The study of hexanoate-based protic ionic liquids used as lubricants in steel-steel contact

Journal Pre-proof The study of hexanoate-based protic ionic liquids used as lubricants in steel-steel contact

Hong Guo, Thomas Smith, Patricia Iglesias PII:

S0167-7322(19)34651-3

DOI:

https://doi.org/10.1016/j.molliq.2019.112208

Reference:

MOLLIQ 112208

To appear in:

Journal of Molecular Liquids

Received date:

20 August 2019

Revised date:

31 October 2019

Accepted date:

23 November 2019

Please cite this article as: H. Guo, T. Smith and P. Iglesias, The study of hexanoatebased protic ionic liquids used as lubricants in steel-steel contact, Journal of Molecular Liquids(2018), https://doi.org/10.1016/j.molliq.2019.112208

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© 2018 Published by Elsevier.

Journal Pre-proof The Study of Hexanoate-based Protic Ionic Liquids Used as Lubricants in Steel-steel Contact Hong Guo a, Thomas Smith b, Patricia Iglesias a* a

Mechanical Engineering, Kate Gleason College of Engineering, Rochester Institute of Technology, Rochester, 14623, New York, United States

b

School of Chemistry and Materials Science, College of Science, Rochester Institute of Technology, Rochester, 14623, New York, United States

novel

hexanoate-based

protic

ionic

liquids

with

different

ro

Three

of

Abstract

ammonium

cations

(2-

hydroxyethylammonium 2-ethylhexanoate, 2-hydroxymethylammonium 2-ethylhexancate, and 2-

-p

hydroxydimethylammonium 2-ethylhexanoate) were synthesized, characterized, and studied as neat lubricants under a steel-steel sliding contact mode. The ionic liquid with the 2-hydroxyethylammonium

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cation presented the highest values of density, viscosity, contact angle and thermal stability. The high

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value of viscosity and contact angle in the 2-hydroxyethylammonium salt can be attributed to hydrogen bonding. The three protic ionic liquids, especially 2-hydroxyethylammonium 2-ethylhexanocate and 2hydroxymethylammonium 2-ethylhexanocate, outperformed other studied lubricants showing lower and

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stable friction coefficient. The improved lubrication performance is attributed to the strong physical adsorption of the ionic liquids on the steel-steel surfaces to form ordered lubrication layers due to the

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electrostatic interaction. In addition, the formation of carbon- and oxygen-enriched layers on worn steel surfaces helped to reduce friction. The formation of this tribolayer may be due to the high contact

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temperature and pressure conditions that promote the reaction of the active elements of steel and functional groups ionic liquids.

Keywords: Protic ionic liquid, Steel-steel contact, Lubricants, Friction 1. Introduction Friction and wear between the moving industrial machinery components under harsh working conditions, and the consequent machine break-down or parts replacement result in enormous energy consumption and economic losses [1–3]. Therefore, in modern lubrication systems, high-performance *Corresponding author. Email address: [email protected] Tel: 585-475-7694

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Journal Pre-proof lubricants and lubricant additives are indispensable to lower friction and lessen subsequent wear. Due to their unique physicochemical characteristics; specifically, negligible vapor pressure, non-flammability, high thermal stability and conductivity, ionic liquids (ILs), which are salts with melting points below 100°C, have been the focus of considerable research. Since they can be easily accumulated on the surfaces of rubbing pairs and form ordered layers in liquid state, ILs have shown great potential as lubricants [4–9] or lubricant additives [10–12]. In addition, in the boundary lubrication regime, ILs can

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form protective interfacial films that result from reactions between active functional groups in ILs such as amino, hydroxyl, carboxyl, thiol, phosphate, phosphite, and halogens and metal oxide or metal surfaces.

