Structural strategies to design bio-ionic liquid: Tuning molecular interaction with lignin for enhanced lubrication

Journal of Molecular Liquids 280 (2019) 49–57

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Structural strategies to design bio-ionic liquid: Tuning molecular interaction with lignin for enhanced lubrication Liwen Mu a,c, Xiaofeng Ma b, Xiaojing Guo d, Minjiao Chen a, Tuo Ji c, Jing Hua a, Jiahua Zhu c,⁎, Yijun Shi a,⁎ a

Division of Machine Elements, Luleå University of Technology, Luleå 97187, Sweden College of Science, Nanjing Forestry University, Nanjing 210037, PR China Intelligent Composites Laboratory, Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH 44325, USA d Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, PR China b c

a r t i c l e

i n f o

Article history: Received 6 December 2018 Received in revised form 7 January 2019 Accepted 5 February 2019 Available online 6 February 2019 Keywords: Lignin Lubrication Hydrogen bond Amino acid Structure Ionic liquids

a b s t r a c t Lignin strengthened ionic liquids (ILs) have shown high potential to be used as high performance green lubricants. Strengthened lignin-ILs molecular interaction is an effective approach to improve their lubrication properties. The molecular interactions of ILs' cation and anion containing different functional groups with lignin and efficiency on the lubricating properties have rarely been studied yet. In this work, a series of novel green lubricants with dissolved lignin in [Choline][Amino Acid] ([CH][AA]), [Tetramethylammonium][Glycine] ([N1111] [Gly]) and [Tetrabutylammonium][Glycine] ([N4444][Gly]) ILs have been synthesized and their tribological properties were systematically investigated. The longer alkyl chain in cation without reciprocal H-bond interaction between ILs' cation and anion has the positive effect on the anti-wear properties. In addition, the less steric effect and more negative natural charges of amino acid anion synergistically contribute to the stronger H-bond interaction between lignin and choline base ILs, which enhances lubrication film strength and thus resulting in the better tribological property of ILs/lignin green lubricants. Specifically, the wear volume loss of the steel disc lubricated by [N4444][Gly] with the addition of 15% lignin is only 12% of the one lubricated by pure [N4444][Gly]. This work presents a method to tune molecular interaction between lignin and ILs via the structural design of ILs' cation and anion, which are revealed as the key factor that bridges the individual components and improves overall lubricating properties. © 2019 Published by Elsevier B.V.

1. Introduction Molecular interactions exist wildly in the nature and help us to understand material's structure and physicochemical behavior [1–3]. Hydrogen bond (H-bond) is one kind of the most studied molecular interactions among many uncharged materials, which behave generally stronger than ordinary dipole-dipole and van der Waals' force [4–6]. Hbond is a directional attractive interaction between electron-deficiently hydrogen and a region of high electron density [7–9]. Depending on the combinations of the proton donors and acceptors, the H-bond strength ranges from 1 to 170 kJ/mol [10]. The strength of H-bond is directly related to all the elements affecting the acidity of the proton donor, the basicity of the proton accepter, and the accessibility of the donor and acceptor [11]. For example, the H-bond strength increases with the acidity of the proton donor and the basicity of the proton acceptor [12]. When the size of donors and acceptors are too large, they can't achieve close proximity, leading to a weak H-bond strength. Among

⁎ Corresponding authors. E-mail addresses: [email protected] (J. Zhu), [email protected] (Y. Shi).

https://doi.org/10.1016/j.molliq.2019.02.022 0167-7322/© 2019 Published by Elsevier B.V.

these factors, the chemical and stereo structures of the donors and acceptors are essential in determining the strength of the H-bond. [13] Tribology is briefly described as the ‘Science and technology of interacting surfaces in a state of relative motion and the practices thereto’ [14]. This very broad scope embraces in detail the processes of friction, lubrication and wear in all mechanical contact situations. Besides mechanical design, lubrication is critical to the economics and operational reliability of industrial manufacture and processing. The composition of existing lubricants is usually designed to meet multiple specific criteria in practical operations [15–17]. Different types of additives, such as thickener, anti-oxidant, corrosion inhibitors, friction modifier, anti-wear agent and etc., are added into base oils to promote lubricating properties [18–20]. The strong adhesion of the lubricants to the friction surfaces and the superior mechanical strength of lubrication films at the molecular scale are two main factors for achieving excellent lubricating properties [21,22]. H-bond interaction seems an effective approach to improve the overall lubrication properties [23]. For example, H-bond network of phosphoric acid and water molecules adsorbed on the two friction surfaces (sapphire and ruby) could behave stable superlubricity behavior [24]. The friction and wear resistance of reduced graphene oxide (rGO)/poly(ethlyne glycol)200 was improved

