free Ionic Liquids as Renewable, Environmentally Friendly and High-Performance Lubricant Additives Rashi Gusain, Sanjana Dhingra, and Om Prakash Khatri Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03347 • Publication Date (Web): 08 Jan 2016 Downloaded from http://pubs.acs.org on January 11, 2016

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Industrial & Engineering Chemistry Research

Fatty Acids Constituted Halogen-Free Ionic Liquids as Renewable, Environmentally Friendly and HighPerformance Lubricant Additives

Rashi Gusain,1,2 Sanjana Dhingra,1 and Om P. Khatri1,2,*

1

Chemical Science Division, CSIR - Indian Institute of Petroleum, Dehradun - 248005, India

2

Academy of Scientific and Innovative Research (AcSIR), New Delhi, India

* Corresponding Author Email: [email protected]

1

ACS Paragon Plus Environment


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Abstract Stearic, oleic and linoleic acids having variable degree of unsaturation (n = 0, 1 and 2) are selected as model fatty acids to synthesize halogen-free, renewable and environmentally friendly ionic liquids. The preparation of fatty acid ionic liquids are confirmed by NMR (1 H and

13

C) and FTIR analyses. The fatty acid ionic liquids as additive to the polyol ester lube

base oil exhibited remarkably improved friction and anti-wear properties for the steel tribopairs. The fatty acid anions exhibiting inherent negative charge, promptly interact with the engineering surfaces compared to that of polyol ester and vegetable oils. Therefore, an addition of 2% fatty acid ionic liquids to the polyol ester, which is composed of fatty acid esters, showed remarkable improvement in both the friction-reducing (28-60%) and the antiwear properties (20-28%) under the boundary lubrication regime. The magnitude of frictionand wear-reduction are largely controlled by a degree of unsaturation in the fatty acid anions. Further, copper strip corrosion tests revealed the non-corrosiveness of fatty acid ionic liquids. A facile, scalable and economic approach to synthesize the environmentally-friendly ionic liquids constituting renewable fatty acid anions promise immense potential for the lubrication applications.

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ToC Graphic

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Introduction Environment protection and ecotoxicity are of paramount importance. Lubricants, the indispensable materials for engineering surfaces including automotive components, microelectromechanical devices and heavy industries, are gaining large attention because of their adverse effect to the environment and the ecosystem.1-2 In order to increase the efficacy of the lubricants, different types of chemical additives are blended to various lube base oils. Among them, the zinc dialkyldithiophosphate (ZDDP) has been considered as most effective antiwear and antioxidant additive to the lubricant system since 1941.3 Over the last two decades, the stringent regulations for environment protection have been alarming to reduce the use of ZDDP-based additives because of their toxicity to the aquatic wildlife, adverse effects to humanhealth, emission of ash component by their thermal decomposition and poisoning of automotive exhaust gas catalyst components.4-6 In this context, a lot of efforts have been directed to search new additives as a replacement to ZDDP. Furthermore, these new additives should exclude phosphorus, sulfur and heavy metals as constituent elements. Ionic liquids, poorly

coordinated

salts of bulky organic cation

and

inorganic/organic anion, exhibit immense potential for lubricant applications because of their remarkable and favourable physicochemical properties such as good conductivity to take away the heat from the contact interfaces, inherent polarity to facilitates their interaction with tribo-interfaces, negligible volatility to avoid environment exposure and high thermal stability.7-9 The irregular shape and sizedifference of constituted cation and anion provides low shearing when ionic liquids are under the sliding-stress and reduces the friction.10 The ionic liquids are highly versatile and the diverse range of cations/anions provides ample opportunities to design the

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task-specific ionic liquids for variable engineering surfaces. Liu and co-workers demonstrated for the first time the use of alkylimidazolium tetrafluoroborate ionic liquid as a novel lubricant for different engineering surfaces viz. steel, aluminium, copper, silicon, sialon and Si3N 4, providing friction-reducing, anti-wear and high load carrying properties.11 Over the last one decade, several studies have been made revealing the potential of ionic liquids as neat lubricants, additives to various lubricants and thin films on the solid surfaces to reduce the friction and the wear. 7-9,1216