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The most commonly used ILs are aprotic salts (APILs) with halide counter-ions. However, halogen-

-p

containing ILs will break down passive oxide layers and corrode the contacting materials, particularly

re

when the halogen-containing ILs are exposed to moisture. In addition, because of complexity in their synthesis, [13] APILs can be very expensive. In contrast to APILs, protic ionic liquids (PILs) can be

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easily synthesized through proton transfer from a Brønsted acid to a Brønsted base. PILs possess a wide

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range of properties and tunable structures, which may make them ideal alternatives to APILs. Indeed, the study of PILs as novel lubricants and lubricant additives is receiving the attention of numerous

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tribological researchers [14–21].

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Bermudez’s research group [18] found that when the PIL, bis (2-hydroxyethylammonium) succinate, is used as additive to water in sapphire-stainless steel contact, will form a PIL boundary film on steel surface after water evaporates that leads to an ultra-low mean friction value of 0.02. In addition, they investigated the tribological performance of a series of PILs involving the same ammonium-based cation but different carboxylate-based anions under copper-copper contacts [15]. The hydrogen bonds occurring between the hydroxyl substitutes of ammonium groups and sliding surfaces are considered as the reason of lower friction and wear. Also, Ortega Vega et al. [14] examined the friction and wear behavior of aluminum with a series of oleate-based PILs as lubricants. Due to the formation of tribofilm on the aluminum surface during the sliding process, all the PIL exhibited good lubricating and anti-wear performance.

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2. Experimental section 2.1. Materials Ethanolamine (≥99 %), N-methyl ethanolamine (≥98 %), N, N’-dimethylethanolamine (≥99.5 %), and 2ethylhexanoic acid (≥99 %) were purchased from Sigma-Aldrich (USA). Mineral oil, MO, and the commercially available lubricant, MOA, were provided by Repsol (Spain). All the reagents and materials

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were used as received without any further treatment.

protic

ionic

liquids

(PILs),

2-hydroxyethylammonium

-p

Three

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2.2. Preparation of PIL

2-ethylhexanoate

(Eet),

2-

re

hydroxymethylammonium 2-ethylhexanoate (Met), and 2-hydroxydimethylammonium 2-ethylhexanoate

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(Det) were synthesized in our laboratory in accordance with a procedure published in a previous paper [22]. Thus, stoichiometric quantities of 2-ethylhexanoic acid (EHA) were added dropwise to

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ethanolamine (EA), N-methylethanolamine (MEA), and N, N’-dimethylethanolamine (DEA), respectively, cooling in a water bath as required to keep the temperature of the reaction mixture below 80C. The

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synthetic procedure is schematically outlined in Fig. 1.

Fig. 1. Synthesis routes of PIL (A) Eet, (B) Met, and (C) Det.

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2.3. Spectroscopic Characterization The stoichiometry of the PILs was confirmed by proton nuclear magnetic resonance (1H NMR) using a Bruker Ultra-shield 500 MHz spectrometer with chloroform-d (CIL, 99.8 atom % D) as solvent. NMR spectra shown in Fig. 2 confirm the molecular structures of the three PILs [19]. Eet 1H NMR (chloroform-d, 500 MHz): d = 7.43 (s, 1H, NH), 3.77-3.79 (t, 2H, CH2), 2.99-3.01 (t, 2H, N- CH2), 2.04-2.09 (m, 1H, CH), 1.40-1.54 (m, 4H, CH2, CH2), 1.24-1.32 (m, 4H, CH2, CH2), 0.86-

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0.90 ppm (m, 6H, CH3, CH3). The obvious peak shifts of 0.44 ppm (2H, CH2) and 0.46 ppm (2H, N- CH2)

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appear in Fig. 2A (a-c), and confirm that proton transfer occurred in Eet.

