Hydrogen Bond-Driven Tribological Properties of Natural Deep Eutectic Solvents for Green Lubrication

Hydrogen Bond-Driven Tribological Properties of Natural Deep Eutectic Solvents for Green Lubrication | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Hydrogen Bond-Driven Tribological Properties of Natural Deep Eutectic Solvents for Green Lubrication Lei Yuan, Zhaoyang Wang, Nan Kang, Mohamed EL Mansori, Meng Wang, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7635266/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Natural deep eutectic solvents (NADESs) have recently attracted considerable attention as sustainable lubricants due to their excellent tribological performance, biocompatibility, tunable composition, and low toxicity. Nevertheless, the relationship between their molecular structures and lubrication mechanisms remains insufficiently understood. In this work, five NADESs with distinct hydrogen-bonding strengths and hydrogen bond acceptor (HBA) structures were prepared using choline chloride, betaine, or L-carnitine combined with glycerol, urea, or malic acid. Their physicochemical and tribological properties were systematically evaluated. The results indicate that stronger hydrogen-bonding interactions restrict molecular mobility, thereby improving lubrication stability, reducing wear track dimensions, and generating smoother surface morphologies. Moreover, functional groups such as carboxyl moieties in HBAs can coordinate with metal surfaces, further enhancing anti-wear effects. Compared with the commercial lubricant PAO40, the optimized NADES achieved a 32.6% reduction in the friction coefficient and an 89.1% reduction in wear volume. These findings highlight the synergistic role of hydrogen bonding and HBA molecular design in determining lubrication behavior, and provide a theoretical basis for the rational design of high-performance, environmentally friendly lubricants based on NADESs. Deep eutectic solvents Hydrogen bonding Green lubricants Friction Tribo-chemical film Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 1. Introduction As pervasive phenomena, friction and wear arise at the interfaces of relatively moving surfaces in both industrial systems and everyday mechanical operations [ 1 – 4 ] . These interactions tend to result in low energy efficiency, increased maintenance costs, and economic burdens. Lubricants exert a crucial role in mitigating these issues by reducing friction and wear at the interface [ 5 – 7 ] . However, the production and application of most traditional lubricants are often plagued by drawbacks including high energy consumption, intricate processing, inadequate biodegradability, and the volatilization of toxic compounds. Over recent years, deep eutectic solvents (DESs) have emerged as a novel category of green solvents and functional fluids. Consisting of a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), DESs display melting points substantially lower than those of their individual components due to the formation of extensive hydrogen-bonding networks [ 8 – 12 ] . DESs possess a series of attractive properties such as facile and scalable synthesis [ 13 ] , low vapor pressure [ 14 ] , non-flammability, tunable polarity [ 15 ] , and excellent thermal [ 16 ] and chemical stability [ 17 ] . These attributes have facilitated their application in diverse fields, including electrochemistry [ 18 ] , catalysis [ 19 ] , biomass valorization [ 20 ] , and separation science [ 21 ] . In addition, DESs have also demonstrated great potential as sustainable lubricants. In 2010, Lawes et al. first proposed choline chloride(ChCl)-urea DES and ChCl-ethylene glycol DESs for surface contact lubrication of steel [ 22 ] , finding that their coefficient of friction (COF) was comparable to that of SAE 5W-30 engine oil. Subsequent to this pioneering study, researchers commenced exploring the tribological behavior of various DESs under different conditions. For example, Shi et al [ 23 ] . investigated the lubrication performance of ChCl–urea, ChCl-ethylene glycol, and ChCl–1,2-propanediol DESs for carbon fiber-filled PTFE composites. Their results demonstrated that DESs outperformed both water and hydraulic oil, reducing the COF and wear by approximately 60% and 50%, respectively, compared to dry friction. Hallett et al. [ 24 ] further showed that ChCl–ethylene glycol DES (1:2 molar ratio) could form a stable friction layer on mica surfaces, enhancing lubrication. Li et al. [ 25 ] investigated two hydrophobic DESs-tetrabutylammonium chloride–decanoic acid (C 4 -DES) and methyl trialkylmethylammonium chloride-decanoic acid(C 8 -DES)and found that compared