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

Jan 8, 2016 - Fatty-Acid-Constituted Halogen-Free Ionic Liquids as Renewable, Environmentally Friendly, and High-Performance Lubricant Additives. Rash...
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Fatty-Acid-Constituted Halogen-Free Ionic Liquids as Renewable, Environmentally Friendly, and High-Performance Lubricant Additives Rashi Gusain,†,‡ Sanjana Dhingra,† and Om P. Khatri*,†,‡ †

Chemical Science Division, CSIR - Indian Institute of Petroleum, Dehradun 248 005, India Academy of Scientific and Innovative Research (AcSIR), New Delhi 110 025, India

‡

S Supporting Information *

ABSTRACT: Stearic, oleic, and linoleic acids, which have a variable degree of unsaturation (n = 0, 1, and 2, respectively), were selected as model fatty acids to synthesize halogen-free, renewable, and environmentally friendly ionic liquids. The preparation of fatty acid ionic liquids was confirmed by 1H and 13C NMR and FTIR analyses. The fatty acid ionic liquids as additives to the polyol ester lube base oil exhibited remarkably improved friction and antiwear properties for steel tribopair. The fatty acid anions exhibiting inherent negative charge promptly interact with the engineering surfaces compared with the polyol ester and vegetable oils. Therefore, the addition of 2% fatty acid ionic liquid to the polyol ester, which is composed of fatty acid, showed remarkable improvement in both the friction-reducing (28−60%) and the antiwear (20−28%) properties under the boundary lubrication regime. The magnitudes of friction and wear reduction are largely controlled by the degree of unsaturation in the fatty acid anion. Further, copper strip corrosion tests revealed the noncorrosiveness of the fatty acid ionic liquids. A facile, scalable, and economic approach to synthesize the environmentally friendly ionic liquids containing renewable fatty acid anions promises immense potential for lubrication applications.

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co-workers demonstrated for the first time the use of alkylimidazolium tetrafluoroborate ionic liquid as a novel lubricant for different engineering surfaces, viz., steel, aluminum, copper, silicon, sialon, and Si3N4, providing friction-reducing, antiwear, and high-load-carrying properties.11 Over the last one decade, several studies have revealed the potential of ionic liquids as neat lubricants, additives to various lubricants, and thin films on solid surfaces to reduce friction and wear.7−9,12−16 Most of the ionic liquids being studied for tribological applications contain halogens, phosphorus, and sulfur as constituent elements of the anions and cations, such as halides, tetrafluoroborate, heaxfluorophosphate, phosphates, sulfates, trifluoromethansulfonate, bis(trifluoromethanesulfonyl)imide, phosphonium, sulfonium, etc. These ionic liquids and their byproducts are hazardous to the environment and lead to corrosive events.17,18 In particular, ionic liquids containing BF4−, PF6−, X− (F−, Cl−, Br−, etc.) anions are prone to hydrolyze in the presence of moisture and generate HX, which corrodes the tribointerface.14,19 Further, the high cost of halogen precursors and potential risks linked to disposal of these ionic liquids are considered as major drawbacks and limit their potential for lubricant applications. Thus, halogen-, phosphorus-, and sulfur-free ionic liquids are gaining large interest. Recently, amino acid, tricyanomethanide, dicyanamide, and chelated orthoborate anion-based ionic liquids have been studied for their tribological properties.20−25 Although these ionic liquids are halogen-free, their high cost of precursors, poor thermal stability, presence of phosphorus as a cationic constituent, and tedious