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Met 1H NMR (chloroform-d, 500 MHz): d = 7.82 (s, 1H, NH), 3.83-3.85 (t, 2H, CH2), 2.98-3.00 (t,

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2H, N- CH2), 2.61 (s, 3H, CH3), 2.05-2.10 (m, 1H, CH), 1.36-1.47 (m, 4H, CH2, CH2), 1.23-1.29 (m, 4H, CH2, CH2), 0.86-0.91 ppm (m, 6H, CH3, CH3). The peak shifts of 0.58 ppm (2H, CH2) and 0.69 ppm (2H,

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N-CH2) appear in Fig. 2B (a-c), and demonstrate that proton transfer occurred in Met. Det 1H NMR (chloroform-d, 500 MHz): d=9.77 (s, 1H, NH), 3.62-3.64 (t, 2H, CH2), 2.68-2.70 (t, 2H,

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N- CH2), 2.41 (s, 6H, CH3, CH3), 1.96-2.01 (m, 1H, CH), 1.37-1.41 (m, 4H, CH2, CH2), 1.25-1.32 (m, 4H,

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CH2, CH2), 0.71- 0.76 ppm (m, 6H, CH3, CH3). The peak shifts of 0.18 ppm (2H, CH2) and 0.4 ppm (2H,

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N-CH2) appear in Fig. 2C (a-c), and confirm that proton transfer occurred in Det.

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Journal Pre-proof Fig. 2. NMR spectrum of A (a) Eet, (b) EA, (c) EHA; B (a) Met, (b) MEA, (c) EHA; and C (a) Det, (b) DEA, (c) EHA. Fourier transform infrared (FTIR) spectroscopy analysis was also employed to characterize these PILs. The spectra were measured in the range of 400 cm-1 to 4000 cm-1 with a resolution of 2

cm-1

using

a

SHIMADZU

IRPrestige-21

FOURIER

TRANSFORM

INFRARED

SPECTROPHOTOMETER fitted with a PIKE TECHNOLOGIES diamond anvil.

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2.4. Ionicity of PIL

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Ionic conductivity of Eet, Met and Det was measured using an Accumet Basic AB 30 Conductivity Meter at 23°C, 40°C, 70°C and 90°C.

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2.5. Thermal Analysis

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The glass transition and melting point of PILs were evaluated by differential scanning calorimetry

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(DSC) using a SHIMADZU DSC-60 attached to a refrigerated cooling system. The temperature range was set from -100°C to 100°C with a heating (or cooling) rate of 10°C/min under air (or liquid nitrogen)

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atmosphere.

Thermogravimetric analysis (TGA) was evaluated using a TA Instruments Q500. Each lubricant

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sample was placed in a platinum pan, and the temperature was increased from 20°C to 600°C at a heating

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rate of 10°C/min under air atmosphere. 2.6. Viscosity and Wettability

Viscosity data was gathered within a temperature range of 25 - 100°C using a Brookfield DV2T-LV Viscometer; temperature was controlled with a Thermosel System. Contact angle measurements were carried out on an AISI 52100 steel surface and evaluated for each lubricant at room temperature. A Rame-Hart 250 Goniometer with a DROPimage Advanced software was employed, and the readings were recorded instantaneously over time for each trial until the contact angle was stable. The reading after the fifth minute was collected, and averaged over three measurements for each lubricant.

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Journal Pre-proof 2.7. Friction and Wear Tests The tribological properties of the three PILs under AISI 52100 steel-steel sliding contact mode were evaluated using a custom-designed ball-on-flat reciprocating tribometer. The tribometer includes a pin which holds a protruding ball on the bottom, and a sample holder in which a steel disk is placed. Strain gauges are connected to the arm of the tribometer to measure the resistance force during the sliding motion. Friction coefficient will be then calculated by a LabVIEW program. Steel disks (Ø

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31.7 x 10 mm, Ra ≈ 0.08 m) slid against stationary steel balls (1.5 mm diameter, and Ra ≈ 0.6 m) during each test. The frictional tests were conducted under a normal load of 3 N, which corresponds to a

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maximum Hertz contact pressure of 2.31 GPa, at a sliding speed of 0.03 m/s, frequency of 5 Hz, stroke

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length of 3 mm and a sliding distance of 108 m. For all lubricants, these test conditions correspond to the

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boundary lubrication regime with lambda ratio, λ< 1 [23,24]. For comparison, tribotests were also carried out under the same conditions with the three bases (EA, MEA, and DEA), the acid EHA, the base mineral

lP

oil MO, and the commercially available MOA. To minimize the experimental error, at least three tests

