Tribological evaluation of environmentally friendly ionic liquids derived from renewable biomaterials

Unlike most of the conventional ionic liquids (ILs) derived from non-renewable resources, five environmentally friendly ILs ([Ch][AA] ILs) derived from amino acids (AAs) and choline (Ch) were synthesized using biomaterials by a simple, green route: acid–base reaction of Ch and AAs. The thermal and corrosion properties, as well as viscosity, of the prepared ILs were examined. The results revealed that the anion structure of ILs plays a dominant role in their thermal and viscosity behavior. These ILs exhibited less corrosion toward copper, related to their halogen-, sulfur-, and phosphorus-free characteristics. The tribological behavior of the synthesized ILs was examined using a Schwingungs Reibung und Verschleiss tester, and the results indicated that these ILs exhibit good friction-reducing and anti-wear properties as lubricants for steel/steel contact. Results from energy-dispersive spectroscopy and X-ray photoelectron spectroscopy indicated that the good tribological properties of [Ch][AA] ILs are related to the formation of a physically adsorbed film on the metal surface during friction.


Introduction
Currently, toxicity, safety, and environmental compatibility of lubricants are attracting increasing attention as almost half of the lubricants are estimated to eventually enter the environment by evaporation, leakages, or spills, leading to adverse negative effects on the environment and ecosystem [1,2]. Therefore, environmentally friendly lubricants have attracted extensive attention, and a considerable number of studies have been conducted in this field [3−8]. Generally, environmentally friendly lubricants are divided into two categories according to their raw material sources. The primary environmentally friendly lubricants are derived from nature resources and are utilized in the modern industry, e.g., natural vegetable oils [9], chemically modified vegetable oils [7,8], genetically modified vegetable oils, and synthetic lubricants derived from biomaterials [10−12]. On the other hand, petrochemical-derived lubricants with good biodegradability are considered as the second type of environmentally friendly lubricants, e.g., synthetic esters, polyether, and low-viscosity polyalphaolefins [1]. Among these environmentally friendly lubricants, synthetic lubricants derived from natural resources have attracted extensive scientific attention because of their flexible molecular structures, controllable performance, and renewable features, and recently, various biomaterials have been used as raw materials to develop environmentally benign lubricants [10−12].
Ionic liquids (ILs), which are solely composed of ions, are salts that are liquids with a melting point of around or less than 100 °C [13]. In the past two decades, ILs have attracted extensive attention in sustainable chemistry and industrial applications because of their low flammability, low vapor pressure, excellent thermal stability, and high ionic conductivity [14−16]. Moreover, ILs exhibit excellent tribological performance because of the tribochemical reaction between the frictional pairs. In 2001, Liu and coworkers were the first to report alkylimidazolium ILs as lubricants with excellent friction-reducing and anti-wear properties [17]. Since their report, ILs have been extensively investigated as lubricants and lubricant additives [18,19]. However, most of the conventional ILs reported typically contain tetrafluoroborate (BF 4  ), hexafluorophosphate (PF 6  ), and bis(trifluoromethylsulfonyl)imide (TFSI  ) anions. Halogen-containing ILs are more prone to reaction with water, generating haloid acid; haloid acid can cause severe metal corrosion and environmental pollution [20,21]; the same issues are also observed for sulfur-and phosphorus-containing ILs [22]. Therefore, in view of environmental protection, it is crucial to develop halogen-, sulfur-, and phosphorusfree ILs. Recently, some exploratory studies have been reported. Aspartic-acid-and glutamic-acid-derived ammonium-cation-based ILs have been reported as efficient anti-wear and friction-reducing additives for mineral base oil [23]. Song et al. have prepared tetrabutylammonium-based amino acid ILs and reported excellent tribological performance for steel/steel, steel/copper, and steel/aluminum contacts [24].
Choline (Ch), a water-soluble nutrient, is an important component of lecithin and sphingomyelin, and it is typically categorized as a B-complex vitamin. Ch is a promising candidate as the cation for ILs, and some choline-based ILs have been reported to exhibit low toxicity and facile biodegradability [25]. On the other hand, amino acids (AAs) are common cost-effective biomaterials, which are abundant in nature and readily available in bulk. [Ch][AA] ILs were prepared with Ch and AAs by a simple method. Different from conventional ILs containing halogen-, sulfur-, and phosphorus in their molecules, [Ch][AA] ILs have been reported to be environmentally friendly, sustainable, non-toxic, and biodegradable materials [26,27]. Several studies have reported the preparation of novel [Ch][AA] ILs, and their potential applications in various fields have been investigated [28,29]. Recently, Mu and coworkers have reported lignin- [Ch][AA] ILs as noncorrosive green lubricants [30]. Nevertheless, to the best of our knowledge, few systematic studies on the structure and properties of Ch-based ionic liquids as environmentally friendly lubricants have been reported.
In this study, five [Ch][AA] ILs were designed and synthesized with an identical cation (choline), but different anions (i.e., AAs), by a green route with only water as the by-product. The effects of anion symmetry and alkyl chain length on the viscosity, thermal stability, corrosion, and tribological properties of [Ch][AA] ILs were examined. In addition, the tribological mechanism of the green ILs was discussed on the basis of energydispersive spectroscopy (EDS) and XPS results.