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via the H-bond between the hydroxyl groups of rGO and the oxygen atoms of PEG200 molecules. [25–27] With the development of modern technology and to meet the requirement of environmental protection, green lubricants with sustainable feature has attracted world attention [28]. Over the past decades, many researchers tried to find the alternative lubricant base oils and additives those are less dependent on petroleum products and have less or negligible impacts on the environment. [29,30] For example, low-cost glycerol aqueous solution with good biocompatibility has been used as an excellent green lubricant. [31] Biopolymer materials such as cellulose, hemicellulose and lignin are natural polymers [32–34], which have been used as green lubricant additive, membrane, absorber, piezoelectric materials and so on [35–37]. They are more advantageous than traditional synthetic polymer due to their environmentally benign feature. Meanwhile, H-bond is one of the most ubiquitous molecular interactions in biomaterials especially biopolymers [38,39], which could be beneficial for the lubrication improvement. Among these biopolymers, lignin, a cross-linked polymer with phenylpropanoic monomers, is the second most abundant biopolymer in nature [40]. Fundamental research has focused on converting lignin to value-added chemicals, materials, and alternative fuels, while very few of them have translated into commercial practice during the past decades [41]. The abundant functional groups (hydroxyl and aldehyde group) in lignin are also expected to form reciprocal H-bond easily with base oil [23,42]. Recently, lignin was demonstrated as an effective lubricant additive in ionic liquids (ILs) [21], base oil [23,43], and cutting fluids (metalworking fluids) [44] to reduce the friction coefficient and wear loss of metal/metal contacts due to the presence of effective H-bond between lignin and the lubricant base. Ionic liquids (ILs), defined as molten salts those are entirely ionic in nature, comprising both cationic and anionic species with a melting point below 100 °C [45,46], have attracted great interests as synthetic lubricants since 2001 [47]. Meanwhile, ILs would revolutionize various industrial processes such as CO2 capture and separation, biomass dissolution and absorption heat transformer. [48] The bio-ILs, synthesized from renewable bio-resources [49], have attracted tremendous interests from researchers in the fields of biomass pretreatment and lubrication [50] due to their distinct advantages such as biodegradability, non-toxicity, and environmentally benign feature [51,52]. For bio-ILs contains H-atoms and lone pairs, there is a potential to form reciprocal H-bond with lignin, which will promote the interaction between bio-ILs and lignin [21]. Due to the wide range of cations and anions of ILs, it is expected that there is a large pool to strengthen the H-bond via the selection of suitable cations and anions of ILs. Thus, it is worthwhile to investigate the effect of ILs structures on their H-bond and thus efficiency on the lubricating properties. In this work, five different kinds of amino acid based bioILs were chosen as the base oil and lignin was selected as the lubricant additive. The rheological and tribological properties of lignin/ amino acid bio-ILs lubricants were systematically investigated. The H-bond between lignin and bio-ILs as well as its effect on the resulting lubrication properties were discussed through both theoretical calculation and experimental approaches.

2.2. Synthesis of ILs and lignin/ILs Choline, tetraethylammonium, or tetrabutylammonium hydroxide aqueous solution was added dropwise to equimolar glycine, L-serine, or L-alanine amino acid with ice cooling. Then, the mixture was magnetically stirred at room temperature for 48 h. After reaction, the water in the mixture was removed using a rotary evaporator at 60 °C. Finally, the [N2222][Gly], [N4444][Gly], [CH][Gly], [CH][Ala] and [CH][Ser] were dried in vacuo for 48 h at 70 °C, refer to Fig. 1 for their molecular structures. [N2222][Gly] and [N4444][Gly] are used to investigate the effect of ILs cation's chain length on the lubricating properties of ILs. [CH][Gly], [CH][Ala] and [CH][Ser] are utilized to study the effect of ILs anion's structure on the lubricating properties. Alkali lignin of different weight (0.05, 0.10 and 0.15 g) was added to 1 g [N2222][Gly] at 90 °C and stirred for 24 h with N2 protection, which eventually formed homogeneous solutions and denoted as [N2222][Gly]-5, [N2222][Gly]-10 and [N2222][Gly]15. Same procedure was applied to other amino acid based ILs to prepare the lignin/ILs lubricants. 2.3. Characterization The molecular structures of ILs were analysed by proton nuclear magnetic resonance (1H NMR, Varian Mercury-300) in D2O at 300 MHz. The following abbreviations were used to designate multiplicities: s = singlet, d = doublet, t = triplet, m = multiplet. [N2222] [Gly]: d = 1.13–1.36 (m, 12H, CH3, CH3, CH3, CH3), 3.16 (s, 2H, CH2N), 3.24 (q, J = 7.22 Hz, 8H, CH2, CH2, CH2, CH2). [N4444][Gly]: d = 0.94 (t, J = 7.32 Hz, 12H, CH3, CH3, CH3, CH3), 1.35 (m, J = 7.14 Hz, 8H, CH2, CH2, CH2, CH2), 1.55–1.76 (m, 8H, CH2, CH2, CH2, CH2), 3.01–3.30 (m, 10H, CH2-N, CH2, CH2, CH2, CH2). [CH][Gly]: d = 3.99–4.08 (d, 2H, CH2), 3.45–3.54 (m, 2H, CH2), 3.22–3.32 (m, 2H, CH2-N), 3.18 ppm (s, 9H, CH3, CH3, CH3). [CH][Ala]: d = 1.18–1.20 (d, 3H, CH3), 3.17 (s, 9H, CH3, CH3, CH3) 3.27 (d, J = 7.03 Hz, 1H, CH-N) 3.42–3.54 (m, 2H, CH2) 3.95–4.11 (m, 2H, CH2). [CH][Ser]: d = 3.17 (s, 9H, CH3, CH3, CH3) 3.24–3.36 (m, 1H, CH-N) 3.43–3.54 (m, 2H, CH2) 3.63–3.81 (m, 2H, CH2) 3.97–4.10 (m, 2H, CH2). All these characterization results in Fig. 2 are well consistent with previous literature report [51,53], which indicate the successful synthesis of the desired ILs. The average viscosity of the lubricants within a range of shear rate from 1 to 300 s−1 was reported using a Bohlin CVO 100 rheometer at