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were performed for each lubricant. Before and after each trial, 0.1 mL lubricant was placed between the contact surfaces and no additional lubricant was added during the test. Friction coefficients were recorded

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over time and calculated as average of each trial. After the tests, an Olympus SZX-12 Optical Microscope

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was used to observe the wear scar of the steel ball, and an Olympus BH-2 Optical Microscope was used to measure wear-track width. Fifteen measurements were taken along each wear track. The wear volume of the steel disk was calculated according to the formula in the paper by Qu and Truhan [25]. In order to study the 3D morphology of the wear track, a Nanovea ST 400 profilometer with a resolution rate of 1000 Hz was used. The wear mechanism and surface interactions of the worn steel disks after lubricated with different lubricants were examined by a Tescan Mira3 scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). 3. Results and Discussion In this research, the aim is to investigate the physicochemical and tribological properties of a series of PILs derived from alkyl ammonium carboxylate salts of hydroxyalkylamines with differing basicity and

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Journal Pre-proof propensity to hydrogen bond. Three hexanoate-based PILs with three ammonium-based cations, 2hydroxyethylammonium 2-ethylhexanocate (Eet), 2-hydroxymethylammonium 2-ethylhexanocate (Met), and 2-hydroxydimethylammonium 2-ethylhexanocate (Det) were synthesized, characterized, and studied as lubricants under a steel-steel sliding contact mode. In order to understand the lubricating mechanism, the PILs and their corresponding neutral bases and acid were used as pure lubricants. In addition, a commercially available lubricant (MOA) and its corresponding mineral base oil (MO) were used as

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comparative controls. Differences in infrared spectra can reflect differences in hydrogen bonding in protic ionic liquids like

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those employed in this study. FT-IR absorption spectrum analysis of Eet, Met, and Det at room

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temperature are shown in Fig. 3. The broad peak (Fig. 3 (A)) in the range of 3600-2500 cm-1 present the

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O-H stretching band, and N-H stretching band which is the characteristic structure of the ammonium cation [17,26]. The wide intensity of the absorption in the region of 3500 -3000 cm-1 seen in Fig. 3 (B)

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scales with the extent of intermolecular hydrogen bonding among hydroxyl groups with Eet > Met > Det. molecules. Strong vibrational peaks between 2956 cm-1 and 2856 cm-1 correspond to the C-H stretching

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bands in the alkyl-chains of PILs [27] (CH3/CH2 asymmetric (2956 cm-1/2926 cm-1) and symmetric (2857

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cm-1/2856 cm-1) stretching band) [28]. The peaks in the region of 1600-1700 cm-1 can be attributed to the

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C=O stretch of H-bonded neutral carboxyl and anionic carboxylate groups. One sees that these peaks are most clearly distinguished in Det where the concentration of the carboxylate is greatest.

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Journal Pre-proof Fig. 3. FT-IR spectra of (A) Eet, Met, and Det; and (B) O-H and N-H stretching bond in PILs. The viscosity and temperature dependence of viscosity is also reflective of the extent of hydrogen bonding. Fig. 4 reveals the dynamic viscosity as a function of temperature for MO, MOA, and PILs. As expected, the viscosity of all lubricants decreased with the increasing temperature. It might be noted that the decrease is particularly pronounced for Eet (see Table 1), where its viscosity decreases 97% from 40°C to 100°C. In this series of PILs, Eet exhibits the highest viscosity probably due to the higher number

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of hydrogen bonds between its molecules. Det, with fewer hydrogen bonds, presents a much lower

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viscosity. The viscosities of the free acid, EA, and the free bases (MEA, DEA EHA), are much lower than

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ur

na

lP

re

-p

MO, the commercial lubricant, MOA, and the PILs.

