The study involved the preparation of mixtures of phytic acid ionic liquid and water, which were analyzed for their rheological, frictional, and electrochemical properties. The viscosity and friction characteristics of the mixtures are significantly affected by the water content. Mixtures with low water content (below 30%) result in low friction and negligible wear because of the generation of a close adsorption film and high viscosity. Especially, steel tribo-pairs lubricated with mixtures containing 25% water exhibited friction coefficients lower than 0.06. However, when the water content exceeds 35%, the friction coefficient increases dramatically, and the tribo-surface suffers from severe corrosive wear due to the predominance of the tribo-film in the frictional properties. These results were confirmed through XPS and electrochemical testing.
Research Article
Friction and corrosion properties of phytic acid ionic liquid-water mixtures
https://doi.org/10.21203/rs.3.rs-2989136/v1
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phytic acid
water
friction
electrochemical test
Industrialization has led to a global energy crisis that poses an ever-growing threat to the survival and development of humanity. Up until now, tribological contacts in mechanical systems have been responsible for almost 23% of the world's energy losses[1, 2]. Petroleum-based lubricants are the most commonly utilized liquid lubricants, and they can significantly enhance the tribological performance of moving components, resulting in decreased energy consumption[3]. As environmental concerns continue to rise, the utilization of lubricants derived from mineral oil has been progressively limited due to their inharmonious nature with the ecosystem[4]. There exists an urgent requirement for high-performance lubricants that are environmentally compatible.
Comprising solely of anions and cations, ionic liquids (ILs) are salts made up of organic compounds that melt at temperatures below 100 °C. Their structurally designable nature, high affinity, nonvolatility, and thermal stability make them a promising alternative to petroleum-based lubricants, attracting the interest of many scholars[5-7]. Notwithstanding, conventional ILs that incorporate halogens have the potential to discharge hazardous contaminants and present incongruity with the environment[8, 9]. Therefore, a succession of halogen-free ILs[3, 9-18], specifically those that are bio-based, have surfaced as sustainable lubricants in recent years. Song et al. developed five tetrabutylammonium-based amino acid ILs as lubricants for various surface contacts. Their findings revealed that these amino acid ILs outperformed both poly-alpha-olefin (PAO) and halogen-containing ILs ([C6mim][NTf2]) in terms of their lubrication capabilities[19]. Gusain and colleagues discovered that four tetrabutylammonium-fatty acid ILs, when used as lubricants, demonstrated notably reduced friction (18-50%) in comparison to polyol ester[20]. The preparation of a series of dicarboxylic acid ILs by Dong et al. revealed their enhanced lubricity in comparison to monocarboxylic acid ionic liquids, particularly in steel-copper and steel-aluminum contacts[21]. By utilizing different anions, namely octanoate, 2-ethylhexanoate, decanoate, and hexadecanoate, Fan et al. created several quaternary ammonium fatty acid ILs. These ILs have demonstrated potential as lubricants for distinct metal tribo-pairs[22]. The researchers Battez et al. produced three ILs with varying anions, specifically octanoate, laurate, and palmitate, and analyzed the impact of the carbon chain length on the ILs' characteristics, including biodegradability, toxicity, and lubricity. The findings indicated that these innovative ILs surpassed the halogen-containing ionic liquid ([N8881][TFSI])[23]. By employing distinct amines, Kreivaitis synthesized three oleic acid ILs as lubricants for steel-steel contact, and demonstrated their remarkable ability to yield extremely low friction coefficients[24]. The production of green lubricants for elevated-temperature use has been achieved by Reeves et al. through the combination of 1,3-diakylimidazolium and trihexyl(tetradecyl)phosphonium cations with carboxylate anions, namely salicylate, saccharinate, and benzoate[25]. Li and colleagues synthesized five choline amino acid-based ILs that exhibit comparable lubrication performance to PAO. Notably, the ILs containing long alkyl chains and guanidyl groups exhibit superior tribological properties[8].