2. Experimental section 2.1. Materials Choline hydroxide aqueous solution (48–50 wt% in water) was purchased from Tokyo Chemical Industry. Tetraethylammonium hydroxide solution (40 wt% in water), Tetrabutylammonium hydroxide solution (40 wt% in water), glycine (≥99%), L-serine (≥99%), L-alanine (≥99%), and deuteroxide (D2O, 99.9 atom% D), alkali lignin were purchased from Sigma Aldrich. All chemicals were used as received without further treatment.

Fig. 1. The structure of amino acid bio-ionic liquids.

L. Mu et al. / Journal of Molecular Liquids 280 (2019) 49–57

Fig. 2. 1H NMR spectra of (a): [N2222][Gly], (b): [N4444][Gly], (c): [CH][Gly], (d): [CH] [Ala], and (e): [CH][Ser].

25 °C. The lubricant sample was dried in vacuo for 24 h at 70 °C before the viscosity test. The water content in the lubricant samples is lower than 1%. Fourier transform infrared-attenuated total reflection (FT-IRATR) spectra were recorded with a Thermo Scientific Nicolet 380 series spectrometer. An Optimol SRV-III oscillating friction and wear tester was used to evaluate tribological properties of the green lubricants under boundary lubrication conditions based on ASTM D 6425 protocol. During the test, the upper steel ball (52100 bearing steel, diameter 10 mm, surface roughness (Ra) 20 nm) slides under reciprocating motion against a stationary steel disc (100CR6 ESU hardened, Ø24 mm × 7.9 mm, and surface roughness (Ra) 120 nm). The disc was supplied by Optimol Instruments Prüftechnik GmbH, Germany. The ball was provided by SKF, Sweden. Before each test the device and specimens were cleaned with acetone and ethanol. All tests were conducted under the load of 258 N (3 GPa Maxium Hertzian pressure) at room temperature (25 °C), a sliding frequency of 50 Hz, and an amplitude of 1 mm. The friction coefficient curves were recorded automatically with a data acquiring system linked to the SRV-III tester. After the tests, the wear volumes of the lower discs, wear diameters of the upper ball and corresponding root mean square (RMS) surface roughness were determined using an optical profiling system (Zygo 7300). Three duplicate friction and wear tests were carried out to minimize the experimental error.