to ester-based oils, these DESs significantly reduced COF (by 29% and 36%) and wear (by 91% and 94%, respectively). Mechanistic studies indicated that DESs exert their anti-friction effects primarily via polar group adsorption and the formation of ultrathin tribo-chemical films. Despite these promising advancement, tribological studies on DESs remain fragmented. Most studies to date have focused on one or two specific DES systems, with limited efforts devoted to systematically investigate how variations in HBA and HBD components modulate lubrication behavior. Furthermore, natural deep eutectic solvents (NADESs), a subclass of DESs derived from renewable and biocompatible sources such as sugars, amino acids, organic acids, and choline or betaine—have received limited attention in lubrication applications. Compared with synthetic DESs, NADESs offer enhanced environmental sustainability, low toxicity, and superior biodegradability, rendering them particularly appealing as green lubricants. As illustrated in Fig. 1 , this study provides the first systematic investigation of how hydrogen bond strength, regulated by different hydrogen bond donors (HBDs) and acceptors (HBAs), governs the tribological performance of natural deep eutectic solvents (NADESs). Five representative NADESs were synthesized, covering a broad spectrum of hydrogen-bonding interactions and physicochemical properties. Their viscosity, contact angle, and key thermodynamic parameters were comprehensively characterized, followed by tribological tests using a friction pair of 45# steel (AISI 1045) and GCr15 bearing steel (AISI 52100). By correlating molecular composition with friction-reducing and anti-wear properties, the results elucidate how hydrogen-bonding networks influence film formation, surface interactions, and lubrication mechanisms. This work not only provides a scientific basis for the rational design of high-performance, environmentally benign NADES-based lubricants but also opens new opportunities for extending their application in tribology and sustainable material engineering. 2. Materials and experimental methods 2.1 Materials Choline chloride, urea, betaine, malic acid, and anhydrous ethanol employed for cleaning were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Betaine and L-carnitine were purchased from Shanghai McLean Biochemical Technology Co., Ltd., with a purity greater than 99%. All reagents were used as received without further purification. A commercial base oil, specifically poly alpha olefin (PAO40; supplied by Tiancheng Meijia, China), was selected as a reference. Its frictional performance was comparatively analyzed against that of the DESs. 2.2 Synthesis of NADESs As shown in Table S1 , the corresponding masses of ChCl and glycerol were weighed in a molar ratio of 1:2 and introduced into a sealed glass container. The mixture was heated to 80 ˚C and stirred at 400 rpm for 1.5 hours until a homogeneous transparent liquid formed, denoted as NADES-1, NDES-2, NADES-3, NADES-4, NADES-5. For brevity, all DESs mentioned in the following text are replaced with abbreviations (Table S1 ). 2.3 Characterization of NADESs Thermogravimetric analysis (TGA) was carried out using a thermal analysis instrument (TG-DSC 3+, Mettler Toledo, Switzerland) with a temperature range of room temperature to 600 ˚C, a heating rate of 10 ˚C/min in a nitrogen atmosphere. Differential Scanning Calorimetry (DSC) testing was conducted using a thermal analysis instrument (TG-DSC 3+, Mettler Toledo, Switzerland) with a temperature range of -80 ˚C to 100 ˚C, a heating rate of 10 ˚C/min. Dissolve approximately 30 mg of the test substance in 1 mL of DMSO (99.9%) in a 5 mm diameter nuclear magnetic resonance tube. Nuclear magnetic resonance hydrogen spectra ( 1 H-NMR) were obtained on a Bruker Avance III 400 MHz spectrometer equipped with a 5 mm BBFOZ gradient intelligent probe. All 1H-NMR data were analyzed using MestReNova software (Mestrelab Research). Contact angles of the NADESs samples on 45# steel substrates were measured using the German KRUSS optical contact angle measuring instrument DSA30S. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FIR) analysis of the NADESs sample was performed using a Fourier transform infrared spectrometer (Spectrum 3, PerkinElmer, USA). The scanning range was set from 650 to 4000 cm − 1 , with a scanning rate of 20 scans per minute. The dynamic viscosities of the 5 NADESs were measured at 25°C using a rotational rheometer (MCR302, Anton Paar, measuring head model CP20/MRD). 