INTRODUCTION Environmental 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 on the environment and the ecosystem.1,2 In order to increase the efficacy of lubricants, different types of chemical additives are blended with various lube base oils. Among them, zinc dialkyldithiophosphate (ZDDP) has been considered as the most effective antiwear and antioxidant additive to lubricant systems since 1941.3 Over the last two decades, the stringent regulations for environmental protection have been alarming to reduce the use of ZDDP-based additives because of their toxicity to aquatic wildlife, adverse effects on human health, emission of ash components 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 the search for new additives as replacements for ZDDP. Furthermore, these new additives should exclude phosphorus, sulfur, and heavy metals as constituent elements. Ionic liquids, which are poorly coordinated salts of bulky organic cations and inorganic/organic anions, exhibit immense potential for lubricant applications because of their remarkable and favorable physicochemical properties such as good conductivity to take away heat from the contact interfaces, inherent polarity to facilitate their interaction with tribointerfaces, negligible volatility to avoid environmental exposure, and high thermal stability.7−9 The irregular shape and size difference between the constituent cation and anion provides low shearing when ionic liquids are under sliding stress and reduces the friction.10 Ionic liquids are highly versatile, and the diverse ranges of cations and anions provide ample opportunities to design taskspecific ionic liquids for variable engineering surfaces. Liu and © 2016 American Chemical Society

Received: Revised: Accepted: Published: 856

September 8, 2015 January 7, 2016 January 8, 2016 January 8, 2016 DOI: 10.1021/acs.iecr.5b03347 Ind. Eng. Chem. Res. 2016, 55, 856−865

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Industrial & Engineering Chemistry Research synthesis procedures are major concerns, which edge their potential for lubricant applications. Vegetable oils are considered as potential alternatives to petroleum-based lubricants because of their renewability, biodegradability, and excellent inherent lubricity.26,27 Chemically, vegetable oils are triglyceride esters of various fatty acids and glycerol. The presence of long-alkyl-chain fatty acid esters in vegetable oils provides desirable tribological properties. The fatty acids are prone to interact with the metallic surface under tribostress and form a tribochemical boundary thin film, which reduces the friction and the wear significantly.28−30 Iglesias and co-workers for the first time synthesized oleate anion-constituted ionic liquids with protic cations and exploited their thermophysical properties.31 Protic ammonium carboxylate ionic liquids showed significant reduction of friction and wear for copper compared with poly(α-olefin) (PAO), which was attributed to the formation of a stable boundary thin film.32 Qu and co-workers compared the tribological properties of carboxylate anion-based ionic liquids with those of organophosphate and sulfonate ionic liquids. Because of long alkyl chain, carboxylate anion exhibited good solubility in PAO synthetic lube base oil and form an antiwear thin film that reduces both the friction and the wear.33 Tetraalkylammonium fatty acid ionic liquids used as lubricants exhibited significantly lower friction compared with polyol ester lube base oil.34 A motive of the present study is to utilize renewable fatty acids for the development of halogen-, phosphorus-, and sulfur-free ionic liquids. In this context, three different anions, viz., stearate, oleate, and linoleate, which are key constituents of vegetable oils and have variable unsaturation (n = 0, 1, and 2, respectively), were selected to probe their effects on the physicochemical, friction, and wear characteristics.

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

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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) were used to synthesize three different ionic liquids. All of the chemicals were used without further purification. Pentaerythritol tetraoleate (polyol), supplied by Mohini Organics Pvt Ltd., was chosen as a synthetic lube base oil for this study. Synthesis of Fatty Acid Ionic Liquids. Tetrabutylammonium cation-based ionic liquids having three variable fatty acid anions were synthesized by a facile and scalable approach. Tetrabutylammonium stearate (TBA-ST) was prepared by stirring an aqueous solution of sodium stearate and TBA-Br in equimolar quantities for 18 h. Dichloromethane (DCM) was added to the reaction mixture to extract the synthesized ionic liquid, followed by washing several times with distilled water to remove the nonreactant content and sodium halide byproduct. DCM was removed by distillation under reduced pressure, and finally TBA-ST ionic liquid was dried under vacuum for 48 h at 80 °C. In order to synthesize tetrabutylammonium oleate (TBA-OL) ionic liquid, the anion precursor sodium oleate was prepared by mixing oleic acid (0.5 M) with an aqueous solution of NaOH (0.5 M) at 60 °C for 3 h. In the subsequent step, TBA-Br (0.5 M) was added to an aqueous solution of sodium oleate (0.5 M), and this mixture was stirred for 18 h. The synthesized TBA-OL ionic liquid was extracted and purified using DCM. Tetrabutylammonium linoleate (TBA-LN) ionic liquid was synthesized by following the TBA-OL preparation procedure using linoleic acid (0.5 M) as the anionic precursor instead of oleic acid.