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of the lubrication film resulting in effective interfacial separation between metal/metal contacts to reduce friction and wear [42]. It will be interesting and important to see the difference of the H-bond between different ILs with and without lignin. The wavenumber shift of the N\\H peak in the Fourier transform infrared spectroscopy is an indicator of the change in intermolecular interactions [57,58]. The N\\H peak shifts towards lower wavenumbers indicate the stronger H-bond between lignin and ILs. Fig. 3 is the N\\H group wavenumber of amino acid ILs lubricants filled with different lignin loading. It can be seen that the wavenumber of N\\H peak decreases with the addition of lignin in choline based ILs, which means that lignin has the positive effect on the increase of H-bond strength between lignin and choline based ILs. In addition, the wavenumber of N\\H peak decreases furtherly with the increment in the lignin loading, which means that the stronger H-bond is achieved between lignin and ILs with the increase of lignin loading. The N\\H peak wavenumber shift in [CH][Ser] ILs with the increase of lignin is largest in choline based ILs. The reason will be discussed in the following DFT calculation section. There are no wavenumber shift of N\\H peak in [N2222][Gly] ILs and small wavenumber increase in [N4444][Gly] ILs with the increase of the lignin loading, which imply lignin could have no obvious or negative effect on the Hbond strength in ammonium based ILs. To have a better understanding of the H-bond between lignin and ILs, electronic structure of ILs was optimized using DFT calculations and the atomic charges of nitrogen and oxygen atoms for different anions were investigated by the natural bond orbital (NBO) analysis at the B3LYP/B3LYP/6–31++G* level of theory, Fig. 4 and Table 1. On the basis of the natural population analysis (NPA), the total natural charges of nitrogen and oxygen atoms in the anion of glycine and alanine are −2.474 e−, which are less negative than that of serine (−3.263 e−). This is to say, the H-bond formation capability between [CH][Gly]/[CH][Ala] and lignin is weaker than that between [CH][Ser] and lignin. In addition, the natural charges of nitrogen in serine (−0.911 e−1) is most negative among the three amino acid, which could explain the largest N\\H peak wavenumber shift of [CH][Ser] with the increase of lignin in IR results. The methyl group (-CH3) in alanine anion reduce the flexibility of anion chains and hinder the interaction between lignin and [CH][Ala]. Therefore, the less steric effect and more negative natural charges synergistically contribute to the stronger H-bond interaction between lignin and choline base ILs. Viscosity is an important characteristic of lubricants. Table 2 shows the viscosity of amino acid based ILs/lignin lubricants within the shear rate of 1–300 s−1. The increase of alkyl chain in the ammonium based ILs will reduce the mobility of cation chains [59], which leads to the higher viscosity of [N4444][Gly] ILs compared with [N2222][Gly] ILs. The viscosity relationship of choline based ILs is in the following order: ν[CH][Ser] N ν[CH][Ala] N ν[CH][Gly], which may be due to the less flexibility of anion chains with the addition of the methyl group (\\CH3) in alanine

2.4. DFT calculation Electronic structure calculations on five kinds of amino acid ILs were performed with the Gaussian 09 C1 package [54] using density functional theory (DFT) at the B3LYP level of theory [55,56]. 6–31++G* basis sets were used for carbon, nitrogen, oxygen, and hydrogen atoms. Frequency calculations were performed to verify that the geometries were minimal. 3. Results and discussion In our previous study, it is found that H-bond is formed between the ILs and lignin, which is beneficial for improving the mechanical strength

Fig. 3. N\ \H peak wavenumber of amino acid ILs lubricants filled with different lignin loading.

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Fig. 4. Optimized structures of amino acid based ILs by the B3LYP method. (Red, white and blue balls represent oxygen, hydrogen and nitrogen atoms, respectively.)

and the stronger H-bond formation between choline cation and serine anion (one more H-bond active site in serine anion, \\CH2\\OH) in the serine anion [60,61]. Generally, the viscosity increases after dissolving lignin in ILs and continuously increases with the increasing mass fraction of lignin. The viscosity relationship of choline based ILs is in following order: ν[CH][Ser]-5/10/15 N ν[CH][Ala]-5/10/15 N ν[CH][Gly]-5/10/15. The viscosity of [CH][AA]/lignin lubricants may be attributed to three factors, the thickening effect of lignin itself, [CH][AA] viscosity and H-bonding interaction of ILs/lignin. Enhanced H-bond in the system is helpful to increase the viscosity [62]. [CH][Ser] has one more H-bond active site (\\OH group) than that of [CH][Gly] and [CH][Ala]. So the H-bond between [CH][Ser] and lignin will be stronger than those of [CH][Gly] and [CH][Ala], and thus lead to the higher viscosity. In addition, the viscosity relationship of choline based ILs is in the following order: ν[CH][Ser] N ν[CH][Ala] N ν[CH] [Gly]. The larger viscosity of choline based ILs will make the positive contribution on the viscosity of the ILs/lignin lubricant. The viscosity enhancement of the lignin additive in different ammonium based ILs is similar as that in choline based IL system. The viscosity of [N4444][Gly]-5/10/15 is larger than that of [N2222][Gly]-5/10/15. The viscosity of [N4444][Gly]/ lignin will increase sharply, when the lignin loading increases up to 15%. The thickening effect of the polymer itself is more obvious with the increase of polymer concentration [63,64]. The friction coefficient (μ) evolution during one-hour friction test and average μ with the presence of lignin/amino acid ILs at 3.0 GPa pressure were shown in Fig. 5 and Table 3. Apparently, unstable friction coefficient was observed by using pure choline and tetraethylammonium based ILs,

Table 1 Natural charges on the nitrogen and oxygen atoms for different anion in the amino acid based ILs by the B3LYP method. Atomic charge (e−1) No. N O