Most of ionic liquids being studied for tribological applications, contain halogens,

phosphorous and sulfur as constituent elements of anions and cations such as halides, tetrafluoroborate, heaxfluorophosphate, phosphates, sulfates, trifluoromethansulfonate, bis(tri-fluoromethanesulfonyl)imide, phosphonium, sulfonium etc. These ionic liquids and their by-products are hazardous to the environment and leads to the corrosive events.17,18 Particularly, BF 4- , PF6- , X- (F-, Cl-, Br- etc.) anions-constituted ionic liquids are prone to be hydrolyzed in the presence of moisture and generates HX, which corrodes the tribo-interfaces.14,19 Further, high cost of halogen-precursors and potential risks linked to disposal of these ionic liquids are considered as major drawbacks and limit their potential to lubricant applications. Thus, the halogen-, phosphorus- and sulfur-free ionic liquids are gaining large interest. Recently, amino acids, tricyanomethanide, dicyanamide and chelated orthoborate anions based ionic liquids have been studied for their tribological properties.20-25 Although, these ionic liquids are halogen-free, however, high cost of precursors, poor thermal stability, presence of phosphorus as cationic constituent and tedious synthesis procedure are major concerns, which edge their potential to the lubricant applications. Vegetable oils are considered as potential alternative to the petroleum-based lubricants because of their renewability, biodegradability and excellent inherent lubricious properties.26-

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Chemically, vegetable oils are triglyceride esters of variable fatty acids and glycerol. The

presence of long alkyl chain fatty acid esters in the vegetable oils provides desirable tribological properties. The fatty acids are prone to interact with metallic surface under the tribo-stress and form tribo-chemical boundary thin film, which reduces the friction and the wear significantly.28-30 Iglesias et al., for the first time synthesized the oleate anion constituted ionic liquids with protic cations and exploited the thermophysical properties.31 The protic ammonium carboxylate ionic liquids showed significant reduction of friction and wear for the copper compared to that of polyalphaolefins (PAO) and were attributed to the formation of stable boundary thin film.32 Qu et al. have compared the tribological properties of carboxylate anion based ionic liquids with organophosphate and sulfonate ionic liquids. Owing to long alkyl chains carboxylate anions exhibited good solubility in PAO synthetic lube base oil and forms antiwear thin film, which reduced both the friction and the wear.33 The tetraalkylammonium fatty acid ionic liquids as lubricants exhibited significantly lower friction compared to polyol ester lube base oil.34 Herein, a motive of this study is to utilize the renewable fatty acids for development of halogen-, phosphorus- and sulfur-free ionic liquids. In this context, three different anions: stearate, oleate and linoleate, which are key constituents of vegetables oils, having variable unsaturation (n = 0, 1 and 2) are selected to probe their effect on physico-chemical, friction and wear-characteristics.

Experimental Section Chemicals and Materials. Tetrabutylammonium bromide (TBA-Br, 99%, Loba Chemie), sodium salt of stearic acid (96%, Acros Organics), oleic acid (Extra pure, Loba Chemie) and linoleic acid (97%, Alfa aesar) are used to synthesize three different ionic liquids. All chemicals are used without further purification. The

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pentaerythritol tetraoleate (polyol), supplied by Mohini Organics Pvt Ltd., is chosen as a synthetic lube base oil for this study. Synthesis of Fatty Acids Ionic Liquids: Tetrabutylammonium cation-based ionic liquids having three variable fatty acid anions are synthesized by a facile and scalable approach. The tetrabutylammonium stearate (TBA-ST) is prepared by stirring the aqueous solution of sodium stearate and TBA-Br in equimolar quantities for 18 hours. The dichloromethane (DCM) is added into the reaction mixture to extract out the synthesized ionic liquid and then washed it several times with distilled water to remove the non-reactant content and sodium halide by-product. DCM is removed by distillation under the reduced pressure and finally TBA-ST ionic liquid is dried under the vacuum for 48 hours at 80 °C. In order to synthesize the tetrabutylammonium oleate (TBA-OL) ionic liquid, an anionic precursor, sodium oleate is prepared by mixing the oleic acid (0.5 M) with an aqueous solution of NaOH (0.5 M) at 60 °C for 3 hours. In the subsequent step, TBA-Br (0.5 M) is added to an aqueous solution of sodium oleate (0.5 M) and stirred for 18 hours. The synthesized TBA-OL ionic liquid is extracted and purified using the DCM. The tetrabutylammonium linoleate (TBALN) ionic liquid is synthesized by following the TBA-OL preparation procedure using linoleic acid (0.5 M) as an ingredient to anionic precursor, instead of oleic acid. Characterization of Ionic liquids: Synthesis of all ionic liquids is confirmed by the Fourier Transform Infrared (FTIR) and Nuclear Magnetic Resonance (NMR; 1H and

13

C) analyses. FTIR spectrum of each ionic liquid is taken using a Thermo-Nicolet

8700 Research spectrometer at a resolution of 4 cm-1 to probe the presence of characteristics chemical functional groups. The NMR spectra of ionic liquid samples are recorded by using Bruker Av III 500 MHz Spectrometer. The CDCl 3 solvent is used to prepare the NMR samples. Thermal stability of ionic liquids is probed by