The ILs mentioned above have their origins primarily in two types of biomaterials, namely fatty acids and amino acids. The viscosity of individual ILs is specific and cannot be adjusted to suit different lubricating environments. Therefore, due to its low viscosity and pressure-viscosity coefficients, water serves as a cost-effective and environmentally friendly material that can be incorporated into ionic liquids in order to adjust their friction and viscosity properties[26]. Phytic acid is a phosphorus-rich substance that is naturally present in various foods such as beans and corn. It serves as a fire-resistant and lubricating agent, and is also a sustainable and eco-friendly material[27, 28]. The present study involves the in-situ preparation of a phytic acid based ionic liquid in water. The objective is to manipulate the friction and viscosity of the phytic acid ionic liquid-water mixtures through varying the water content. Additionally, the corrosion properties of the mixtures will be investigated.
2.1 Chemicals
Phytic acid solution (70wt% in H2O) was procured from Shanghai Macklin Biochemical Co., Ltd. Triethanolamine (97wt%) was furnished by Sigma-Aldrich.
As per the research article[29], the formation of the ionic liquid occurred in the presence of water through the reaction between phytic acid and triethanolamine at a molar ratio of 1:12. The resulting phytic acid ionic liquid-water mixture was prepared with varying mass concentrations of water (10%, 15%, 20%, 25%, 35%, 40%, 45%, 50% and 70%) by adjusting the amount of water added.
2.2 Rheological measurement
An Anton Parr rheometer (MCR 302) was utilized to determine the rheological properties of the ionic liquid-water mixture. The temperature of the lubricant was maintained at 30℃ during the course of the experiments, while the shear rate was logarithmically incremented from 0.1 to 1000 s-1.
2.3 Friction test
The frictional properties of a range of ionic liquid-water blends were analyzed through experimentation using an Optimol SRV-V tester. The tribo-pairs were composed of two components: a ball with a diameter of 10 mm and a disc with a diameter of 24 mm and a thickness of 7.9 mm. Both components were constructed from AISI52100 steel, which possesses an elastic modulus of 208 GPa [30]. The surface roughness (R) of the ball and disc were measured to be 25 nm and 23 nm, respectively. Under the test conditions of 30 ℃ temperature, 5 N load, 25 Hz frequency, and 1 mm amplitude, the friction tests were conducted thrice. The quantification of wear volumes was conducted via a non-contact 3D Optical Surface Profiler. To evaluate the worn surfaces, a combination of techniques was employed, including scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS), Raman microscopy, and X-ray Photoelectron Spectroscopy (XPS).
2.4 Electrochemical test
Electrochemical analyses were conducted using a DONGHUA DH7000C electrochemical workstation to scrutinize the corrosion mechanisms. The experimental setup employed a traditional three-electrode system, consisting of a working electrode made of 20# carbon steel plate with a test area of 1 cm2, a reference electrode of saturated Ag/AgCl, and a counter electrode made of platinum sheet. A combination of ionic liquid and a 1 wt% NaCl solution was utilized as the electrolyte, with the latter being designated as a reference. Prior to the commencement of the test, the open circuit potential (OCP) was measured for 30 minutes to ensure a reliable outcome. The measurement of the potentiodynamic polarization curve was conducted in a cathode to anode direction, with a scanning rate of 0.100 mV s-1 and an E value of EOCP ± 250 mV. Electrochemical impedance spectroscopy (EIS) was performed with a sine signal amplitude of 5 mV, within a frequency range of 100 kHz to 0.1 Hz, to attain a comprehensive understanding of the system's electrochemical behavior.
3.1 Rheological properties
The viscosity trends of several combinations of ionic liquid and water as influenced by shear rate are graphically presented in Figure 1, while Table 1 provides the respective average viscosities (η). The mixture containing 10% water manifests notably higher viscosity than the other mixtures. The viscosity displays an initial exponential drop when the water content rises from 10% to 15%, and subsequently, it decreases gradually. Additionally, as the shear rate increases, the viscosity of the mixture containing 10% water undergoes a slight decrease, implying that this mixture displays shear thinning behavior of non-Newtonian fluids. Conversely, the other mixtures exhibit almost constant viscosities and behave as Newtonian fluids. The rheological outcomes indicate that the viscosity of the phytic ionic liquid-water mixture can be regulated by modifying the water content.