2.4 Calculation of intermolecular interaction energy Quantum mechanical calculations were carried out to explore the intermolecular interactions. All calculations were completed using the Gaussian 09 software package. The M06-2x [ 26 ] functional and 6–31 + g(d) basis set were employed for structure optimization and single - point energy calculations. The Multiwfn 3.8 program was utilized to analyze the distribution of electrostatic potential (ESP) on the molecular surface. The interaction energy was calculated according to the following formula: where E int represents the interaction energy, A and B represent individual molecules, A-B represents the complex formed following the interaction between A and B, and E BSSE represents the basis set superposition error. 2.5 Stability testing To evaluate the corrosion behavior of the prepared NADESs, the 45# steel used in this experiment was immersed in NADESs for testing. The polished and cleaned 45# steel specimens were completely submerged in NADESs at room temperature (25°C) for 7 days, and photos were taken daily to record the surface changes of the 45# steel. Additionally, the FTIR spectra of the NADESs before and after friction were compared to investigate whether the hydrogen - bond interactions and components of NADESs changed after lubrication. 2.6 Friction Testing The friction performance of NADESs was evaluated using the rotary module of a ball disc friction and wear tester (UMT-2, Bruker, USA). The friction ball (upper sample) is made of GCr15 bearing steel (φ = 6.35 mm, Ra = 30 nm), while material of the friction disc (lower sample) is 45# steel (φ = 40 mm). Bearing steel and 45# steel are widely used in various mechanical parts [ 27 ] . The surface of 45# steel was polished with sandpaper of 100 mesh, 500 mesh, 1000 mesh, and 2000 mesh, respectively. After polishing, the disc was cleaned with ethanol, and its surface roughness was measured using a white light interferometer, with the roughness controlled it within 20 nm. Before testing, the steel balls and discs were cleaned with ethanol using ultrasonic waves. The test parameters were set as follows: the rotation radius of 4 mm, rotation speed of 60 r/min, and friction duration of 30 minutes under different loads, with the COF recorded throughout. The loads used in this study were 50 N (2.32 GPa), 100 N (2.93 GPa), and 150 N (3.35 GPa), respectively. The calculation formula for Hertz contact stress is where ρ is the comprehensive curvature radius and E is the comprehensive elastic modulus. The experimental temperature is 20°C and the relative humidity is 50%. Each experiment should be repeated at least 3 times to eliminate accidental errors and ensure the accuracy of experimental data. 2.7 Analysis of worn surfaces Upon completion of the friction tests, the width and depth of the wear marks were measured for 3 times using a white light interferometer (Verified XL, ZYGO, USA), and the wear volume and wear rate were subsequently calculated. To elucidate the lubrication mechanism of the NADESs, the morphology and chemical composition of the worn surface of the chassis after friction were analyzed. Firstly, images of the wear tracks were taken using an optical microscope (Smart Zoom 5, Zeiss, Germany). The elemental composition was then characterized using a Gemini300 thermal field emission scanning electron microscope. Then, XPS (K-Alpha, Thermo Fisher Scientific, USA) was used to analyze the chemical composition of the friction surface and the possible formation of friction film. 3. Results and discussion 3.1 Characterization of NADESs The formation of hydrogen bonds and the successful synthesis of the five NADESs were confirmed by FTIR spectroscopy, as evidenced by characteristic peak shifts associated with functional group interactions [ 28 ] . Figure 2 (a) shows the FTIR spectra of ChCl, glycerol and NADES-1 respectively. The characteristic peaks at 3286 cm⁻¹ and 1029 cm⁻¹ are attributed to the -OH and C-O groups of glycerol respectively. In the FTIR spectrum of NADES-1, we found that the hydroxyl group in glycerol shifted to a higher wavenumber, appearing at 3305 cm⁻¹. Also, the C-O peak of glycerol also exhibited a hypsochromic shift, moving from 1029 cm⁻¹ to 1036 cm⁻¹. These observations demonstrate that hydrogen bonds were formed between ChCl and glycerol, indicating the successful synthesis of NADES-1. Figure 2 (b) shows the FTIR spectra of ChCl, urea, and NADES-2 respectively. From the figure, the peaks at 1675 cm⁻¹ and 1589 cm⁻¹ in urea correspond to the C = O and -NH groups. After the formation of NADES, they shifted to different wavenumbers, moving to 1660 cm⁻¹ and 1605 cm⁻¹ respectively, which provides evidence for the formation of hydrogen bonds between ChCl and urea. Similarly, for NADES-3, the -OH and C = O peaks of malic acid shifted from 2873 cm⁻¹ and 1691 cm⁻¹ to 2915 cm⁻¹ and 1722 cm⁻¹ respectively in Fig. 2 (c). For NADES-4, the C = O peak of betaine and the -OH and C-O peaks of glycerol shifted from 1612 cm⁻¹, 3288 cm⁻¹, and 1029 cm⁻¹ to 1624 cm⁻¹, 3300 cm⁻¹, and 1038 cm⁻¹ respectively in Fig. 2 (d). In the case of NADES-5, the COO- peak of L-carnitine and the C-O peak of glycerol shifted from 1373 cm⁻¹ and 1029 cm⁻¹ to 1391 cm⁻¹ and 1039 cm⁻¹ respectively in Fig. 2 (e).Collectively, the observed shifts in these characteristic peaks across all five NADESs confirm the formation of hydrogen bond interactions between the respective HBA and HBD components, thereby validating the successful synthesis of the targeted NADESs. Figure S1 presents the ¹H-NMR spectra of the five synthesized NADESs along with their corresponding individual components. In the ¹H-NMR spectra, the formation of hydrogen bonds reduces the electron shielding of active hydrogen atoms, resulting in a downfield shift (higher chemical shift) in their resonance signals [ 29 ] . As shown in Figure S1 , the active hydrogen signals of the HBDs in all NADESs exhibit such downfield shifts due to hydrogen bond formation with the HBAs. The specific chemical shift values are summarized in Table S2. Based on the FTIR and ¹H-NMR characterizations, we also calculated the interaction energies ΔE of the five NADESs using quantum mechanical calculations (QC). As shown in Table 1 and Fig. S2, the interaction energies ΔE of the NADESs are 22.45, 22.95, 24.04, 23.24, and 25.71 kcal/mol respectively. It was noteworthy that the value of the interaction energy ΔE mentioned above is calculated based on NADESs with a molar ratio of 1:1, because NADESs with a molar ratio of 1:1 have a lower solidification point, rendering them unsuitable for lubrication applications. However, the above calculation results can still represent the relative size of NADESs with a molar ratio of 1:2. Therefore, when ChCl is used as the HBA, the hydrogen-bond strength follows the sequence: NADES-3 > NADES-2 > NADES-1. When glycerol acts used as the HBD, the hydrogen bond strength follows the order: NADES-5 > NADES-4 > NADES-1. These results are consistent with the trend observed in the ¹H-NMR spectra. Table 1 Physico-chemical properties of NADESs. Sample Interaction energies ΔE (kcal/mol) Melting temperature(°C) Decomposition temperature (°C) Viscosity (MPa·s) Contact angle on 45# steel (°) NADES-1 22.45 -77.6 263.4 296 85.2 ± 0.5 NADES-2 22.95 21.8 242.3 611 80.4 ± 0.3 NADES-3 24.04 -37.3 241.5 54619 45.2 ± 0.6 NADES-4 23.24 -71.1 266.2 1809 71.4 ± 1.6 NADES-5 25.71 -55.6 200.7 6445 69.9 ± 0.6 The melting temperature and decomposition temperature are key characteristics that define the temperature range within which NADESs remain in a liquid state [ 30 ] . The melting temperatures of the five NADESs were analyzed using differential scanning calorimetry (DSC). As shown in Table 1 and Fig. S3, the melting temperatures of NADES-1, NADES-2, NADES-3, NADES-4, and NADES-5 are − 77.6°C, 21.8°C, -37.3°C, -71.1°C, and − 55.6°C, respectively. When compared to the melting temperatures of the raw materials used for synthesis (glycerol: 17.8°C, ChCl: 302°C, urea: 133°C, malic acid: 101°C, betaine: 293°C, and L-carnitine: 200°C), the synthesized NADESs exhibit substantially lower melting temperatures, which is one of the most distinct features of NADESs. Additionally, we investigated the thermal stability of the five synthesized NADESs using thermogravimetric (TG) and differential thermogravimetric (DTG) analyses. The TG curves and corresponding DTG curves are shown in Fig. S4. The results indicate that all five NADESs undergo initial decomposition above 200°C and complete decomposition at approximately 300°C. Clearly, they all exhibit strong thermal stability, remaining stable under relatively high-temperature conditions. Both the DSC and TG characterizations confirm that NADESs possess a broad operating temperature range. Viscosity reflects the internal resistance of molecules during fluid flow, affecting their ability to form a lubricating film, and is the most important performance indicator of lubricating oils [ 31 ] . The dynamic viscosities of the five NADESs were measured using a rheometer. As shown in Fig. S5(a) and Table 1 , at a shear rate of 100 1/s, the viscosities are 0.296, 0.611, 54.619, 1.809, and 6.445 Pa·s, respectively. It can be observed that when the hydrogen bond acceptor (HBA) is the same (NADES-1, NADES-2, NADES-3), adjusting the hydrogen bond strength by modifying the hydrogen bond donor (HBD) leads to a positive correlation between viscosity and