An Amino Acid Functionalized Ionic Liquid as A Multifunctional Lubricant Additive in Water-Glycerol

proton lubri-Abstract: Water-glycerol as one promising alternative for mineral oil can be applied as a green lubricant but has poor lubricity and strong corrosivity. It is desirable to design multifunctional water soluble lubricant additives. Protic ionic liquids (PILs) show considerable potential due to their facile preparation and environmental friendliness. Herein, an eco-friendly PIL ([osar][mea]) was facilely prepared from an amino acid derivate and investigated its anticorrosion and lubrication performances in the glycerol solution. Furthermore, the wear traces were measured using SEM/EDS and XPS for exploring the lubrication mechanism. The prepared PIL can rapidly increase the corrosion inhibition ability of water-glycerol as its concentration increases over CMC, and the anion playing a key role in the light of DFT calculations. Furthermore, [osar][mea] can greatly enhance the lubrication capability especially of water-glycerol while its concentration in the glycerol solution exceed 3%. The lubricity reduced with the increasing load. By the means of using SEM/EDS and XPS evaluation of the wear traces, we can speculate the possible lubrication mechanism may be the presence of the [osar][mea] adsorption film and the tribofilm containing complex nitrogen compounds.


Introduction
Traditional petroleum lubricants exhibit widespread applications in transportation vehicles, mining, electric power, metallurgy and other industrial fields and represent over 90 share of worldwide lubricant market 1 3 . They are derived from nonrenewable petroleum resources and display several drawbacks including high flammability, poor biodegradability and tendency to contaminate environment 4 6 .
Glycerol is an abundant green material obtained from biodiesel and can improve the fluidity in subzero and antimicrobial properties of water 7 9 . Besides, water-glycerol as an aqueous lubricant exhibits several advantages like environment-friendliness, fire-resistance and excellent cooling ability. With energy exhaustion and environment depravation, it has been one of the most promising alternatives and used in metal working and hydraulic transmission 10 12 . Nevertheless, water-glycerol s disadvantages of poor lubricity and high corrosivity impede their extensive applications 13,14 . Hence, there is a pressing need for green additives which can remarkably enhance anticorrosion and lubricating properties of water-glycerol.
Ionic liquids ILs exhibit enormous potential as lubricant additives for their unique physicochemical characteristics including high reactivity, sturdy surface adsorption and controllable structure 15 18 . Early studies focus on halogencontaining ILs which cause corrosion to metal in the presence of water and are only suitable for aqueous lubrication of non-metal materials 19 . Recently, a novel family of halogen-free ILs from amino acid, ricinoleic acid, organic phosphate and ibuprofen have been used as lubricant additives to improve tribology performances of aqueous lubricants 20 24 . However, there are still some problems such as complex preparation and environment compatibility.
In this study, an amino acid functionalized IL was prepared from a renewable biomaterial: N-oleoyl sarcosine. It belongs to protic ionic liquids PILs synthesized through proton transfer from a Brønsted acid to a Brønsted base and displays potential benefits including facile synthesis and environmental friendliness 25 . Current work focused on the applications of the PIL as a multipurpose additive for aqueous lubrication and probe its anticorrosion and lubri-