Characterization of [Ch][AA] ILs
Before characterization, all of the [Ch][AA] ILs were dried under vacuum at 65 °C for 48 h. The structures of [Ch][AA] ILs were confirmed by 1 H NMR (400 MHz) spectroscopy. Differential scanning calorimetry (DSC 200F3, Netzsch) was employed to record the glass transition temperatures (T g ). First, samples were heated to 120 °C to evaporate the solvent from the ILs, cooled to 100 °C with liquid nitrogen, and finally heated to 100 °C at a rate of 10 °C ·min -1 . The decomposition temperatures (T d ) of the samples were recorded on an STA449F3 instrument (TGA, Netzsch) at a heating rate of 10 °C ·min -1 under nitrogen. An SYP1003-III kinematic viscosity tester was used to measure the kinematic viscosity of the prepared ILs at 40 and 100 °C .

Copper strip corrosion test
The copper strips used in this experiment were 10 mm in length, 10 mm in width, and 3 mm in thickness. Before corrosion tests, all of the copper strips were polished using an abrasive paper, followed by cleaning by ultrasonication in acetone for 10 min. The samples were immersed in the IL solutions and heated at 100 °C for 72 h. After the test, the copper strips were washed with acetone, and the corrosion level was confirmed according to the corrosion standard tint board.

Tribological tests
The friction and wear tests were carried out using an Optimol SRV-IV oscillating reciprocating friction and wear tester. The upper ball with a diameter of 10 mm was composed of AISI 52100 steel (hardness of approximately 59-61 HRC). The upper ball reciprocally slides against the lower stationary discs (Φ 24 mm  7.9 mm) at an amplitude of 1 mm. The lower stationary discs were composed of AISI 52100 steel, with a hardness of approximately 61-64 HRC. All tests were conducted at 20 °C for 30 min at a frequency of 25 Hz, a load of 100 N, and a relative humidity of 30%-40%. The volume loss of the lower disc was measured using a MicroXAM-3D non-contact surface mapping microscope profilometer. Scanning electron microscopy (SEM) and EDS analysis images were recorded on an SEM instrument (JSM-5600LV, JEOL). The chemical composition of the wear scars was confirmed by X-ray photoelectron spectroscopy (XPS), and XPS profiles were recorded on a PHI-5702 electron spectrometer (Perkin-Elmer, USA). Before SEM and XPS analysis, the lower discs were cleaned by ultrasonication in acetone for 20 min to remove the residual lubricants on the surface.

Thermal analysis
Figure 2(a) shows the DSC curves of the prepared ILs, and Table 1 summarizes the T g of the ILs. As can be observed in Fig. 2(a), all five ILs do not exhibit melting temperatures in the range of the measured temperature, and their T g values ranged between 62.2 and 48.2 °C .
[Ch][Gly] exhibited the lowest T g of 62.2 °C . With the increase in the anion size (from glycine, alanine,   [29,32]. The thermal decomposition properties of the ILs were examined by measuring the weight loss as a function of temperature. Figure 2(b) shows the results, and Table 1 summarizes the T d of ILs. From Fig. 2(

Viscosity
Viscosity is defined as the resistance of a fluid to flow, reflecting the manner in which molecules interact to resist motion. Viscosity affects the ability of a lubricant to form a lubricating film, which is considered as one of the most important properties of lubricants. Viscosity is closely related to the chemical structures of the lubricant, as well as their molecular size and shape.
To investigate the structure-viscosity relationship of the AA-Ch-based ILs, the kinematic viscosity of the synthesized ILs were measured at 40 and 100 °C .

Corrosion
The widely employed copper strip corrosion test is a straightforward method to measure the corrosion of lubricants. In this study, the copper strip corrosion test was carried out to investigate the corrosion properties of the synthesized [Ch][AA] ILs. Figure 3 shows the photographs of the copper strips after the test, and the test strips are compared with the standards to determine the corrosion level. From the copper strips tested with the green ILs, almost no corrosion was detected, with a corrosion grade of 1a or 1b in Fig. 3. The non-corrosive property of these ILs is possibly related to the absence of halogen, phosphorus, and sulfur in their molecules.

Tribological properties
The tribological properties of the synthesized ILs were tested using a Schwingungs Reibung und Verschleiss (SRV) tester. A conventional halogen-containing IL, e.g., [C 6 mim][NTf 2 ], was used for comparison. Figure 4 shows the evolution of the friction coefficient of these lubricants at room temperature. All lubricants exhibited a relatively short running-in time. Similar trends were observed between the wear scar diameter (WSD) of the ball lubricated with ILs and the wear volume of the discs ( Table 2). The anti-wear  property of [Ch][AA] ILs is thought to be related to the effective boundary films between friction pairs. To characterize the boundary films, in situ average contact resistance measurement was carried out using the SRV tester [36,37]. Figure 6 shows the contact resistance change using  Figure 7 shows the SEM micrographs and 3D optical microscopic images of steel discs lubricated with different ILs: All wear scars were obtained under the same conditions. As can be observed in the SEM micrographs, the worn surfaces lubricated by  (Fig. 9). The peaks of Fe2p were observed at approximately 725.1 and 711.2 eV (Fig. 9 [38]. The iron surface may easily undergo oxidation during sliding, with the formation of an oxide layer. A N1s absorption peak was not observed in the XPS spectrum, indicating that the excellent tribological properties of [Ch][AA] ILs may lead to the formation of a physically adsorbed film rather than the tribochemical reaction films on the friction pairs. During the tribological process, the carboxyl groups from AA anions are easily adsorbed on the positively charged metallic surface via electrostatic attractions, which effectively prevent the frictional pairs from direct contact [24,39]. This physically adsorbed film may have been thoroughly cleaned by ultrasonication, leading to the absence of N in the EDS and XPS images. The halogen-, sulfur-, and phosphorus-free ILs were more prone to form a physically adsorbed film on friction pairs. This result is in good agreement with those reported previously [23,24,30].

Conclusion
In this study, five environmentally friendly halogen-,