hydrogen bond strength in NADES-1, NADES-2, and NADES-3. Similarly, when the HBD is the same (NADES-1, NADES-4, NADES-5), the same pattern is observed. In addition, we also measured the viscosity of commercial lubricating oil PAO40, which was 0.5 Pa·s. The affinity of lubricants to sample surfaces is evaluated by their wettability, which also affects their lubrication performance [ 32 ] . To assess this, we measured the contact angles of the five NADESs on 45# steel at 25°C, as shown in Fig. S5(b) and Table 1 . The results indicate that the contact angles of NADES-1, NADES-2, NADES-3, NADES-4, NADES-5 and PAO40 are 85.2°, 80.4°, 45.2°, 71.4°, 69.9° and 30.2° respectively. For NADESs, the contact angle and viscosity of these NADESs do not correspond. Among NADES-1, NADES-2, and NADES-3, NADES-3 exhibited the smallest contact angle. Typically, liquids with higher viscosities tend to have larger contact angles, however, this is not the case here. As shown in the electrostatic diagram in Fig. S6, the red area represents positive electrostatic potential, while the blue area represents negative electrostatic potential. The smaller contact angle of NADES-3 may be due to the strong electronegativity of the carboxyl group, which promotes coordination with the metal surface. When comparing NADES-1 and NADES-2, it is evident that NADES-2 has a smaller contact angle, despite the lower electronegativity of urea compared to glycerol. This suggests that the amino group in urea is more likely to form coordination with metals, further reducing the contact angle. Additionally, in NADES-1, NADES-4, and NADES-5, the contact angle is in the following order: NADES-5 > NADES-4 > NADES-1. This may be attributed to the higher electronegativity of L-carnitine compared to betaine, and betaine’s higher electronegativity than ChCl. Moreover, the chloride ions in ChCl may be encapsulated by glycerol, limiting their interaction with the metal surface, resulting in a relatively large contact angle for NADES-1. In conclusion, both the electronegativity of the NADES components and their coordination strength with metals play a crucial role in determining the contact angle. 3.2. Stability of steel in NADESs To investigate the tribological properties of NADESs with varying hydrogen bond strengths, the five types of NADESs were divided into two groups based (Table S1 ) on variations in hydrogen bond donors (HBD) or hydrogen bond acceptors (HBA). The first group consists of NADES-1, NADES-2, and NADES-3, which share the same HBA but differ in HBD. The second group consists of NADES-1, NADES-4, and NADES-5, characterized by the same HBD but distinct HBAs. Figure 3 (a) shows a schematic diagram of the friction experiments using GCr15-45# steel friction pair. 3.3 Tribological test analysis To investigate the tribological properties of NADESs with different hydrogen bond strengths, five types of NADESs were divided into two groups based (Table S1 ) on the changes in hydrogen bond donors (HBD) or hydrogen bond acceptors (HBA). The first group consists of NADES-1, NADES-2, and NADES-3, sharing the same HBA but with different HBD. The second group consists of NADES-1, NADES-4, and NADES-5, with the same HBD but different HBAs. Figure 3 (a) shows a schematic diagram of friction experiments using GCr15-45# steel friction pair. As shown in Fig. 3 (c), when ChCl serves as the HBA, the friction coefficient of NADES-1 exhibits multiple significant fluctuations throughout the lubrication process, with numerous peaks and valleys evident in the friction curve, and an average friction coefficient of 0.1153. This indicates that NADES-1 exhibits lubrication instability throughout the entire friction stage. In contrast, NADES-2 displays relatively minor fluctuations, with no obvious peaks or valleys in its friction curve. Instead, it shows a stable decreasing trend, with an average friction coefficient of 0.0969. This improvement can be attributed to the higher hydrogen bond strength in NADES-2, which renders the intramolecular hydrogen bonds more stable and less prone to cleavage, thereby limiting molecular mobility. Compared to NADES-1, NADES-2 exhibits higher viscosity, enhanced stability, and a lower friction coefficient. In addition, as shown in Table 1 , the amino groups in urea can coordinate with metals, which strengthens the affinity of NADES-2 for metal surfaces and reduces friction. Although NADES-3 exhibits the strongest hydrogen bonding interactions and contains carboxyl groups that form strong coordination with metals, theoretically it should have a lower friction coefficient due to its smaller contact angle. However, the actual results are