Analysis and tribological performance
The DMol3 module from Materials Studio software performs DFT calculations using CGA/BLYP functional with a DNP basis set to optimize the geometries of cation and anion of osar mea and obtain the energies of their orbitals involving the highest occupied molecular orbit HOMO and the lowest unoccupied molecular orbit LUMO .
The platinum ring detachment method was performed to determine the surface tension for different mass concentrations of osar mea in water-glycerol solutions using a Q100 Surface tension meter. The test was conducted three times at 25 , and the average value was calculated as the final result.
The based fluid was mixed with the osar mea at different mass fractions 0 , 0.5 , 1.0 , 3.0 and 6.0 to obtain the corresponding aqueous lubricants whose viscosities 40 are described in Table 1. The anticorrosion characteristics of the aqueous lubricants were explored using cast iron chips test referring to DIN 51360 Part 2 method 30 . The dry cast iron chips on round filter were wetted by the tested solution and storied at 25 for 12 h. The rust spots on the filter were observed as the test result.
The tribology behavior of the water lubricants was studied using an Optimol Schwingung Reibung Verschleiss SRV -V tester. The test specimens were a 10 mm diameter AISI52100 steel ball and a 24 mm diameter AISI52100 steel disc. The tests in this work were performed for 30 min at 25 and 125 N. The frequency and the amplitude was 50 Hz and 1 mm respectively. Tribology properties of 3.0 osar mea were further investigated under different loads 50 N, 100 N and 125 N . According to the literature 31 , all the friction tests are in the boundary lubrication regime. The wear volumes were determined by a non-contact 3D Optical Surface Profiler. The worn surfaces were assessed by a scanning electron microscopy with an energy dispersive X-ray spectroscopy SEM/EDS and an X-ray photoelectron spectrometry XPS .

Surface tension and corrosion inhibition
The surface tensions of water-glycerol with varying concentrations of osar mea are indicated in Fig. 3. The surface tension firstly decreases quickly and subsequently increase slowly with the rising concentration. The critical micelle concentration CMC of osar mea in water-glycerol can be speculated to approach 0.5 in that the surface tension falls to the valley of the curve as the concentration of osar mea reaches 0.5 32 .
The results of corrosion test are presented in Fig. 4. If only small amount of osar mea less than 0.5 is added to water-glycerol, the number and area of rust spots remains as large as water-glycerol. However the number and area of rust spots significantly decreases while the concentration of osar mea exceeds 0.5 . Rust spot is absent when the concentration of osar mea increases to 3 . The above results show the anticorrosion property of osar mea . According to the DFT calculations, the energetic values of the HOMO and LUMO E H and E L for the cation and anion were deduced and listed in Table 2. The difference between E L and E H is defined as ∆E. As shown in Table 2, the anion of osar mea display smaller E L and ∆E than contrasted with the cation. Referring to the previous paper 33 , the smaller E L and ∆E demonstrates the stronger adsorption and the higher inhibition efficiency. Accordingly, the anion of osar mea predominates the inhibition efficiency. The corrosion inhibition of osar mea may result from the adsorbed osar mea film hindering water from contacting the metal surface. In addition, multilayer molecules begin to adsorb onto the surface and form a thicker adsorption film providing more excellent corrosion protection as the additive concentration increases over CMC 34,35 .

Tribology properties
We studied the influence of the concentration of osar mea on the tribology properties of water-glycerol. Figure  5a displays the mean friction coefficients and wear volumes of the base fluid with osar mea at varying concentrations. Both the average friction coefficient and the wear volume significantly decreases after the blend of the glycerol solution with osar mea . They decrease by 36.3 and 81.6 respectively when the concentration of osar mea increases from 0 to 6 . However, there is no marked change in either the friction coefficient or the wear volume as the osar mea concentration exceeds 3 . Moreover, the wear volume remains invariable with the rising concentration of osar mea from 3 to 6 ,   maybe because the adsorption amount of osar mea on the contact surface has approaches or attains saturation when the concentration of osar mea reaches 3 . The change of friction behavior with time was indicated in Fig.  5b. At the initial period of about 200 s in the friction process, the friction coefficient of 0.5 osar mea stays at a low level of about 0.12 but quickly increases to as high level as that of the glycerol solution, because very low concentration of osar mea has difficulty in forming a stable lubricating film. Each of the glycerol solutions with osar mea undergoes a running-in stage. The running-in time increases by about three times when the concentration of osar mea increases from 0.5 to 6 . A high friction stage occurs in the running-in process when the concentration of osar mea exceeds 0.05 .
We further researched the tribological properties of 3 osar mea at different loads. Figure 6a presents the dependence of varying loads on the mean friction coefficients and wear volumes. With the increase of load from 50 N to 125 N, the average friction coefficient nearly remains unchangeable, but the wear volume increases by about two times. The possible reason was proposed as follows. The increasing load remarkably decreases the thickness of lubrication film but increases the direct contact of asperities 36 . Figure 6b presents the change of friction behavior with time at different loads. There is no obvious running-in stage in the friction process at 50 N. At higher load 100 N or 125 N , the running-in time increases to about 800 s, and there is a wide friction fluctuation at the initial stage. Evidently, it is unfavorable for osar mea as water based