Sum

1 2 3 4

Gly

Ala

Ser

−0.879 −0.805 −0.790

−0.881 −0.804 −0.789

−2.474

−2.474

−0.911 −0.795 −0.776 −0.781 −3.263

which is evidenced by the sharp jump of friction coefficient during the friction process, Fig. 5(a, i, m and q). [N4444][Gly] IL behaves smooth and the lowest average friction coefficient (0.068) during the friction process, which indicates a superior lubricating performance. Lignin could lower the friction coefficient of [N2222][Gly], but it induces some friction coefficient fluctuation with jumps at the initial stage of [N2222] [Gly]-10/15. Lignin could stabilize the friction coefficient of [N4444] [Gly], but it increases the average friction coefficient, which could be attributed to the weaker H-bond at the higher lignin loading and has been discussed in the IR section. The addition of lignin definitely helps to reduce the friction coefficient in choline bases ILs and it decreases with the increase of lignin loading. Meanwhile, the friction coefficient could be stabilized all through the testing period with the addition of lignin, and the stabilizing effect makes it more powerful with the increase of lignin loading. Specifically, the friction coefficient lubricated by [CH][Gly]15 is only 63.4% of pure [CH][Gly]. Fig. 6 shows the disc wear volume loss lubricated by different amino acid lubricants. For ammonium based ILs, the disc wear volume of [N4444][Gly] is smaller than that of [N2222][Gly]. The wear volume relationship of the disc lubricated by choline based ILs is in following order: W[CH][Ser] N W[CH][Ala] N W[CH][Gly]. [CH][Gly] has been demonstrated the strongest affinity to metal surface among three choline bases ILs in previous literature report [65], which positively contributes to the formation of mechanically strong liquid film and thus effectively prevents the direct contact between steel ball and steel disc to reduce friction coefficient and wear loss [66]. It is observed that lignin could be helpful to improve the anti-wear properties of amino acid ILs and the wear volume loss decreases continuously with increase of lignin fraction. Specifically, the wear volume of the steel disc lubricated by [N4444][Gly]-15 is only 12% of the one lubricated by pure [N4444][Gly].

Table 2 Viscosity of amino acid based ILs/lignin lubricants (Pa·s). Lignin

[N2222][Gly]

[N4444][Gly]

[CH][Gly]

[CH][Ala]

[Ch][Ser]

0 5 10 15

0.0379 0.0750 0.1000 0.1594

0.1293 0.2144 0.5744 2.2914

0.0350 0.1182 0.1363 0.2189

0.0623 0.1667 0.2170 0.6287

0.1100 0.4903 0.7915 0.8526

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Fig. 5. Friction coefficient of disc lubricated by lignin/amino acid ILs lubricants. Load: 3.0 GPa, testing duration: 1 h, room temperature. (a)–(d): [N2222][Gly], [N2222][Gly]-5, [N2222][Gly]-10, [N2222][Gly]-15. (e)–(d): [N4444][Gly], [N4444][Gly]-5, [N4444][Gly]-10, [N4444][Gly]-15. (i)–(l): [CH][Gly], [CH][Gly]-5, [CH][Gly]-10, [CH][Gly]-15. (m)–(p): [CH][Ala], [CH][Ala]-5, [CH] [Ala]-10, [CH][Ala]-15. (q)–(t): [CH][Ser], [CH][Ser]-5, [CH][Ser]-10, [CH][Ser]-15.

Table 3 Average friction coefficient of disc lubricated by lignin/amino acid ILs lubricants. Load: 3.0 GPa, testing duration: 1 h, room temperature. Lignin 0 5 10 15

[N2222][Gly]

[N4444][Gly]

[CH][Gly]

[CH][Ala]

[CH][Ser]

0.101 ± 0.006 0.091 ± 0.002 0.082 ± 0.003 0.095 ± 0.004

0.068 ± 0.003 0.073 ± 0.002 0.074 ± 0.004 0.080 ± 0.003

0.112 ± 0.008 0.100 ± 0.009 0.083 ± 0.003 0.070 ± 0.004

0.118 ± 0.009 0.113 ± 0.006 0.111 ± 0.004 0.101 ± 0.005

0.123 ± 0.004 0.113 ± 0.005 0.104 ± 0.004 0.091 ± 0.005

To get a better understanding of the interaction between lignin and ILs (cation and anion), the enhancement of anti-wear properties is calculated and shown in Table 4. In the glycine based ILs/lignin lubricant, the effect of the cation structure on the anti-wear property enhancement at the lignin loading of 5 and 10% is [CH][Gly] N [N4444][Gly] N [N2222][Gly]. One more hydroxyl group in the choline cation will provide more H-bond formation site and form stronger H-bond than ammonium based ILs/lignin. The longer alkyl chain in the tetrabutylammonium cation will form the mechanically stronger liquid film without the reciprocal H-bond between cation/anion and lignin, which leads to a lower wear volume loss. [N4444][Gly]-15 achieve the largest enhancement of anti-wear property and its lubrication regime will be discussed in the following section. The enhancement relationship of choline ILs with different amino acid anion at different lignin loadings is [CH][Ser] N [CH] [Gly] N [CH][Ala]. The interaction mechanism in the section of DFT calculations indicates that the denser H-bond interaction and longer alkyl chain in cation without reciprocal H-bond interaction will generate better anti-wear properties.