exactly the opposite. Specifically, the friction coefficient of NADES-3 is higher than those of NADES-1 and NADES-2, with an average of 0.1333. This phenomenon can be attributed to the exceptionally high viscosity of NADES-3, which slows mass transfer and hinders the recombination of hydrogen bonds, thereby resulting in higher friction. During the experiment, NADES-3 exhibited turbidity during friction, which dissipated upon standing. The infrared characterization in Fig. S8 verifies that the composition of NADES-3 remains unchanged before and after friction. Despite its high coefficient, NADES-3 demonstrates greater stability, with an almost linear friction curve. This may be due to its high viscosity and strong intermolecular hydrogen bonding interactions, which facilitate the maintenance of a stable equilibrium state even when the hydrogen bonds are cleaved. Therefore, it can be inferred that stronger hydrogen bonding interactions enhance the lubrication stability of NADES. In the second group of NADESs, there is no significant difference in the coefficient of friction (COF) among the three NADESs in Fig. 3 (d). However, the fluctuation of NADES-1 is still the largest, while NADES-4 shows good stability with an average friction coefficient of 0.125. NADES-5 maintains high stability throughout the entire friction process, with an average friction coefficient of 0.123. As the hydrogen bond strength increases (Fig. S2), the stability of NADES is enhanced. In addition, compared with ChCl in NADES-1, betaine in NADES-4 and L-carnitine in NADES-5 constrain molecular mobility after forming hydrogen bonds with glycerol. In contrast, the chloride ions in ChCl exhibit relatively high mobility, which accounts for the superior stability of NADES-4 and NADES-5. Furthermore, we also chose commercial lubricant PAO40 as a comparison. As shown in Fig. 3 , the average friction coefficient of PAO40 is 0.1437, which is considerably higher than that of NADESs. Compared with NADES-2, the friction coefficient of PAO40 is reduced by 32.6%, highlighting the excellent lubrication performance of NADESs. Figure 4 presents the optical micrographs of the wear tracks. Due to its strong hydrogen-bond interactions, high viscosity, and metal surface interaction, NADES-3 exhibits a distinct boundary after wear, with extremely faint wear tracks, indicating minimal wear. In the second group of NADESs, a clear trend is observed: as the hydrogen-bond strength increases, the boundaries of the wear tracks become increasingly distinct, smooth, and uniform. Among these, NADES-5 displays the most regular and distinct boundary, with the most uniform wear and the worn surface retaining its metallic luster. This phenomenon can be attributed to the strong hydrogen-bond strength of NADES-5, which enhances the restricted movement of molecules, leading to a stable and high-strength molecular adsorption film that effectively reduces wear. In comparison, PAO40, being non-polar, cannot form an anti-wear molecular layer. Its wear track has a rough and unclear boundary, with uneven wear. The wear volume is also a crucial indicator for evaluating the lubrication effect. 3D topography images of the wear tracks are shown in Fig. 5 (a), from which the width and depth of the wear track were obtained. As shown in Fig. 5 (b) and (c), among the NADESs in the first group, NADES-2 exhibits the largest wear dimensions (both in terms of width and depth). Its wear width is 440 µm, and the depth is 6.3 µm. This may be due to the presence of urea in NADES-2, which may break down the iron oxide protective layer, leading to greater wear. In contrast, NADES-3 shows the smallest wear dimensions, with a width of only 218 µm and a depth of 1.2 µm. This can be attributed to its strong hydrogen-bond strength, where the carboxyl group forms a robust coordination and reaction with iron, generating an iron carboxylate protective layer that effectively reduces wear under load. In the second group of NADESs, a comparison reveals that NADES-1, with its lower hydrogen-bond strength, experiences the most severe wear. As the hydrogen-bond strength increases, the wear dimensions decrease to varying degrees. Furthermore, the structural features of HBAs play a crucial role in wear morphology: longer chains like those in L-carnitine, provide elastic compressibility, thereby reducing wear depth. The carboxyl groups in both betaine and L-carnitine can form coordination bonds with the metal, contributing to a reduction in wear track dimensions. Compared with NADESs, the commercial lubricant PAO40 exhibits a significantly higher wear depth of 10.0 µm which is 88% higher than that of