Fig. 4. Average dynamic viscosity as a function of temperature for MO, MOA and PILs. Inlet exhibits the viscosity of MO, MOA and Det. Table 1. The viscosity of each lubricant. Lubricant 25°C Dynamic Viscosity 40°C (cP) 100°C

MO 92.78 42.54 5.90

MOA 95.22 42.89 5.95

EA 19.02 10.06 2.25

MEA 10.96 6.35 1.85

DEA 3.75 2.44 1.24

EHA 6.52 4.34 1.59

Eet 8943.75 2405.56 59.40

Met 926.86 301.58 14.72

Det 61.74 27.74 3.71

Since the motional frequency of molecules increases with the temperature, the density of a PIL will vary with temperature. Table 2 reports the density of PILs at different temperatures. The density of each

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Journal Pre-proof PIL decreases with increasing temperature. In addition, at the same temperature, the higher number of hydrogen bonds in Eet results in greater density than Met, which possesses an intermediate value; or Det which has the lowest density. Table 2. Density of PILs. Eet

Met

Det

25°C

1.09

0.97

0.95

40°C

1.01

0.94

0.91

100°C

0.97

0.92

0.88

Density (g/cm3)*

of

PIL

-p

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*Densities of PILs were measured by weighing a known volume under different temperatures.

Thermal characteristics of lubricants are of great importance. A good lubricant needs to be in a liquid

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state and thermally stable over a broad temperature range. All the three PILs in this study go through

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glass transition, crystallization, and melting processes, as displayed in Fig. 5. In Det, the glass transition occurs at -79°C, followed by a strong exothermic peak at -17°C due to “cold” crystallization [29], and an

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intense peak for melting at -0.4°C. As heating of the sample is continued, a small peak (deflection in the

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baseline) of unknown origin occurs around 42°C. In the cooling cycle, a small baseline deflection is observed at about 63°C. During cooling, Det shows recrystallization and glass transition at -10.3°C, and -

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86.5°C respectively. Eet and Met do not exhibit the strong cold crystallization and melting behavior shown by Det. However, they exhibit low-temperature glass transitions and similarities in the subtle peaks on the heating and cooling cycles. Data are summarized in Table 3. As expected, Eet presents the highest glass transition temperature since it has a higher degree of hydrogen bonding. The glass transition of Met and Det decreases in concert with the lower extent of hydrogen bonding in these materials [30].

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Fig. 5. DSC curves of Eet, Met and Det.

Glass Transition (°C)

Melting (°C)

Eet

-50.1

*

Met

-61.4

*

Det

-78.8

-0.4†

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Cooling Cycle

Baseline deflection (°C) ‡

Baseline deflection (°C) ‡

Weak recrystallization exotherm (°C)

Glass Transition (°C)

70.10

64.61

-4.07

-53.69

87.27

59.56

-4.05

-66.46

42.15

62.85

-10.3

-86.5

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PIL

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Heating Cycle

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Table 3. Thermal properties of Eet, Met, and Det.

*No crystal melt observed †Strong cold crystallization exotherm was observed prior melting at -17°C. ‡These changes in heat capacity may be associated with a melt of a liquid crystalline mesophase.

Thermal gravimetric analysis of all lubricants was carried out and the results are given in Fig. 6 and Table 4. Base oil, MO, and the commercially available MOA showed higher thermal stability than the PILs, with the onset of decomposition occurring at temperatures of 291°C, and 299°C, respectively. The PILs, Eet and Met, experienced decomposition (due to volatilization of components) at temperatures of 177°C and 175°C, respectively. Det experienced substantial mass loss decomposition at 141°C. Comparing the decomposition temperature of Eet, Det, and their neutral bases and acid, the thermal

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Journal Pre-proof property of the corresponding base played an important role on the thermal stability of the resulting PIL. The mass loss in Eet, Met and Det is no doubt associated with the reversal of the acid/base proton

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exchange equilibrium.

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Fig. 6. Thermogravimetric analysis curves of different lubricants.