Fig. 7 presents the three-dimensional (3D) surface profiles of the wear tracks on disc after friction test by using different lubricants. The wear track by amino acid ILs is obviously deeper than the tracks lubricated by ILs/lignin, which further confirms the superior antiwear properties of amino acid ILs/lignin. Larger lignin fraction is more effective in lubrication as revealed by the shallower wear tracks observed. From 1st to 4th columns at the same row in Fig. 7, the smallest wear volume was obtained at the 15% lignin loading. Comparing the 3D images from first/second rows of Fig. 7, [N4444] [Gly]/lignin shows relatively smaller and shallower wear tracks than those of [N 2222 ][Gly]/lignin, which further verifies its better anti-wear properties. Among the choline based lubricant with lignin, it can be seen that [CH][Ser]/lignin shows the smallest and shallowest wear tracks. Ball wear diameter is another important anti-wear property index for the lubricant evaluation. Fig. 8 shows the ball wear diameter lubricated by different amino acid based ILs lubricants under the pressure of 3.0 GPa. It should be noted here that [N4444][Gly] has the lowest ball wear diameter among the amino acid bade ILs. In addition, lignin could be helpful to decrease the ball wear diameter of amino acid based ILs. It is observed that the ball wear diameter decreases continuously with the increase of lignin fraction. Fig. 9 shows the 3D microscopic images of ball wear tracks by using different lubricants. In the first column, the ball wear diameter lubricated by [N4444][Gly] is the smallest in the five pure amino acid based ILs. From Fig. 9(a–d), the wear track with pure [N2222][Gly] is obviously deeper than the tracks for lignin/[N2222 ][Gly]. The ball wear track Table 4 Enhancement in anti-wear properties with the addition of lignin. ILs

Fig. 6. Disc wear volume lubricated by different amino acid based lubricants. Load: 3.0 GPa, testing duration: 1 h, room temperature.

[N2222][Gly] [N4444][Gly] [CH][Gly] [CH][Ala] [CH][Ser]

Enhancement (%) 5

10

15

16.0 22.8 35.6 18.2 75.7

33.1 37.0 47.8 24.0 81.3

39.9 88.2 74.3 30.6 85.1

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Fig. 7. 3D surface profile of disc wear tracks by using different lubricants. (a)–(d): [N2222][Gly], [N2222][Gly]-5, [N2222][Gly]-10, [N2222][Gly]-15. (e)–(d): [N4444][Gly], [N4444][Gly]-5, [N4444] [Gly]-10, [N4444][Gly]-15. (i)–(l): [CH][Gly], [CH][Gly]-5, [CH][Gly]-10, [CH][Gly]-15. (m)–(p): [CH][Ala], [CH][Ala]-5, [CH][Ala]-10, [CH][Ala]-15. (q)–(t): [CH][Ser], [CH][Ser]-5, [CH][Ser]10, [CH][Ser]-15.

decreases gradually from left figure to right figure in each row. These results further prove lignin has the positive effect on the anti-wear properties of lubricants. The minimum film thickness Hc is an important comprehensive property index of lubricant physical property (such as viscosity) and operating conditions (such as applied load and sliding velocity) [67], which could be determined using the Hamrock–Dowson equation (Eq. (1)). [68] A larger H c could help to reduce metal/metal

contacts and promote anti-wear capability of the lubricants, and vice versa. Table 5 shows the calculated film thickness for amino acid based ILs under the mean sliding speed of 0.1 m/s (according to the 50 Hz reciprocating motion and amplitude of 1 mm). These results demonstrate the positive contribution of lignin and more effective lignin in enhancing lubricant film thickness. It is apparent that the largest lubricant film thickness was obtained at [N 4444 ] [Gly]-15, which also has the best anti-wear properties among the amino acid based ILs lubricants.
 H c ¼ 3:63Rx U 0:68 G0:49 W −0:073 1−e−0:68k

ð1Þ

where k is the ellipticity parameter, and U¼

Fig. 8. Ball wear diameter lubricated by different amino acid based lubricants. Load: 3.0 GPa, testing duration: 1 h, room temperature.