Table 1 The mean viscosities of various mixtures of phytic acid ionic liquid and water
|
Water content of the mixture / % |
10 |
15 |
20 |
25 |
30 |
35 |
40 |
45 |
50 |
70 |
|
η / Pa·s |
30.4 |
4.45 |
1.32 |
0.477 |
0.212 |
0.106 |
0.0587 |
0.0318 |
0.0188 |
0.00322 |
3.2 Friction properties
Table 2 The mean viscosities of various mixtures of phytic acid ionic liquid and water
The instantaneous friction coefficients of various phytic acid ionic liquid-water mixtures are depicted in Figure 2, and the corresponding average friction coefficients are enumerated in Table 3. With a water content of 10%, the average friction coefficient maintains a relatively high value of 0.208. An exponential decline in viscosity occurs as the water content rises from 10% to 15%, leading to a remarkable reduction in the mean friction coefficient by 38.9%. When the water content further increases to 25%, the average friction coefficient gradually decreases by 60.1%. Throughout the entire process of friction, it is evident that the instantaneous friction coefficient remains relatively stable when the water content falls within the range of 15% to 25%. However, there is a slight fluctuation in the instantaneous coefficient when the water content is at 10%. This can be attributed to the inconsistent viscosity of non-Newtonian fluids with shear thinning properties. Upon reaching a water content of 30%, there is a sudden increase in the average friction coefficient, returning to a higher value of 0.221. Following that, there is a nearly linear rise in the mean friction coefficient as the water content increases. Meanwhile, the instantaneous coefficient becomes more and more unstable because the lubrication regime approaches BL and even turns into BL. Concurrently, the instantaneous coefficient becomes increasingly unstable due to the lubrication regime approaching BL and even transitioning into BL. The fluctuation of the friction curve of water is most pronounced, and its average friction coefficient exceeds that of all mixtures by a significant margin.
Table 3 The average friction coefficients of various mixtures of phytic acid ionic liquid and water
|
Water content of the mixture / % |
10 |
15 |
20 |
25 |
30 |
35 |
40 |
45 |
50 |
70 |
100 |
|
Average friction coefficient |
0.208 |
0.127 |
0.104 |
0.083 |
0.221 |
0.250 |
0.278 |
0.309 |
0.340 |
0.370 |
0.448 |
In addition, an investigation was conducted to examine the friction characteristics of a solution composed of ionic liquid and water, with a water content of 25%, under varying loads. The instantaneous development of the friction coefficients over time are depicted in Figure 3 and the resulting average friction coefficients are listed in Table 4. The average friction coefficient undergoes an initial decrease to a value less than 0.06 as the load is increased to 10 N. Nevertheless, as the load further escalates to 50 N, the friction coefficient subsequently increases to 0.088. It is worth mentioning that as the load is elevated to 50 N, the friction coefficient undergoes a prolonged running-in time before it eventually stabilizes. This means that it takes a significant amount of time for the coefficient to reach a consistent state. Maybe because the adsorption layer consisting of the phytic acid ionic liquid predominates the properties of lubrication film. The aforementioned results imply that the variations in friction properties of different blends of phytic acid ionic liquid and water are in accordance with the classical stribeck curve, and the phytic acid ionic liquid exerts a positive influence on the reduction of friction.
Table 4 The average friction coefficients of the mixture containing 25% water under different load conditions
|
Load / N |
5 |
10 |
50 |
|
Average friction coefficient |
0.083 |
0.059 |
0.088 |
The wear volumes for diverse phytic acid ionic liquid-water blends are portrayed in Figure 4. In cases where the water content is less than 25% or equivalent to it, the wear scars are too slight for the profilometer to register the wear volume. At a water content of 30%, a discernible wear mark emerges, yet the wear volume experiences a reduction of 62.2% in comparison to that of pure water. This implies that the phytic acid ionic liquid still has a constructive effect on enhancing the antiwear property at this juncture. As the water content rises to 35%, the wear volume surpasses that of pure water by a small margin. Furthermore, as the water content continues to increase, the wear volume of the mixture experiences a rapid surge, eventually reaching a staggering 399% increase compared to pure water at a water content of 70%. It is unexpected that the wear and friction findings do not demonstrate absolute consistency.