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using a thermal analyzer (Diamond, PerkinElmer) under the flow of nitrogen (50 mL.min-1) at thermal rate of 10 °C.min-1. The viscosity and density of ionic liquids are measured on Stabinger viscometer (Anton Paar, model SVM3000) at variable temperatures. The reported values of viscosity of different samples at variable temperature are average of three measurements along with their standard deviation. The error bar of viscosity is provided to understand the repeatability of the results. Corrosion Tests: The copper strip corrosion tests are carried out to evaluate the corrosion properties of fatty acid ionic liquids by following the ASTM D130 standard method. In typical experiment, freshly polished and cleaned copper strips are dipped in the 2% ionic liquid blend in polyol at 100 °C. After 3 hours, Cu strips are washed with hexane and ethanol using ultrasound bath for 10 minutes each in the subsequent order. This was followed by structural and elemental analyses of Cu strips using field emission scanning electron microscopy (FESEM, FEI Quanta 200F) and Energy Dispersive X-ray Spectroscopy (EDX) coupled to FESEM, respectively. Lubrication Properties of Ionic Liquids: The lubrication properties of fatty acids anion-based ionic liquids, blended in polyol ester lube base oil, are probed using four-ball tribo-tester (Ducom Instruments, India). In a typical tribo-test, three steel balls (f = 12.7 mm) are clamped in a pot containing lube sample and fourth ball is rotated over these three stationary balls at specific applied normal load. All tribo-tests are conducted following the ASTM D4172 standard test method under 392 N load and 1200 rpm rotating speed for one hour. The temperature of lube pot is maintained throughout the experiment by thermo-couple sensor at 75 °C. Each tribo-test is repeated for 2-3 times, since the repeatability is an important parameter in the tribology. The changes of friction as a function of contact time encompass an inherent error and the degree of inherent error is depending on the stability of developed thin

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film between the contact interfaces. The average coefficient of friction along with standard deviation is computed based on multiple measurements (2-3 measurements), where both inherent and statistical errors are considered. The reported wear scar diameter on steel ball is average of nine measurements (three balls of each measurement) along with standard deviation. The error bars of friction and wear results are provided to understand the repeatability of the results. The surface features of worn area on the steel balls are probed by FESEM measurements. In order to understand the nature of tribo-chemical thin film deposited under the tribo-stress, the elemental composition and distribution of elements on the worm surfaces are examined by EDX coupled to FESEM.

Results and Discussion O O

TBA-ST

N

O N

O

TBA-OL

O O

TBA-LN

N

Figure 1: Structural illustration of TBA-ST, TBA-OL and TBA-LN ionic liquids. 9



Stearic, oleic and linoleic acids are key constituents of vegetable oils and are selected for this study to prepare their ionic liquids with tetrabutylammonium cation. The stearate anion is fully saturated with all methylene units, whereas oleate and linoleate anions contain one and two double bonds (unsaturation sites), respectively, as shown in the Figure 1. These ionic liquids are prepared by a facile and economic approach using equimolar quantities of sodium salt of each fatty acid and the TBA-Br. (c)

% Transmittance, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TBA-LN

(b)

TBA-OL

(a)

TBA-ST 3000

2500

2000

1500

1000

-1 Wavenumber, cm

Figure 2: FTIR spectra of (a) TBA-ST, (b) TBA-OL and (c) TBA-LN ionic liquids. The synthesis of fatty acids anion-based ionic liquids is confirmed by FTIR and NMR (1H and

13

C) characterization. Figure 2 shows FTIR spectra of TBA-ST, TBA-OL and

TBA-LN ionic liquids. The strong and broad vibrational modes in the range of 30002800 cm-1 are assigned to methylene and methyl asymmetric and symmetric stretches of long alkyl chains of stearate, oleate and linoleate anions and TBA cation. The

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presence of unsaturation (double bonds) in TBA-OL and TBA-LN are supported by appearance of vibrational peaks at ~3005, ~1650 and ~994 cm-1, ascribed to =C-H stretch, -C=C- stretch, and =C-H bending modes, respectively. The absence of these vibration modes in the TBA-ST, rules out the presence of unsaturation. These ionic liquids exhibit a strong carbonyl (C=O) band in the range of 1765-1742 cm-1, associated with carboxylate group of anionic moieties. Further, appearance of asymmetric and symmetric COO- stretches in the range of 1628-1635 and 1490-1485 cm-1, respectively, confirmed the carboxylate groups in the ionic liquids.35-36 The vibrational modes in the range of 1470-1370 cm-1 are assigned to bending modes of methylene and methyl units of ionic liquids. The C-N stretch in the range of 11151108 cm-1 is attributed to the TBA cation. These vibrations confirmed the synthesis of TBA-ST, TBA-OL and TBA-LN ionic liquids. The 1 H and