Figure 2. FTIR spectra of (a) TBA-ST, (b) TBA-OL, and (c) TBA-LN ionic liquids.

Characterization of the Ionic Liquids. The syntheses of the ionic liquids were confirmed by Fourier transform infrared (FTIR) and 1H and 13C NMR analyses. The FTIR spectrum of each ionic liquid was 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 the ionic liquid samples were recorded on a Bruker Avance III 500 MHz spectrometer. CDCl3 was used as the solvent to prepare the NMR samples. The thermal stabilities of the ionic 857

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Industrial & Engineering Chemistry Research Table 1. 1H NMR Shifts (500 MHz) for TBA-ST, TBA-OL, and TBA-LN Ionic Liquidsa δ, ppm

a

H atom

TBA-ST

TBA-OL

TBA-LN

Ha Hb Hc,f Hd He Hg Hh,l Hi Hj Hk

3.36−3.40, t, 8H 1.67−1.70, m, 8H 1.25−1.28, m, 36H 0.99−1.02, t, 12H 0.86−0.89, t, 3H − − − 1.61−1.62, t, 2H 2.33−2.36, t, 2H

3.34−3.37, t, 8H 1.62−1.68, m, 8H 1.27−1.35, m, 36H 0.87−0.89, t, 12H 0.87−0.89, t, 3H 1.37−1.41, m, 4H 2.72−2.81, m, 4H 5.33−5.36, m, 2H 1.99−2.03, q, 2H 2.34−2.37, t, 2H

3.32−3.35, t, 8H 1.61−1.64, m, 8H 1.25−1.30, m, 36H 0.98−1.01, t, 12H 0.86−0.88, t, 3H 1.41−1.48, m, 4H 2.61−2.68, m, 6H 5.30−5.42, m, 4H 1.94−2.08, q, 2H 2.34−2.37, t, 2H

The 1H and 13C NMR spectra for the three ionic liquids are shown in Figures S1−S3 in the Supporting Information.

liquids were probed using a thermal analyzer (Diamond, PerkinElmer) under a flow of nitrogen (50 mL·min−1) at thermal rate of 10 °C·min−1. The viscosities and densities of the ionic liquids were measured on a Stabinger viscometer (Anton Paar model SVM3000) at various temperatures. The reported viscosity values for the different samples at various temperatures are averages of three measurements along with their standard deviations. The error bars for the viscosity are provided to understand the repeatability of the results. Corrosion Tests. Copper strip corrosion tests were carried out to evaluate the corrosion properties of the fatty acid ionic liquids according to the ASTM D130 standard method. In a typical experiment, freshly polished and cleaned copper strips were dipped in the 2% ionic liquid blend in polyol at 100 °C. After 3 h, the Cu strips were washed with hexane and then ethanol using an ultrasound bath for 10 min each. This was followed by structural and elemental analyses of the Cu strips by field-emission scanning electron microscopy (FESEM) using an FEI Quanta 200F scanning electron microscope and by energydispersive X-ray spectroscopy (EDX) using an EDX spectrometer coupled to the scanning electron microscope, respectively. Lubrication Properties of the Ionic Liquids. The lubrication properties of the fatty acid anion-based ionic liquids blended with polyol ester lube base oil were probed using a fourball tribotester (Ducom Instruments, Bengaluru, India). In a typical tribotest, three steel balls (ϕ = 12.7 mm) were clamped in a pot containing the lube sample and the fourth ball was rotated over these three stationary balls at a specific applied normal load. All of the tribotests were conducted following the ASTM D4172 standard test method under a load of 392 N and a rotation speed of 1200 rpm for 1 h. The temperature of lube pot was maintained throughout the experiment using a thermocouple sensor at 75 °C. Each tribotest was repeated two or three times since repeatability is an important parameter in tribology. The changes in friction as a function of contact time encompass an inherent error, and the degree of inherent error depends on the stability of the thin film developed between the contact interfaces. The average coefficient of friction along with the standard deviation was computed on the basis of multiple measurements (two or three measurements), and both inherent and statistical errors were considered. The reported wear scar diameter on the steel balls lubricated with individual sample is average of nine measurements (three balls for each measurement) along with its standard deviation. The error bars for the friction and wear results are provided to understand the repeatability of the results. The surface features of the worn areas on the steel balls were probed by FESEM measurements. In order to understand

Figure 3. Viscosities of TBA-OL and TBA-LN ionic liquids as functions of temperature. These values are averages of three measurements along with their standard deviation.

the nature of the tribochemical thin film deposited under the tribostress, the elemental compositions and distributions of elements on the worn surfaces were examined by EDX.