η0 U e FN ; G ¼ α film E0 ; W ¼ 0 2 : E 0 Rx E Rx

where Rx (unit: m) is the ball diameter, Ue (m/s) is the entrainment speed, η0 is the apparent viscosity of lubricant, E′ (GPa) is the effective Young's modulus, αfilm is pressure-viscosity coefficients, FN (N) is the applied load and e is 2.71828. The value or approximation of these parameters could be obtained in our previous papers [69,70]. The α film of similar ILs and ILs/lignin at 25 °C would not change a lot [47]. So the approximate α film of ILs and ILs/lignin is 8.0 GP−1. FN in this work is 258 N, Ue is 0.1 m/s, Rx is 0.005 m and E′ is 210 GPa.

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Fig. 9. The 3D microscopic images of ball wear tracks by using different lubricants. (a)–(d): [N2222][Gly], [N2222][Gly]-5, [N2222][Gly]-10, [N2222][Gly]-15. (e)–(d): [N4444][Gly], [N4444][Gly]5, [N4444][Gly]-10, [N4444][Gly]-15. (i)–(l): [CH][Gly], [CH][Gly]-5, [CH][Gly]-10, [CH][Gly]-15. (m)–(p): [CH][Ala], [CH][Ala]-5, [CH][Ala]-10, [CH][Ala]-15. (q)–(t): [CH][Ser], [CH][Ser]-5, [CH][Ser]-10, [CH][Ser]-15.

Root mean square (RMS) surface roughness of worn balls and disc are shown in Tables 6 and 7. It is well known that the friction coefficient and anti-wear properties increase as the rate of solid contacts increases due to greater surface roughness [71,72]. As shown in Tables 6 and 7, it is found that the addition of lignin could reduce the RMS value of the worn surface as compared with pure ILs, which will lead to the positive effect of lignin on the lubrication properties. With respect to [N2222] [Gly]. The lambda value is calculated (λ, Tallian parameter) according to the Eq. (2), which reveals the lubricating regime of contacts, depending on the velocities and roughness of the surfaces measured after testing. Boundary lubrication regime is indicated for lambda b1 and mixed lubrication for lambda between 1 and 3 [73]. The lubrication transfers from boundary regime to mixed regime, indicating that solid asperity

Table 5 Calculated film thickness for amino acid based ILs under the tested roughness and sliding speed of 0.1 m/s (nm).

contact is reduced with a lubricating film and to decrease the friction coefficient and wear. Hc λ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 Rball þ Rdisc 2

ð2Þ

Table 8 shows the λ for amino acid ILs/lignin under the tested roughness and sliding speeds of 0.1 m/s. It is evident from Table 8 that lignin could increase λ of amino acid ILs based lubricant. Most of λ is lower than 1, which shows the boundary lubrication regime. In the boundary regime, lubricant forms a coherent chemisorbed or physisorbed layer which could separate metal/metal contacts effectively and thus reduce friction and wear [74]. Interestingly, the lubrication regime of [N4444] [Gly]-15 is mixed lubrication regime (λ = 1.245), which reduce the

Table 6 Root mean square (RMS) surface roughness of worn balls (Rball) (unit: nm).

Lignin

[N2222][Gly]

[N4444][Gly]

[CH][Gly]

[CH][Ala]

[CH][Ser]

Lignin

[N2222][Gly]

[N4444][Gly]

[CH][Gly]

[CH][Ala]

[Ch][Ser]

0 5 10 15

12 18 22 31

27 38 74 189

11 25 28 38

16 32 38 78

24 66 92 96

0 5 10 15

294 323 245 418

236 221 214 95

439 403 332 109

398 389 362 273

402 382 225 107

56

L. Mu et al. / Journal of Molecular Liquids 280 (2019) 49–57

Table 7 RMS surface roughness of worn disc (Rdisc) (unit: nm). Lignin

[N2222][Gly]

[N4444][Gly]

[CH][Gly]

[CH][Ala]

[Ch][Ser]

0 5 10 15

316 456 267 715

267 264 257 119

739 502 363 154

763 415 388 358

1112 536 354 233

Table 8 Lambda value (λ) for amino acid ILs/lignin under the tested roughness and sliding speeds of 0.1 m/s. Lignin

[N2222][Gly]

[N4444][Gly]

[CH][Gly]

[CH][Ala]

[Ch][Ser]