13

C NMR analyses are carried out to confirm the molecular structure of

synthesized organic salts. All proton shifts are extracted in the Table 1. The 1H NMR spectra exhibited signal at extreme upfield position at ~0.86-0.89 ppm, attributed to the methyl protons at terminal sites of fatty acid anions and TBA cation as demonstrated in the Figure S1-S3 (ESI). The respective carbons in

13

C NMR gave signal at ~13-14 ppm. Fatty acid

anions contain long alkyl chain constituting methylene units, which gave proton and carbon signals in the range of 1.25-1.64 and 16-30 ppm, respectively. The electronegative oxygen atoms in the COO- group attract the electron, as a result, neighbouring protons showed downfield chemical shifts. The methylene proton next to carboxylate group exhibited high chemical shift of 2.33-2.37 ppm. Further, the

13

C NMR signal owing to COO- group and

neighbouring CH2 units appeared at ~175-180 ppm and ~33-38 ppm, respectively. The hydrogen near the double bonds are deshielded because of free movement of the electrons in the pi-bond, consequently, proton close of double bond shifted to 5.30-5.42 ppm and

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correspondingly carbon NMR signal appeared in the range of 125-130 ppm for TBA-OL and TBA-LN ionic liquids. The proton close to the ammonium center in the tetrabutylammonium cation has strong influence over electron density, which deshielded the protons and causes high chemical shift in the protons nearby the cationic center. As a result, protons of methylene units directly bonded to nitrogen shifted to ~3.3-3.5 ppm. On moving away from cationic centre, the chemical shift progresses toward the right side i.e. upfield position.

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Table 1. 1H NMR shifts (δ, ppm, 500 MHz) for TBA-ST, TBA-OL and TBA-LN ionic liquids# Ionic Liquids

Ha

Hb

Hc & Hf

Hd

He

Hg

Hh & Hl

Hi

Hj

Hk

δ, ppm

δ, ppm

δ, ppm

δ, ppm

δ, ppm

δ, ppm

δ, ppm

δ, ppm

δ, ppm

δ, ppm

TBA-ST

3.36-3.40t, 8H

1.67-1.70m, 8H

1.25-1.28m, 36H

0.99-1.02t, 12H

0.86-0.89t, 3H

-

-

-

1.61-1.62t, 2H

2.33-2.36t, 2H

TBA-OL

3.34-3.37t, 8H

1.62-1.68m, 8H

1.27-1.35m, 36H

0.87-0.89t, 12H

0.87-0.89t, 3H

1.37-1.41m, 4H

2.72-2.81m, 4H

5.33-5.36m, 2H

1.99-2.03q, 2H

2.34-2.37t, 2H

TBA-LN

3.32-3.35t, 8H

1.61-1.64m, 8H

1.25-1.30m, 36H

0.98-1.01t, 12H

0.86-0.88t, 3H

1.41-1.48m, 4H

2.61-2.68m, 6H

5.30-5.42m, 4H

1.94-2.08q, 2H

2.34-2.37t, 2H

# The position of each type of hydrogen based on 1H NMR spectra are demonstrated in the Figure S4 (ESI)

13



350

Kinematic Viscosity, mm 2.s-1

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TBA-OL

280

210

140

TBA-LN

70

0 40

50

60

70

80

90

100

Temperature, oC

Figure 3: Viscosity of TBA-OL and TBA-LN ionic liquids as a function of temperature. These values are average of three measurements along with their standard deviation.

Table 2: Melting temperature and trapped water content of fatty acid ionic liquids Ionic Liquid

TBA-ST

TBA-OL

TBA-LN

Melting Point, °C

53.3

-15.3

-31.3

Water Content, ppm

260

40

550

The physical state of fatty acids ionic liquids are strongly influenced by the chemical structure of constituted anion. The long alkyl chain with full of saturated methylene units in stearate anion provides solid-like structure to TBA-ST ionic liquid and is attributed to the van der Waals interaction between the methylene units of neighbouring alkyl chains. The introduction of unsaturation in long alkyl chain of anionic moiety leads to bending of chain at the double bond site and distort the packing orientation. Consequently, van der Waals interactions between the sterically hindered methylene units reduces in the oleate and linoleate anions (Figure 1), and provides liquid phase to the TBA-OL and TBA-LN ionic 14


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liquids. The TBA-LN exhibits two double bonds; as a result each molecule bents at two positions and that significantly reduced the van der Waals interaction between their methylene units because of steric constraints. Therefore, the TBA-OL ionic liquid exhibited higher viscosity (Figure 3) compared to TBA-LN ionic liquid and is attributed to their packing orientation. The melting points of fatty acid ionic liquids are further supported by their packing orientation. The TBA-ST melts at 53 °C owing to large range of van der Waals interaction between methylene units of stearate anions. Whereas, TBA-OL and TBA-LN ionic liquids melt at significantly low temperature: -15 and -31 °C, respectively (Table 2). The presence of unsaturation sites in both TBA-OL and TBA-LN ionic liquids distort their packing orientation, as a result reduction of their melting point temperatures.