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RESULTS AND DISCUSSION Stearic, oleic, and linoleic acids are key constituents of vegetable oils and were selected for this study to prepare their ionic liquids with tetrabutylammonium cation. The stearate anion is fully saturated with all methylene units, whereas the oleate and linoleate anions contain one and two double bonds (unsaturation sites), respectively, as shown in Figure 1. Each of these ionic liquids was prepared by a facile and economical approach using equimolar quantities of the sodium salt of the corresponding fatty acid and TBA-Br. The syntheses of the fatty acid anion-based ionic liquids were confirmed by FTIR and 1H and 13C NMR 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 3000−2800 cm−1 are assigned to methylene and methyl asymmetric and symmetric stretches of the long alkyl chains of the stearate, oleate, and linoleate anions and the TBA cation. The presence of unsaturation (double bonds) in TBA-OL and TBA-LN is supported by the appearance of vibrational peaks at ∼3005, ∼ 1650, and ∼994 cm−1, ascribed to the C−H stretch, −CC− stretch, and = C−H bending modes, respectively. 858

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1490−1485 cm−1, respectively, confirmed the presence of 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 the ionic liquids. The C−N stretch in the range of 1115−1108 cm−1 is attributed to the TBA cation. These vibrations confirmed the syntheses of TBA-ST, TBA-OL, and TBA-LN ionic liquids. The 1H and 13C NMR analyses were carried out to confirm the molecular structures of the synthesized organic salts. All of the proton shifts are listed in Table 1. The 1H NMR spectra exhibited signals at extreme upfield positions of ∼0.86−0.89 ppm, attributed to the methyl protons at the terminal sites of the fatty acid anions and TBA cation, as demonstrated in Figures S1−S3 in the Supporting Information). The respective carbons in the 13C NMR spectra gave signals at ∼13−14 ppm. Fatty acid anions contain long alkyl chains comprising methylene units, which gave proton and carbon signals in the ranges of 1.25−1.64 and 16−30 ppm, respectively. The electronegative oxygen atoms in the COO− group attract electrons, and as a result, the neighboring protons show downfield chemical shifts. The methylene proton next to the carboxylate group exhibited high chemical shift of 2.33−2.37 ppm. Furthermore, the 13C NMR signals due to the COO− group and neighboring CH2 units appeared at ∼175−180 and ∼33−38 ppm, respectively. The hydrogens near the double bonds are deshielded because of free movement of the electrons in the π bond. Consequently, protons close to the double bond shifted to 5.30−5.42 ppm, and the corresponding carbon NMR signals appeared in the range of 125−130 ppm for TBA-OL and TBALN ionic liquids. The nitrogen center in the tetrabutylammonium cation has a strong influence over the electron density, which deshields the protons near the cationic center and causes a high chemical shift in these protons. As a result, protons of methylene units directly bonded to nitrogen shifted to ∼3.3−3.5 ppm. Moving away from the cationic center, the chemical shift progressed upfield.

The absence of these vibrational modes in the spectrum of 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 the carboxylate groups of the anionic moieties. Furthermore, the appearance of asymmetric and symmetric COO− stretches in the range of 1628−1635 and Table 2. Melting Temperatures and Trapped Water Contents of the Fatty Acid Ionic Liquids melting point, °C water content, ppm

TBA-ST

TBA-OL

TBA-LN

53.3 260

−15.3 40

−31.3 550

Figure 4. TGA patterns of TBA-ST, TBA-OL, and TBA-LN ionic liquids. Thermal rate: 10 °C· min−1 under a nitrogen flow.