EA

MEA

DEA

EHA

Eet

Met

Det

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Table 4. The thermal stability of all lubricants. MO

MOA

Onset temperature (°C)

291.21

298.53

100.36

76.81

154.33

177.48

174.82

141.44

112.3

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Lubricant

The ionicity of a PIL is reflected in its conductivity, which provides a qualitative measurement of the degree of ionization of a PIL [30]. The temperature dependence of the ionic conductivity of the PILs is shown in Fig. 7. The conductivity of Det is 680 µS/cm at room temperature and remains almost constant up to 70°C, and decreases to about 253 µS/cm at 90°C. The conductivity of Met and Eet is much more temperature dependent. For Met, conductivity increases dramatically from 405 µS/cm at room temperature to 1780 µS/cm at 70°C. Above this temperature, the conductivity of Met drops to 1110

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Journal Pre-proof µS/cm at 90°C. The temperature dependence of the conductivity of Eet exhibits a sigmoidal profile, starting at 95 µS/cm at room temperature and saturating at 748 µS/cm at 90C. The PILs being studied in the present work are salts of weak bases (Ethanolamine, N-methyl ethanolamine, N, N’-dimethylethanolamine) and a weak acid, 2-ethylhexanoic acid. Accordingly, proton transfer is an equilibrium reaction and the degree of proton transfer (ionization) is dependent upon the relative basicity of the three alkanol amines [31]. Given the temperature dependence of this dynamic

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equilibrium, the variation in the ionic conductivity observed in Fig. 7 is not unexpected. From ambient temperature to 90°C, the viscosity of all three PILs decreases, and given increased

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mobility at lower viscosity, increased conductivity would be expected. Fig. 7 shows that conductivity of

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Met and Det increases up to 70°C and drops significantly at 90°C. This decrease that occurs despite a

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lower viscosity may be attributed to a reduction in ionicity driven by reversal in the equilibrium for transfer of a proton from EHA to MEA and DEA. For Eet, the conductivity increases throughout the

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range of temperatures studied. While reversal in the equilibrium for transfer proton from EHA to EA is

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likely to be occurring above 70°C, the reduction in viscosity in the highly hydrogen-bonded Eet system is

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much greater; and thus, increased mobility compensates for reduced ionicity.

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Journal Pre-proof Fig. 7. Ionic conductivity of Eet, Met and Det. The wettability of a lubricant reflects its affinity to the metal surface, which in turn affects its friction behavior. The wettability of each lubricant on a steel surface was determined by measuring the contact angle and is reported in Table 5. For each neutral base, the contact angle turned to zero immediately after the testing droplet touched the steel surface, whereas, the initial contact angle of the acid, EHA, was about 10°, and shifted to zero after ~20 seconds. These results indicate strong dipolar interactions between the

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base or acid molecules and this steel surface, extremely thin layers were observed on the steel surface

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after tests finished.

The images in Table 5 shows that Det exhibits the lowest contact angle of the three PILs. This

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indicates that Det has a stronger interaction with the steel surface than do Met and Eet [17], and may

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provide a uniform lubricant film on substrate to protect against severe friction and wear [32]. The

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decreasing contact values of Det < Met < Eet may be related to differing extent of proton transfer and hydrogen bonding in these salts, with Det having the most ionic character, and the least propensity for

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hydrogen bonding. When comparing the three PILs (see Table 1 and Table 2), one sees a clear

and higher contact angle.

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relationship between the viscosity, density and contact angle. The more viscous PIL has higher density

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Table 5. Contact angles of studied lubricants on AISI 52100 steel surfaces. Lubricant

Contact Angle (after 5 min)

Average Contact Angle (Standard Deviation)

MO

22.62° ( ± 0.29)

MOA

37.73° ( ± 0.40)

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36.29 ° ( ± 0.25)

Met

30.96° ( ± 0.01)

Det

22.60° ( ± 0.03)

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Eet

The friction behavior of the neat PILs under reciprocating steel-steel contact mode was investigated

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using a ball-on-flat tribometer, and compared to EHA and their corresponding alkanol amine precursors.