100

TBA-ST TBA-OL TBA-LN

80

% Weight

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0 100

200

300

o Temperature, C

Figure 4. TGA patterns of TBA-ST, TBA-OL and TBA-LN ionic liquids. Thermal rate: 10 °C. min-1 under the nitrogen flow. Further, the thermal stability of fatty acid ionic liquids is explored in the range of 30-350 °C. These ionic liquids are stable up to 200 °C and then decomposed with increasing of

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temperature (Figure 4). All three ionic liquids exhibited similar thermal decomposition pattern under the nitrogen atmosphere irrespect to presence of variable unsaturation sites.

Figure 5: FESEM images and corresponding overlay elemental mapping of copper strips exposed to 2% blends of (a) TBA-OL and (c) TBA-BF4 ionic liquids in the polyol ester lube base oil at 100 °C for 3 hours. The brown, green and blue pixels represent oxygen, copper and fluorine, respectively. High-resolution FESEM images of copper strips exposed to (d) TBA-BF4 ionic liquid explicitly demonstrates the development of corrosion pits, whereas (b) TBA-OL exposed sample rules out the development of corrosion pits. The corrosion behavior of 2% blend of TBA-OL ionic liquid in the polyol lube base oil is probed following the ASTM D130 copper strip test method. For a comparative study, TBABF4 ionic liquid is examined for the corrosion behaviour. The changes in structural features and elemental distribution of copper strip samples exposed to each ionic liquid are probed by electron microscopy and elemental mapping. Fig. 5 shows FESEM images and corresponding overlay elemental mapping of copper strips exposed to (a-b) TBA-OL and (c-d) TBA-BF4 ionic liquids. The copper strip exposed to TBA-BF4 ionic liquid exhibited corrosion pits (Fig. 16


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5c-d) of 500 nm - 2 µm size and are distributed throughout the sample surface. The corresponding elemental distribution overlay explicitly illustrates uniform distribution of fluorine and oxygen (Fig 5cii) revealing the role of BF4 anion in development of corrosion pits. However, the surface features of copper strip exposed to TBA-OL ionic liquid shows no significant changes (Fig 5a-b). These results suggested that fatty acid ionic liquids, which are halogen-free, do not corrode the metal surfaces, whereas presence of halogen facilitates the corrosive events.

Coefficient of Friction

0.12

Polyol

0.5% TBA-OL

1.0% TBA-OL

1.5% TBA-OL

2.0% TBA-OL

2.5% TBA-OL

(a)

0.10

0.08

0.06

0.04

0

10

20

30

40

50

60

(b)

0.08

74

72

0.06

70

0.04 68

0.0

0.5

1.0

1.5

2.0

2.5

Concentration of TBA-OL in Polyol, % w/v

17


)

0.10

Kinematic Viscosity, mm 2.s-1 (

)

Rolling Contact Time, Minutes

Average Coefficient of Friction (

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



640

Average Wear Scar Diameter, mm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(c)

600 560

520

480 440

400 0.0

0.5

1.0

1.5

2.0

2.5

Concentration of TBA-OL in Polyol, % w/v

Figure 6: (a) Changes in coefficient of friction as a function of TBA-OL dosing in the polyol ester lube base oil. (b) Changes in average coefficient of friction and kinematic viscosity as a function of TBA-OL dosing in the polyol ester lube base oil. (c) Changes in average wear scar diameter as a function of TBA-OL dosing. Load: 392 N, rotating speed: 1200 rpm, temperature: 75 °C, test duration: 1 hour. The average coefficient of friction and wear scar diameter along with their standard deviation are computed based on multiple measurements, where both inherent and statistical errors are considered. The friction and wear properties of fatty acid ionic liquids are examined by following the four-ball tribo-tests under the boundary lubrication regime. Figure 6 shows the optimization of ionic liquid concentration in the polyol ester based on coefficient of friction. The average coefficient of friction with polyol ester for steel tribo-pairs is found to be 0.085 under the load of 392 N. The TBA-OL as an additive to polyol ester showed significant decrease of coefficient of friction. An increasing dose of TBA-OL in the polyol showed gradual reduction of coefficient of friction. A 2% dose (w/v) of TBA-OL exhibited maximum reduction of friction (60%) compared to that of polyol ester lube base oil. These results suggest that TBAOL as an additive played an important role to improve the lubrication properties of polyol

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ester by reducing the friction between steel tribopairs under the rolling contact. The viscosity of polyol ester lube base oil increased gradually with increasing the dose of TBA-OL (Figure 6b). However, beyond 2% dose of TBA-OL, the viscosity of polyol ester blend increased sharply on further addition of TBA-OL ionic liquid. Consistent with viscosity result, the coefficient of friction increased, when the dose of TBA-OL was beyond 2%. This might be because of internal resistance to shear provided by viscous blend of TBA-OL, as a result, 2.5 % TBA-OL exhibited higher coefficient of friction. Figure 6c shows the reduction of wear scar diameter (WSD) with increasing the dose of TBA-OL and 2% dosing revealed the maximum reduction (24%) of WSD.