Figure 5. (a, c) FESEM images (i) and corresponding overlays of elemental maps (ii) of copper strips exposed to 2% blends of (a) TBA-OL and (c) TBA-BF4 ionic liquids with polyol ester lube base oil at 100 °C for 3 h. The brown, green, and blue pixels in (aii) and (cii) represent oxygen, copper, and fluorine, respectively. (b, d) High-resolution FESEM images of copper strips exposed to (b) TBA-OL and (d) TBA-BF4 ionic liquids. The image for the TBA-BF4-exposed sample explicitly demonstrates the development of corrosion pits, whereas the image for the TBA-OL-exposed sample rules out the development of corrosion pits. 859

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Industrial & Engineering Chemistry Research The physical state of the fatty acid ionic liquid is strongly influenced by the chemical structure of the constituent anion. The long alkyl chain with fully saturated methylene units in the stearate anion provides a solidlike structure to TBA-ST ionic liquid and that is attributed to the van der Waals interactions between the methylene units of neighboring alkyl chains. The introduction of unsaturation in the long alkyl chain of the anionic moiety leads to bending of the chain at the double-bond site, which distorts the packing orientation. Consequently, van der Waals interactions between the sterically hindered methylene units are reduced in the oleate and linoleate anions (Figure 1), resulting in a liquid phase for the TBA-OL and TBA-LN ionic liquids. TBA-LN exhibits two double bonds, and as a result, each molecule is bent at two positions. This significantly reduces the van der Waals interactions between their methylene units because of steric constraints. Therefore, TBA-OL ionic liquid exhibits higher viscosity than TBA-LN ionic liquid (Figure 3), which is attributed to their packing orientations. The melting points of the fatty acid ionic liquids are further supported by their packing orientations. TBA-ST melts at 53 °C because of the large range of van der Waals interactions between the methylene units of stearate anions, whereas TBA-OL and TBA-LN ionic liquids melt at significantly low temperatures of −15 and −31 °C, respectively (Table 2). The presence of unsaturation sites in both TBA-OL and TBA-LN ionic liquids distorts their packing orientations, resulting in a reduction of their melting point temperatures. Furthermore, the thermal stabilities of the fatty acid ionic liquids were explored over the range of 30−350 °C. These ionic liquids are stable up to 200 °C and then decompose with increasing temperature (Figure 4). The three ionic liquids exhibited similar thermal decomposition patterns under a nitrogen atmosphere irrespective of their different numbers of unsaturation sites. The corrosion behavior of a 2% blend of TBA-OL ionic liquid with the polyol lube base oil was probed according to the ASTM D130 copper strip test method. For a comparative study, TBA-BF4 ionic liquid was examined for corrosion behavior. The changes in structural features and elemental distribution of the copper strip samples exposed to each ionic liquid were probed by electron microscopy and elemental mapping. Figure 5 shows FESEM images and corresponding overlay elemental maps 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 (Figure 5d) that were 500 nm to 2 μm in size and distributed throughout the sample surface. The corresponding elemental distribution overlay explicitly illustrates a uniform distribution of fluorine and oxygen (Figure 5cii) revealing the role of the BF4 anion in the development of corrosion pits. However, the surface features of copper strips exposed to TBA-OL ionic liquid show no significant changes (Figure 5a,b). These results suggested that fatty acid ionic liquids, which are halogen-free, do not corrode the metal surfaces, whereas the presence of halogen facilitates the corrosive events. The friction and wear properties of the fatty acid ionic liquids were examined by following the four-ball tribotests under the boundary lubrication regime. Figure 6 shows the optimization of the ionic liquid concentration in the polyol ester based on the coefficient of friction. The average coefficient of friction with polyol ester for steel tribopair was found to be 0.085 under a load of 392 N. The use of TBA-OL as an additive to polyol ester showed a significant decrease in the coefficient of friction. Increasing the dose of TBA-OL in the polyol showed a gradual reduction in the coefficient of friction. A 2% (w/v) dose of

Figure 6. (a) Changes in the coefficient of friction as a function of TBAOL dose in the polyol ester lube base oil. (b) Average coefficient of friction and kinematic viscosity as functions of TBA-OL dose in the polyol ester lube base oil. (c) Average wear scar diameter as a function of TBA-OL dose. Test parameters: load, 392 N; rotation speed, 1200 rpm; temperature, 75 °C; test duration, 1 h. The average coefficient of friction and wear scar diameter along with their standard deviations were computed on the basis of multiple measurements, and both inherent and statistical errors were considered.