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The friction behavior of MO and the commercial lubricant, MOA was also evaluated. Fig. 8 reveals the

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average friction coefficient after sliding 108 m using each lubricant. The use of any of the three PILs reduces the friction coefficient as compared to the MO and MOA under the same conditions. Eet and Met

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provide a significant improvement in the friction coefficient, and maintain a relatively stable friction (see inlet of Fig. 8). However, severe oscillations in friction are observed during the wear process when MO

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and MOA are used as lubricants, probably due to the stick-slip phenomenon between the steel ball and

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steel disk at the low speed used, where the neighboring asperities undergo junctions being formed and then broken, leading to a fluctuation in friction coefficient. The stick-slip phenomenon also occurs in the late friction stage using PILs, but the fluctuation is milder and is caused by the unstable lubrication films. Although Det shows a relatively high friction coefficient at the beginning, it decreases gradually with time and stabilizes around 0.058. In comparison to MO, friction reductions of 63.8 %, 69.5 %, and 43.8 % were obtained by using Eet, Met and Det, respectively. Compared to PILs themselves, acid and base components of the PILs (EA, MEA, DEA, and EHA) showed higher friction values. Under the same testing conditions EHA exhibited better lubricating performance than the neutral bases, with a reduction of 32.4 % as compared to MO.

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Fig. 8. Average friction coefficient of steel-steel contact after lubricated by each lubricant. Inlet

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shows the evolution of friction coefficient with time for MO, MOA, Eet, Met, and Det.

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Fig. 9 summarizes the wear volumes of the steel disks treated with the different lubricants. Steel disks

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lubricated with Eet exhibited the least wear loss with wear reductions of 80.5 % and 65.2 % as compared to MO and MOA, respectively. Met also exhibited good anti-wear behavior with wear reductions of 62.3

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% and 32.7 % with respect to MO and MOA, respectively. The base, EA, exhibited wear loss two order of magnitude higher than that of Eet. Neat bases MEA or DEA also showed higher wear volumes than their

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corresponding PILs, Met and Det. With a wear reduction in the steel disk of 50.0 % with respect to MO,

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the free acid, EHA, provided significant lubrication benefit. This wear reduction may be attributed to the adsorption of the carboxyl group (-COOH) of the free acid on the metal surface with friction force being lowered by the alkyl chains absorbed at the interface [33,34].

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Fig. 9. Wear volume of the steel disks after lubricated with different lubricants.

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Fig. 10 shows the optical micrographs of the worn balls after testing. The wear scars obtained using

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PILs are smaller than those obtained with the neat bases, free acid, MO or MOA. The ball lubricated with Eet showed a very tiny, superficial wear scar [see Fig. 10 (G)], Fig. 10 (B) and (C) show clear abrasive

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(D)] is small but relatively rough.

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marks on the worn surfaces lubricated with MOA and EHA. The wear scar obtained using EA [Fig. 10

Fig. 10. Optical images of worn balls after lubricated with (A) MO, (B) MOA, (C) EHA, (D) EA, (E) MEA, (F) DEA, (G) Eet, (H) Met, and (I) Det.

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Journal Pre-proof Optical micrographs of worn disk surfaces are shown in Fig. 11. Abrasive traces and plastic deformation along the borders of the wear tracks can be observed in most images. In comparison to the wear tracks obtained when lubricated with MO and MOA, the wear tracks obtained with PILs are narrower and more uniform. This confirms the substantive lubrication abilities of Eet, Met and Det. Lubricating with Eet leads to the narrowest wear track and minimal abrasive marks along the sliding direction with plastic deformation at the borders. The wear width and depth after a test lubricated with Eet

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were 41.8 % and 66.2 % narrower than in the disk surfaces lubricated with MO, and 29.6 % and 50.4 % narrower than the disk surfaces lubricated with MOA. These differences can be clearly observed in the

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3D and 2D images of the worn surfaces in Fig. 12. The significant difference in the wear morphology or

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friction behavior (see Fig. 8) between MO and Eet is suspected to be caused by their different lubrication

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mechanisms. Under the conditions studied, all the lubricants are in a boundary lubrication regime where the λ values are smaller than 1 [35]. Accordingly, when using Eet as lubricant, it is postulated that the

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difference in friction and wear may be due to the formation of tribofilms that protect the steel surfaces.