Table 3: Physicochemical properties of fatty acid anions-based ionic liquids blended in the polyol ester lube base oil Sample Description

Kinematic viscosity, mm2.s-1

Viscosity Density at 15°C,

At 40 °C

At 100 °C

Index

g.mL-1

Polyol Ester

67.86 ± 0.15

12.39 ± 0.08

183

0.929

Polyol + 2% TBA-ST

69.33 ± 0.42

12.52 ± 0.01

182

0.930

Polyol + 2% TBA-OL

71.07 ± 0.36

12.74 ± 0.01

181

0.928

Polyol + 2% TBA-LN

67.22 ± 0.49

12.31 ± 0.01

184

0.929

19



0.12

Polyol 2% TBA-OL

(a)

2% TBA-ST 2% TBA-LN

0.10


0.08

0.06

0.04

0.02 0

10

20

30

40

50

60

Wear Scar Diameter, mm

Rolling Contact Time, minutes

Average Coefficient of Friction

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(bii)

630

560

490

420

0.10

(bi)

0.08

0.06

0.04

0.02

Polyol

TBA-ST

TBA-OL

TBA-LT

Figure 7: (a) The evolution of coefficient of friction with contact time for the polyol ester lube base oil and 2 % of TBA-ST, TBA-OL, and TBA-LN ionic liquids individually blended

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in the polyol ester. Comparison of (bi) coefficient of friction and (bii) WSD of steel balls lubricated with polyol ester and 2% blend of each ionic liquid having variable fatty acid anion. Load: 392 N, rotating speed: 1200 rpm, temperature: 75 °C and test duration: 1 hour. The average coefficient of friction and wear scar diameter along with their standard deviation are computed based on multiple measurements.

In order to understand the role of ionic liquid structure on their lubrication properties, herein three ionic liquids having variable fatty acid anions: stearate with full of saturated methylene units, oleate and linoleate exhibiting one and two unsaturation sites, respectively, are selected. The viscosity index of polyol ester remains unchanged on addition of 2% fatty acid ionic liquids (Table 3). Figure 7 illustrates the friction and wear characteristics of 2% w/v blend of each ionic liquid (TBA-ST, TBA-OL, and TBA-LN) in the polyol ester under the load of 392 N. The 2% TBA-ST ionic liquid exhibited 28% reduction in both friction and WSD compared to that of polyol ester lube base oil. However, TBA-OL and TBA-LN ionic liquids showed 60 and 33% reduction in the friction, respectively. The polyol ester lube base oil has intrinsic lubricious nature owing to presence of four oleate functionalities in the form of ester. In spite of that, 2% fatty acid ionic liquids significantly improved the friction-reducing characteristic of the polyol ester. Among these three ionic liquids, oleate anion showed maximum reduction (60%) in coefficient of friction. Further, a steady-state pattern of coefficient of friction for TBA-OL (Figure 7a) as a function of contact time suggested the formation of uniform and stable low-shear strength thin film of ionic liquids on the tribo-interfaces, which reduces the friction. The correlation between the degree of unsaturation in the fatty acid anion and correspondingly wear scar diameter on the steel balls are shown in Figure 7bii. The addition of 2% ionic 21



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liquid consistently improved the wear-reducing performance by reduction of WSD in the range of 20-28%. The TBA-ST, is noted as the most effective additive providing 28% reduction in WSD compared to that of polyol ester lube base oil. The wear results suggested that increasing the degree of unsaturation has negative effect on the performance of ionic liquids for the wear reduction. The TBA-ST exhibited smallest wear scar, which gradually increased with increasing the number of unsaturation (Figure 7bii). The free fatty acids as additives to sunflower vegetable oil showed similar wear trends.28 These results suggested that anionic moiety of ionic liquids primarily determine the anti-wear properties by forming a tribo-chemical thin film under the boundary lubrication regime. The polyol ester (pentaerythritol tetraoleate) lube base oil is chemically prepared by esterification of pentaerythritol with oleate moieties. Under the boundary lubrication conditions, polyol ester decomposed to free acids which interact with iron surface and forms the tribo-chemical thin film.37 Even, vegetable oils, which are rich sources of fatty acids in forms of triglycerides, also provides lubricous properties and forms the thin film of fatty acids under the tribo-stress.28 Herein, our motive was to provide inherent polar nature to the fatty acids in the form of ionic liquids, so they can promptly interact with the steel surface under the tribo-stress. The polyol and vegetable oils initially break-down into the fatty acids under the tribo-stress and then interact with metal surfaces. While these ionic liquids, composed of fatty acids anions, exhibit inherent negative charge and could easily interact with the steel surface under the tribo-stress. Therefore the addition of 2% fatty acids ionic liquid to the polyol, which already had fatty acids in form of ester, showed remarkable improvement in both the friction-reducing and anti-wear properties under the boundary lubrication