TBA-OL exhibited maximum reduction of friction (60%) compared with polyol ester lube base oil. These results suggest that TBA-OL as an additive played an important role in improving 860

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Industrial & Engineering Chemistry Research Table 3. Physicochemical Properties of Fatty Acid Anion-Based Ionic Liquids Blended with the Polyol Ester Lube Base Oil kinematic viscosity, mm2·s−1 sample description

at 40 °C

at 100 °C

viscosity index

density at 15 °C, g·mL−1

polyol ester polyol + 2% TBA-ST polyol + 2% TBA-OL polyol + 2% TBA-LN

67.86 ± 0.15 69.33 ± 0.42 71.07 ± 0.36 67.22 ± 0.49

12.39 ± 0.08 12.52 ± 0.01 12.74 ± 0.01 12.31 ± 0.01

183 182 181 184

0.929 0.930 0.928 0.929

Figure 8. Comparison of coefficients of friction and WSDs of steel balls lubricated with 2% individual blends of TBA-OL ionic liquid and ZDDP in the polyol. Test parameters: load, 392 N; rotation speed, 1200 rpm; temperature, 75 °C; test duration, 1 h. The average wear scar diameter along with its standard deviation was computed on the basis of multiple measurements.

the lubrication properties of polyol ester by reducing the friction between steel tribopair under the rolling contact. The viscosity of polyol ester lube base oil increased gradually with increasing dose of TBA-OL (Figure 6b). However, beyond the 2% dose of TBA-OL, the viscosity of the polyol ester blend increased sharply upon further addition of TBA-OL ionic liquid. Consistent with the 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 the viscous blend of TBA-OL, causing the blend with 2.5% TBA-OL to exhibit a higher coefficient of friction. Figure 6c shows the reduction in wear scar diameter (WSD) with increasing dose of TBA-OL, and the 2% dose revealed the maximum reduction in WSD (24%). In order to understand the effect of the ionic liquid structure on the lubrication properties, herein three ionic liquids having various fatty acid anions (stearate with fully saturated methylene units and oleate and linoleate exhibiting one and two unsaturation sites, respectively) were selected. The viscosity index of polyol ester remains unchanged upon addition of 2% fatty acid ionic liquid (Table 3). Figure 7 illustrates the friction and wear characteristics of a 2% (w/v) blend of each ionic liquid (TBA-ST, TBA-OL, or TBA-LN) with polyol ester under a load of 392 N. The blend with 2% TBA-ST ionic liquid exhibited a 28% reduction in both friction and WSD compared with polyol ester lube base oil. However, TBA-OL and TBA-LN ionic liquids showed 60 and 33% reductions in the friction, respectively. The polyol ester lube base oil has intrinsic lubricity due to the presence of four oleate functionalities in the form of esters. In spite of that, addition of 2% fatty acid ionic liquid significantly

Figure 7. (a) Evolution of the coefficient of friction with contact time for the polyol ester lube base oil blended with 2% TBA-ST, TBA-OL, or TBA-LN ionic liquid. (b) Comparison of (i) coefficients of friction and (ii) WSDs of steel balls lubricated with polyol ester blended with 2% ionic liquids having various fatty acid anions. Test parameters: load, 392 N; rotation speed, 1200 rpm; temperature, 75 °C; test duration, 1 h. The average coefficient of friction and wear scar diameter along with their standard deviations were computed on the basis of multiple measurements. 861