3.3 Surface analysis
The wear scars produced from different combinations of phytic acid ionic liquid and water are depicted in Figure 5, including their 3D profiles and longitude depth curves. Clearly, a lower water content in the blend is responsible for producing a smaller wear scar and shallower depth than that observed with pure water. Notably, the presence of only 25% water leads to no discernible wear trace. High water content (45% and 70%) results in considerably greater wear scar and depth compared to pure water. The corresponding SEM photographs are shown in Figure 6. On a smaller observation scale, there is still no evidence of wear in the steel samples that were lubricated with the lubricant containing 25% water. As the water content continues to rise beyond 30%, the wear scar becomes more prominent, showing a multitude of deeper and wider furrows. Interestingly, the wear traces produced by the phytic acid ILs containing high water content (45% and 70%) are relatively larger when compared to pure water. Nevertheless, it is important to highlight that the wear trace resulting from pure water showcases visible pits and cracks, while the former does not exhibit any such anomalies, potentially due to the advantageous influence of phytic acid ionic liquid, which is high in phosphorus and impedes the initiation and propagation of micro-pitting.
The XPS spectra of the wear traces are depicted in Figure 7. It can be observed that the F2p and O1s peaks across all spectra exhibit remarkable similarity, suggesting that the F2p peaks at 710.1 eV and 723.1 eV may have arisen from the fusion of iron oxides with the O1s peaks at 530.6 eV and 531.8 eV[33]. Apart from pure water, the XPS spectra of wear scars lubricated with phytic acid ionic liquid-water mixtures exhibit comparable N1s peaks at 399.9 eV and P2p peaks at 133.4 eV. These peaks may indicate the presence of nitrogen oxide compounds and iron phosphate, respectively[34, 35]. It can be concluded from the aforementioned findings that a tribo-reaction takes place between the ionic liquid of phytic acid and the tribo-surface. Additionally, as the water content rises to a high level, the friction properties are primarily influenced by the tribo-film.
3.4 Electrochemical properties
The Tafel curves of the mixtures of ionic liquid and water were illustrated in Figure 8, and the corresponding electrochemical parameters are presented in Table 5. As a result of the introduction of phytic acid ionic liquid to water, the corrosion potential has been observed to shift negatively. This indicates that the ionic-water combination has a greater tendency towards corrosion than water alone from a thermodynamic perspective[18]. Despite the fact that the introduction of phytic ionic liquid to water causes a slight rise in the corrosion current density at first, the presence of ionic liquids with water content exceeding 35% only slightly speeds up the corrosion process. Nevertheless, as the water content further decreases, the corrosion current density experiences a significant decline. At a water content of 25%, the current density decreases exponentially and is lower than that of the blank, indicating that ionic liquids containing 25% and 15% water act as corrosion inhibitors[36]. 
The corrosion properties of different ionic liquid-water blends were further investigated through Electrochemical Impedance Spectroscopy (EIS) test. The Nyquist and Bode plots are illustrated in Figure 9. The Nyquist plot (Figure 9a) shows that the ionic liquids with 15%, 25%, and 35% water content have two prominent capacitive loops in the high- and low-frequency ranges, which correspond to the protective corrosion layer and the electric double layer[37], respectively. The ionic liquids that contain 15% and 25% water exhibit larger capacitive loops compared to the one with 35% water. Additionally, their high-frequency capacitive loops are more dominant, suggesting that their corrosion mechanisms are alike. The Bode plots illustrated in Figure 9b and Figure 9c reveal that the ionic liquids containing 15% and 25% water possess a higher phase angle at high frequencies and a greater impedance Z at low frequencies in comparison to the blank and other mixtures. This corresponds to the development of a more durable protective film[38]. Based on the electrochemical and friction tests, it has been established that the presence of water in the ionic liquid at levels above 35% can prompt a corrosive attack on the steel specimens, resulting in more significant wear than that caused by pure water.
3.5 Conclusion
This study involved the preparation of various mixtures of phytic acid ionic liquid and water, and an investigation into their rheological, frictional, and electrochemical properties. The viscosity and friction properties can be managed by altering the amount of water present. When the water content is low (below 30%), the friction coefficient is very low and wear is negligible due to the presence of an adsorption film and high viscosity. However, when the water content exceeded 35%, the lubricating properties were dominated by a tribo-film, resulting in an increase in the friction coefficient and corrosive wear. XPS analyses and electrochemical tests have provided supporting evidence for these findings.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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No competing interests reported.