Surface analyses
The geography and element construction of tribo-surfaces was measured using SEM/EDS. The SEM micrographs and EDS spectra are depicted in Fig. 7. At the high load 125 N , the base fluid causes a large wear scar, while the addition of 3 osar mea makes the wear scar smaller and narrower but there are still lots of wide cracks and deep grooves observed in the tribo-surface. As the load de-creases from 125 N to 50 N, 3 osar mea can afford better lubrication and result a smaller and smoother worn surface where there are only several slight and shallow furrows especially at 50 N. In the EDS spectra, nitrogen as a characteristic element of the studied PIL is significantly detected only after the addition of osar mea at the low load of 50 N, maybe because the lubricating film produced by osar mea is easily destroyed at the high load 100 N and 125 N . Furthermore the chemical construction of wear traces was evaluated by XPS and the spectrograms  are shown in the Fig. 8 the C1s peak at 284.8 eV as calibration . The C1s peaks at 284.8-285.2 eV and 288.6 eV are attributed to C C and C O respectively 37 . The O1s peak at 531.3 eV and the Fe2p peak at 710.4 eV represent the formation of iron oxide 38 . The N1s peak at 400.2 eV only observed in the presence of osar mea stands for the complex nitrogen compounds derived from osar mea and appears more obvious with the decreasing load. Besides, the complex nitrogen compounds are considered to play a key role in the lubrication on the basis of the previous studies 20, 24, 39 41 . According to above surface analyses, the potential lubrication mechanism was speculated as follows. The adsorption film formed by osar mea plays a key lubricating role at the initial friction stage, but it may be continually destroyed and reconstructed in the friction process. Meanwhile, the adsorbed osar mea molecules may triboreact with the metal surface to promote the growth of the tribofilm containing complex nitrogen compounds. The adsorption film and the tribofilm may contribute to the excellent lubricity.

Conclusions
In the present work, an ecofriendly PIL was facilely synthesized from an amino acid derivate and applied as a multipurpose additive in water-glycerol. It can yield a remarkable improvement in the anticorrosion performance of water-glycerol while its concentration increases over 0.5 . Especially as the concentration of osar mea reaches 3.0 , there is nearly no rust spot in the filter paper. DFT calculation deduced that the anion of osar mea play a decisive role in the corrosion inhibition efficiency. Meanwhile, it can significantly improve the lubricity of waterglycerol, and the optimal concentration is 3.0 . Each of the aqueous lubricants undergoes a running-in period which prolongs with the increasing concentration of osar mea , and there is a high friction stage in the running-in time when the concentration exceeds 0.5 . Further investigation on the change of tribology behavior with load reveals that the antiwear and friction reducing abilities reduce with the increasing load. Based on the SEM/EDS and XPS analyses of worn surfaces, the stable lubrication film consisted of the adsorbed osar mea film and the tribofilm containing complex nitrogen compounds.
cal performances of ionic liquids as additives for aqueous systems and molecular dynamics simulations. ACS Appl. Mater. Interfaces 12, 39910-39919 2020 . CC BY 4.0 Attribution 4.0 International . This license allows users to share and adapt an article, even commercially, as long as appropriate credit is given. That is, this license lets others copy, distribute, remix, and build upon the Article, even commercially, provided the original source and Authors are credited.