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Industrial & Engineering Chemistry Research improved the friction-reducing characteristic of the polyol ester. Among these three ionic liquids, oleate anion showed the maximum reduction in the coefficient of friction (60%). Further, a steady-state pattern of the coefficient of friction for TBA-OL (Figure 7a) as a function of contact time suggested the formation of a uniform and stable low-shear-strength thin film of ionic liquid on the tribointerfaces, which reduces the friction. The correlation between the degree of unsaturation in the fatty acid anion and the wear scar diameter on the steel balls is shown in Figure 7bii. The addition of 2% ionic liquid consistently improved the wear-reducing performance by reduction of the WSD in the range of 20−28%. TBA-ST is noted as the most effective additive, providing a 28% reduction in WSD compared with polyol ester lube base oil. The wear results suggested that increasing the degree of unsaturation has a negative effect on the performance of ionic liquids for wear reduction. TBA-ST exhibited the smallest wear scar, which gradually increased with increasing number of unsaturation sites (Figure 7bii). Free fatty acids used as additives to sunflower vegetable oil showed similar wear trends.28 These results suggested that the anionic moiety of the ionic liquid primarily determines the antiwear properties by forming a tribochemical 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 boundary lubrication conditions, polyol ester decomposes to give free acids that interact with the iron surface and form the tribochemical thin film.37 Even vegetable oils, which are rich sources of fatty acids in the form of triglycerides, also provide lubricity and form thin films of fatty acids under tribostress.28 Herein, our motive was to provide an inherent polar nature to the fatty acids in the form of ionic liquids so they can promptly interact with the steel surface under tribostress. The polyol and vegetable oils initially break down into fatty acids under tribostress and then interact with the metal surface, whereas these ionic liquids, composed of fatty acid anions, exhibit inherent negative charge and can easily interact with the steel surface under tribostress. Therefore, the addition of 2% fatty acid ionic liquid to the polyol, which already has fatty acids in the form of esters, showed a remarkable improvement in both the friction-reducing and antiwear properties in the boundary lubrication regime. This was attributed to the rapid response of fatty acid anions to interact with the steel surface compared with fatty acid esters. Furthermore, the tribo performance of the fatty acid ionic liquids was compared with that of the conventional additive ZDDP, which is being widely used for lubricant applications. Figure 8 shows coefficients of friction and WSDs of steel balls lubricated with 2% individual blends of TBA-OL and ZDDP in the polyol. The blend with TBA-OL ionic liquid exhibits a significantly lower coefficient of friction (by ∼65%) than that with ZDDP under identical tribological conditions. This could be attributed to the low shearing of TBA-OL ionic liquid. However, ZDDP revealed better antiwear properties. The WSD of the ZDDP-blend-lubricated steel ball was found to be 10% lower than that of the TBA-OL ionic liquid blend. The enhanced antiwear properties of ZDDP are attributed to the formation of a thin film of glassy polyphosphate on the contact interfaces under tribostress, which protects the steel balls against wear.3,38 Considering the environmental drawbacks of conventional ZDDP additive and the excellent friction-reducing property of TBA-OL, fatty acid ionic liquids could be good alternatives for lubricant applications.

Figure 9. FESEM images of the worn surfaces of steel balls lubricated with (ai, aii) polyol ester lube base oil and 2% individual blends of (bi, bii) TBA-ST, (ci, cii) TBA-OL, and (di, dii) TBA-LN ionic liquids with polyol ester. Test parameters: load, 392 N; rotation speed, 1200 rpm; temperature, 75 °C; test duration, 1 h.

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 tribostress. 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. Furthermore, EDX analyses of steel balls lubricated with fatty acid ionic liquids were carried out to probe the elemental composition of the tribochemical thin film developed on the worn surfaces. The FESEM micrographs and corresponding elemental distribution images of worn surfaces (Figure 10) lubricated with a 2% blend of each ionic liquid explicitly demonstrate the uniform distribution of nitrogen and carbon. In these ionic liquids, carbon and nitrogen are characteristic elements of the anion and cation, respectively, and are uniformly distributed on the worn areas of the steel balls. These results suggested the formation of a tribochemical thin film composed of ionic liquid that protects the 862

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Figure 10. FESEM images and corresponding elemental distributions on the steel ball surfaces lubricated with (ai−aiii) 2% TBA-ST, (bi−biii) 2% TBA-OL, and (ci−ciii) TBA-LN ionic liquid blends with polyol lube base oil.