0 5 10 15

0.027 0.033 0.062 0.037

0.075 0.110 0.221 1.245

0.013 0.039 0.057 0.203

0.019 0.056 0.072 0.174

0.020 0.100 0.219 0.376

potential for asperity contact more effectively and lead to the best antiwear properties. The enhanced lubrication property of the ILs/lignin system could be attributed to the adsorption ability of lubricant on the steel surface and the strength of the lubrication film between metal/metal contacts, in Fig. 10. Firstly, previous tribological study on amino acids and choline based ILs lubricants did not detect nitrogen element on the wear surface by XPS technique [18,75], which means that the tribochemical reaction of ILs with metal surface doesn't occur during the friction process. There are some positive charges on the metal surface during the friction process, due to the release of the low-energy electrons on metal surface from contact convex sites, which form the strong affinity with the negatively charged carboxylic acid group (physical adsorption) [75]. Glycine anion has been demonstrated having the strongest affinity to metal surface among three kinds of anions in ILs in previous literature report [65], which could prevent the direct metal/metal contacts more effectively and thus promotes interfacial lubrication. Secondly, dense H-bond would favour a stronger ILs/lignin interaction, and thus strengthen the lubrication film between metal/metal contacts. The less steric effect and more negative natural charges of amino acid anion synergistically contribute to the stronger H-bond interaction between ILs and lignin, and reduce friction and wear. 4. Conclusions To sum up, a series of novel green lubricants have been developed in this work using amino acid ILs as lubricant base and lignin as additive. The lignin in five kinds of amino acid based ILs has been demonstrated

as effective additive to reduce wear and to stabilize friction coefficient of developed green lubricants. Specifically, the wear volume of the disc by [N4444][Gly]-15 is only 12% of the one lubricated by pure [N4444][Gly]. One more hydroxyl group in the choline cation will provide more H-bond formation site and form stronger H-bond than ammonium based ILs/lignin. Density function theory calculation reveals that the preferable H-bond site in serine with lignin is identified based on the most negative natural charge. In addition, the ILs with the longer carbon length have better lubricating properties than the one with shorter alkyl chain length. The less steric effect and more negative natural charges of amino acid anion synergistically contribute to the stronger H-bond interaction between lignin and choline base ILs, which enhances lubrication film strength and thus improve the tribological property of ILs/lignin green lubricants. Acknowledgements Y.S. is grateful for financial support from Swedish Kempe Scholarship Project [grant numbers JCK-1507, SMK-1740], the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning [Formas, grant number 2016-01098], Swedish Energy Agency foundation [Energimyndigheten, grant number 2017-008200] and the support of the Swedish MISTRA foundation [grant number MI16.23]. J.Z. acknowledges the financial support from the American-Scandinavian Foundation, NSF [grant number CBET-1603264] and the American Chemical Society Petroleum Research Fund [grant number 55570-DNI10]. Partial support from the start-up fund of the University of Akron, the State Key Laboratory of Materials-Oriented Chemical Engineering [grant number KL1503] and National Natural Science Foundation of China [grant numbers 21676291, 21808102] are also acknowledged. References [1] M.-T. Wei, S. Elbaum-Garfinkle, A.S. Holehouse, C.C.-H. Chen, M. Feric, C.B. Arnold, R.D. Priestley, R.V. Pappu, C.P. Brangwynne, Nat. Chem. 9 (2017) 1118. [2] C. Wang, Z. He, X. Xie, X. Mai, Y. Li, T. Li, M. Zhao, C. Yan, H. Liu, E.K. Wujcik, Z. Guo, Macromol. Mater. Eng. 303 (2018) 1700462. [3] Y. Zheng, Y. Zheng, Z. Wang, Y. Cao, Q. Shao, Z. Guo, Green Chem. Lett. Rev. 11 (2018) 217. [4] J. Borges, J.F. Mano, Chem. Rev. 114 (2014) 8883. [5] M. Dong, Q. Li, H. Liu, C. Liu, E.K. Wujcik, Q. Shao, T. Ding, X. Mai, C. Shen, Z. Guo, Polymer 158 (2018) 381. [6] Y. Qian, Y. Yuan, H. Wang, H. Liu, J. Zhang, S. Shi, Z. Guo, N. Wang, J. Mater. Chem. A 6 (2018) 24676. [7] Z. Xiao, T. Duan, H. Chen, K. Sun, S. Lu, Sol. Energy Mater. Sol. Cells 182 (2018) 1. [8] C. Wang, B. Mo, Z. He, X. Xie, C.X. Zhao, L. Zhang, Q. Shao, X. Guo, E.K. Wujcik, Z. Guo, Polymer 138 (2018) 363. [9] C. Wang, B. Mo, Z. He, Q. Shao, D. Pan, E. Wujick, J. Guo, X. Xie, X. Xie, Z. Guo, J. Membr. Sci. 556 (2018) 118. [10] S.J. Grabowski, Chem. Rev. 111 (2011) 2597. [11] Y. He, B. Zhu, Y. Inoue, Prog. Polym. Sci. 29 (2004) 1021. [12] G.R. Desiraju, Acc. Chem. Res. 35 (2002) 565. [13] H. Szatyłowicz, J. Phys. Org. Chem. 21 (2008) 897.

Fig. 10. Schematic illustration of physical adsorption of ILs onto steel surface and reciprocal hydrogen bonding between lignin and ILs. (a): ILs; (b): ILs/lignin.

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