22


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regime. This was attributed to the rapid response of fatty acids anions to interact with steel surface compared to that of fatty acid esters. 0.12

0.10

Wear Scar Diameter,mm


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.08

0.06

440

2% ZDDP

400

360

320

2% ZDDP

2% TBA-OL

0.04

2% TBA-OL 0.02 0

10

20

30

40

50

60

Rolling Contact Time, Minutes Figure 8. Comparison of coefficient of friction and WSD of steel balls lubricated with 2% individual blend of TBA-OL ionic liquid and ZDDP in the Polyol. Load: 392 N, rotating speed: 1200 rpm, temperature: 75 °C, test duration: 1 hour. The average wear scar diameter along with standard deviation is computed based on multiple measurements.

Furthermore, the tribo-performance of fatty acid ionic liquids is compared with conventional additive ZDDP, which is being widely used for lubricant applications. Figure 8 shows coefficient of friction and WSD of steel balls lubricated with 2% individual blend of TBA-OL and ZDDP in the Polyol. The TBA-OL ionic liquid exhibits significantly low (~65%) coefficient of friction compared to that of ZDDP under the identical tribological conditions. This could be attributed to the low shearing of TBA-OL ionic liquid. However, ZDDP revealed better antiwear properties. The 23



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WSD of ZDDP lubricated steel ball is found to be 10% lower than that of TBA-OL ionic liquid. The enhanced antiwear properties of ZDDP is attributed to formation of thin film of glassy polyphosphate on the contact interfaces under the tribo-stress, which protects the steel balls against the wear.3,38 Considering the environmental drawbacks of conventional ZDDP additive and excellent friction-reducing of TBAOL, fatty acid ionic liquids could be good alternates for the lubricant applications.

Figure 9. FESEM images of the worn surfaces of steel balls lubricated with (ai-ii) polyol ester lube base oil and 2 % individual blend of (bi-ii) TBA-ST, (ci-ii) TBA-OL and (di-ii) TBA-LN ionic liquids in the polyol ester. Load: 392 N, rotating speed: 1200 rpm, temperature: 75 °C, test duration: 1 hour.

24


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Figure 9 shows microscopic images of worn steel balls lubricated with polyol ester and 2% blend of each fatty acid ionic liquid. The worn area lubricated by polyol ester exhibited large wear scar with lot of uneven features suggesting the plastic deformation and flow of material under the tribo-stress. The 2% of TBA-ST ionic liquid reduced the WSD and showed comparatively smoother features, although plastic deformation cannot be ruled out. The TBA-LN ionic liquid lubricated ball showed larger wear scar compared to that of with TBA-ST and TBA-OL ionic liquids.

Figure 10: FESEM Images and corresponding elemental distribution on the steel-ball surfaces lubricated with (ai-aiii) 2% TBA-ST, (bi-biii) 2% TBA-OL and (ci-ciii) TBA-LN ionic liquids blend in the polyol lube base oil.

25



Figure 11: (a) FESEM micrograph and (b) corresponding carbon distribution on the steel-ball surface lubricated with 2% TBA-OL ionic liquid blend in the polyol lube base oil. The intense distribution of carbon on the contact area further confirms the development of ionic liquid based tribo-chemical thin film. 1200

Fe

C N

600

(a)

C: 56.6 At% N: 13.3 At% O: 24.0 At% Si: 1.4 At% Cr: 0.2 At% Fe: 4.5 At%

900

O

300

Fe

Si

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cr

Fe

0

Fe

(b)

1590

C: 42.7 At% O: 15.7 At% Si: 0.9 At% Cr: 0.9 At% Fe: 39.8 At%

O

1060

Fe

530

C

Fe

Si

Cr

0 0

2

4

6

8

Energy, eV

Figure 12: EDX spectra of (a) contact and (b) non-contact area of steel-ball surface lubricated with 2% TBA-OL ionic liquid blend in the polyol lube base oil. The presence of Nitrogen on the contact surface reveals the formation of tribo-chemical thin film consisting of TBA-OL ionic liquid. 26