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

The fatty acid anions react with steel tribointerfaces and form the tribochemical thin film. The strength of such a thin film primarily depends on the strength of the interactions between the molecules making up the tribochemical thin film. In TBA-ST, stearate anions interact with steel tribointerfaces through the carboxylate group, and the methylene units in the stearate anion interact with neighboring methylene units of another stearate anion via weak van der Waals interactions, providing a stable structure. The degree of van der Waals interaction increases with as the number of methylene units increases and provides the well-organized solidlike structure because of their close packing. However, the unsaturation sites in the oleate and linoleate anions sterically distort the structure in the molecules. Consequently, all of the methylene units of one chain cannot interact with nearby

surface against undesirable wear and reduces the friction. It is noted that under tribostress, the fatty acid ionic liquid promptly interacts with the steel surface and forms the tribochemical thin film. The high abundance of carbon on the contact area of the steel ball lubricated with the 2% blend of TBA-OL ionic liquid (Figure 11) confirms the participation of the ionic liquid in tribochemical thin film formation, whereas the low abundance of carbon on the noncontact area of the steel ball can be attributed to the adsorption of the ionic liquid at higher temperature under the triboconditions. The strong signatures of carbon, nitrogen, and oxygen in the point spectroscopy data for the tribochemical thin film (Figure 12) explicitly revealed the role of the fatty acid ionic liquid. The magnitude of wear reduction is strongly determined by the degree of unsaturation in the constituent fatty acid anion. 863

DOI: 10.1021/acs.iecr.5b03347 Ind. Eng. Chem. Res. 2016, 55, 856−865

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Industrial & Engineering Chemistry Research

friction and wear reductions are strongly controlled by the degree of unsaturation in the fatty acid anion. The long alkyl chain of stearate anion in the TBA-ST thin film provides a compact and solidlike structure due to the van der Waals interactions between the methylene units of the long alkyl chains in the stearate anions. The presence of unsaturation sites in the tribochemical thin films of TBA-OL and TBA-LN leads to sterically constrained structures that hinder the van der Waals interactions between the methylene units, resulting in a loosely packed structure of the thin film that exhibits poorer antiwear properties than that of TBA-ST. The fatty acid anions exhibiting inherent negative charge promptly interact with the steel surface under tribostress compared with polyol ester and exhibit significant reduction in friction and wear under boundary lubrication.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03347. 1 H and 13C NMR spectra and vibrational frequencies and assignments for TBA-ST, TBA-OL, and TBA-LN (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 12. EDX spectra of (a) contact and (b) noncontact areas of the steel ball surface lubricated with 2% TBA-OL ionic liquid blend with polyol lube base oil. The presence of nitrogen on the contact surface reveals the formation of a tribochemical thin film consisting of TBA-OL ionic liquid.

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

methylene units of another chain in the thin film. This provides a loosely oriented structure that cannot provide good antiwear properties. Such a phenomenon increases with increasing number of unsaturation sites; as a result, the linoleate anion-containing ionic liquid showed poorer antiwear performance than the oleate anion-containing ionic liquid.

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CONCLUSION Renewable and ecofriendly lubricants and additives are gaining large interest for industrial applications. Vegetable oils are well-established lubricating materials since ancient times. The fatty acids of triglyceride esters in vegetable oils provide low friction and wear-reducing properties by forming a tribochemical thin film on the engineering surfaces. In this work, stearic, oleic, and linoleic acids were selected as model fatty acids to synthesize halogen-free ionic liquids. The preparation of tetrabutylammonium ionic liquids having various anions, viz., stearate, oleate, and linoleate, was confirmed by 1H and 13C NMR and FTIR analyses. The physical states and viscosities of the synthesized ionic liquids are found to be primarily controlled by the chemical structures of the constituent anions. The stearate anion, composed of saturated methylene units, provides a solidlike structure to the TBA-ST ionic liquid, whereas the oleate and linoleate anions, having one and two double bonds, respectively, exhibit a viscous liquid phase. As additives, the fatty acid ionic liquids provide remarkably improved friction and antiwear properties for the steel surface compared with polyol ester lube base oil. Elemental mapping of worn surfaces confirmed the deposition of fatty acid ionic liquid thin films under tribostress. The magnitudes of the 864

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