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Although EHA exhibits a uniform wear track, and a multiplicity of sharper abrasive marks on the worn surface [see Fig. 11 (C)] which matches the wear on the worn steel ball seen in Fig. 10 (C). The steel

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disks experienced severe abrasion and corrosion [see Fig. 11 (D), (E) and (F)] when lubricated with the

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free bases EA, MEA, and DEA. Deep furrows are observed in the track lubricated with EA, this may result from direct contact of the asperities between the moving surfaces. Corrosion was also observed when the disks are lubricated with Det.

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Fig. 11. Optical images of wear tracks on the steel disks after sliding 108 m and lubricated with (A)

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MO, (B) MOA, (C) EHA, (D) EA, (E) MEA, (F) DEA, (G) Eet, (H) Met, and (I) Det.

Fig. 12. (A) 3D profilometer images and (B) 2D profiles of wear tracks on the steel disks after lubricated with MO, MOA, and Eet.

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Journal Pre-proof In order to better understand the lubricating mechanism of the PILs, SEM micrographs and EDS were obtained for worn surfaces after tests lubricated with Eet, Det, MO and MOA and the results are summarized in Fig. 13. Abrasive marks can be seen in all worn surfaces [see Fig. 13 (A)], and are most pronounced in the surface lubricated with MOA. A wider wear track with plastically deformed material accumulated along the border is observed in the surface lubricated with MO. In the surface lubricated with Eet, the wear track is narrower and more superficial. EDS results presented in Fig. 13 (B) show a

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slight increase of carbon and oxygen inside of the wear tracks of surfaces lubricated with MO and MOA. This incremental increase in carbon may come from the lubricants; the incremental increase in oxygen

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must be related to oxidation on the worn surfaces. A much higher level of carbon was detected inside of

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the wear track lubricated with Eet. This higher amount of carbon inside the wear track confirms a

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tribochemical reaction between Eet and elements of the steel and may be responsible for its superior antiwear performance. An oxygen-richened tribofilm, that seems to be less effective under the conditions

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studied than the carbon-richened film, is also confirmed when the surfaces are lubricated with Det.

Fig. 13. (A) SEM micrographs of wear tracks on the steel disks and (B) EDS spectrum of the difference between inside and outside the wear track after lubricated with MO, MOA, Eet, and Det.

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Journal Pre-proof 4. Conclusions Three novel hexanoate-based protic ionic liquids, 2-hydroxyethylammonium 2-ethylhexanoate (Eet), 2hydroxymethylammonium 2-ethylhexanoate (Met), and 2-hydroxydimethylammonium 2-ethylhexanoate (Det) were synthesized, characterized, and studied as neat lubricants under a steel-steel sliding contact mode. Eet presents the highest values of density, viscosity, contact angle and thermal stability, probably due to the higher extent of hydrogen bonding among its molecules. All the protic ionic liquids exhibited

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stable friction performance and provided significant reduction in friction coefficient and wear as

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compared to that obtained when lubricated with mineral oil (MO) or a commercial mineral oil-based lubricant containing an unknown, proprietary, lubricating additive (MOA). Specifically, Eet provided

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reduction in friction and wear of 53.6 % and 65.2 % as compared to MOA. The presence of higher

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amount of carbon or oxygen inside of the wear tracks after lubrication with protic ionic liquids supports

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the proposition that the PIL molecules are adsorbed on the steel surfaces and react with the passive oxide

Acknowledgement

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on the steel surface to generate a tribofilm that protects the steel from further wear.

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The authors wish to thank the Mechanical Engineering Department at the Kate Gleason College of

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Engineering at Rochester Institute of Technology for the financial support, and REPSOL for providing the commercially available lubricant and the corresponding base oil. Hong Guo would also like to thank the Gleason Corporation for the Gleason Doctoral Fellowship. References [1]

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Highlights • Three ionic liquids were synthesized, characterized, and studied as lubricants.

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Journal Pre-proof • Ionic liquids outperformed other lubricants showing lower and stable friction.

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• Lubrication performance is attributed to physical adsorption and triboreactions.

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Figure 1

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