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Furthermore, the EDX analyses of steel balls lubricated with fatty acids ionic liquids are carried out to probe the elemental composition of tribo-chemical thin film developed on the worn surfaces. The FESEM micrographs and corresponding elemental distribution images of worn surfaces (Figure 10) lubricated with 2% of each ionic liquid, explicitly demonstrated the uniform distribution of nitrogen and carbon. In these ionic liquids, carbon and nitrogen are characteristics elements of anion and cation, respectively, and are uniformly distributed on the worn area of steel balls. These results suggested the formation of tribo-chemical thin film composed of ionic liquids which protect the surfaces against the undesirable wear and reduces the friction. It is noted that under the tribo-stress, fatty acid ionic liquids are promptly interacts with the steel surface and forms the tribo-chemical thin film. The high abundance of carbon (Figure 11) on the contact area of steel ball lubricated with 2% TBA-OL ionic liquid confirms the participation of ionic liquids in tribo-chemical thin film formation. Whereas, the low intensity of carbon on the non-contact area of steel ball can be attributed to the adsorption of ionic liquid at higher temperature under the tribo-condition. The strong signatures of carbon, nitrogen and oxygen in the point spectroscopy data (Figure 12) of tribochemical thin film explicitly revealed the role of fatty acid ionic liquids. The magnitude of wear-reduction is strongly determined by the degree of unsaturation in the constituent fatty acid anion. The fatty acid anions react with steel tribo-interfaces and form the tribo-chemical thin film. The strength of such thin film primarily depends on the strengths of interactions between the molecules making up the tribo-chemical thin film. In TBA-ST, stearate anion interact with steel tribo-interfaces through carboxylate group and the methylene units in the stearate anion interact with neighbouring methylene units of another stearate anion via weak van der Waals interaction and provides stable structure. The degree of van der Waals interaction increases with increasing the methylene units and provides the well-organized solid-like structure because of their close packing. However, the unsaturation

27



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sites in oleate and linoleate anions sterically distort the structure in the molecules. Consequently, all methylene units of one chain couldn't interact with nearby methylene units of another chain in the thin film and provides loosely oriented structure, which couldn't provide good anti-wear properties. And such phenomenon increases with increasing the number of unsaturation sites; as a result, linoleate anion showed poorer antiwear performance than that of oleate anion constituted ionic liquid.

Conclusion Renewable and ecofriendly lubricants and additives are gaining large interest for industrial applications. Vegetable oils are well established lubricious materials since ancient time. The fatty acids of triglyceride esters in the vegetable oils provide low friction and wear-reducing properties by forming a tribo-chemical thin film on the engineering surfaces. Herein, stearic, oleic and linoleic acids are selected as model fatty acids to synthesize the halogen-free ionic liquids. The preparation of tetrabutylammonium ionic liquids having variable anions: stearate, oleate and linoleate, are confirmed by NMR (1H and

13

C) and FTIR analyses. The physical state

and viscosities of synthesized ionic liquids are found to be primarily controlled by chemical structure of constituted anions. The stearate anion, composed of saturated methylene units, provides solid structure to the TBA-SL ionic liquid, whereas oleate and linoleate anions having one and two double bonds, respectively, exhibited in the viscous liquid phase. As an additive, fatty acid ionic liquid provides remarkably improved friction and anti-wear properties for the steel surface compared to that of polyol ester lube base oil. Elemental mapping of worn surfaces confirmed the deposition of fatty acids ionic liquid thin film under the tribo-stress. The magnitude of friction- and wear-reductions is strongly controlled by a degree of unsaturation in the

28


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fatty acid anion. The long alkyl chain of stearate anion in the TBA-ST thin film provided compact and solid-like structure, attributed to the van der Waals interaction between the methylene units of long alkyl chains in the stearate anion. The presence of unsaturation sites in the tribo-chemical thin film of TBA-OL and TBA-LN leads to sterically constraint structure, which hinder the van der Waals interaction between the methylene units, consequently loosely packed structure of thin film and exhibited poorer anti-wear properties than that of TBA-ST. The fatty acid anions exhibiting inherent negative charge promptly interact with the steel surface under the tribo-stress compare to that of polyol ester and exhibit significant reduction of the friction and the wear under boundary lubrication.

Acknowledgements We kindly acknowledge the Director CSIR-IIP for his kind permission to publish these results. The authors are thankful to CSIR, India for financial support. The analytical support from the Analytical Science Division of CSIR-IIP is kindly acknowledged. R. Gusain thanks the CSIR, India for the fellowship support.

Supporting Information 1

H and 13C NMR spectra of TBA-ST, TBA-OL and TBA-LN ionic liquids

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Fatty-Acid-Constituted Halogen-Free Ionic Liquids as Renewable

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