NADES-2, indicating the poor lubrication performance of PAO40. Figure 5 (d) shows the wear amount of the wear marks. Compared with PAO40, NADES-1, NADES-3, NADES-4, NADES-5, the wear volume of NADES decreased by 41.3%, 89.1%, 64.8% and 76.2% respectively, indicating that the anti-wear effect of NADESs is significant. Due to the presence of urea in its composition, NADES-2 damages the protective layer of iron oxide, resulting in relatively severe wear that is comparable to that of PAO40. The wear reduction behavior is related to the interactions between polar functional groups (hydroxyl (R-OH), amino (R-NH), etc.) on NADES and surfaces. According to the model proposed by Hardy and Doubleday [ 33 ] , polar molecules are adsorbed onto the surface to form a closely - packed, vertically - oriented monolayer. In contrast, PAO40 lacks such polar effects, resulting in a significantly larger wear volume compared to the NADESs. To investigate the load-bearing capacity of NADESs, their further examined their friction performance under 100 N and 150 N loads were further examined. Figure 6 (a) and (b) show the friction coefficient of NADESs at 100 N. When the load increases to 100 N, the friction coefficient of NADESs remains almost unchanged compared to those under 50 N. As shown in Fig. 6 (a), the average friction coefficients of NADES-1 and NADES-2 are 0.1150 and 0.1094, respectively, with significant fluctuations. But with the enhancement of hydrogen bonding, the friction coefficient curve of NADES-3 shows stability consistent with 50 N, with an average friction coefficient of 0.1350. Figure 6 (b) shows the friction coefficient of the second group of NADESs under 100 N, with the average friction coefficients of NADES-4 and NADES-5 being 0.1201 and 0.1257, respectively. The friction coefficient curve of NADES-5 is also a straight line with almost no fluctuations, while NADES-4 and NADES-1 have significant fluctuations. The fluctuation of commercial lubricating oil PAO40 is the largest, with an average friction coefficient of 0.1335, indicating that the lubrication stability of PAO40 is significantly inferior to that of the NADESs. As shown in Fig. 6 (c) and (d), when the load increases to 150 N, the average friction coefficient of NADESs remains stable and almost unchanged. The friction curves of NADES-3 and NADES-5 remain stable due to their high hydrogen bond strength, with average friction coefficients of 0.1301 and 0.1233, respectively. The friction curve of NADES-1 shows multiple sudden increases in friction coefficient, with an average friction coefficient of 0.105. This may be attributed to the weakest hydrogen bond strength and insufficient load bearing capacity, as hydrogen bonds are constantly breaking and reformation due to incomplete destruction. The NADES-2 exhibits the lowest friction coefficient, at 0.0984. The average friction coefficient of NADES-4 is slightly lower than that of NADES-5, at 0.12. Commercial lubricants exhibit the worst lubrication effect, with an average friction coefficient of 0.128. As shown in Fig. 7 , with increasing the load, the friction coefficient of NADESs remained nearly stable at higher loads (100 N and 150 N). The load dependent pattern of the COF may not be typical. However, this phenomenon has been observed in numerous studies on dry and lubricated sliding contacts of various bulk materials [ 34 ] . In conclusion, the friction coefficients of the two NADESs remained relatively stable throughout the entire test process and did not exhibit a sudden increase in friction coefficient, indicating that NADESs have excellent load-bearing capacity. Figure 8 compares the wear dimensions and wear volume of NADES and PAO40 under loads of 100 N and 150 N. The corresponding optical photos and 3D morphology images are shown in Fig. S9 and S10. In the second group of NADESs, the long chains in L-carnitine provide elastic compressibility, resulting in its minimum depth. Compared with PAO40, the depth of wear marks after NADESs lubrication decreased significantly, which is speculated to be related to the adsorption lubrication film formed by polar molecules in NADESs, effectively reducing vertical wear. Unlike the changing trend of COF, the wear volume of NADESs increases with the increase of load, consistent with the trend predicted by the classical Archard wear law. Compared with commercial lubricant PAO40, NADESs exhibit excellent wear resistance, with NADES-3 showing the best performance. Specifically, compared with PAO40, the wear volume was reduced by 87.6% and 74.3% under loads of 100 N and 150 N, respectively. 4. Analysis of wear surface and lubricating mechanism To gain a deeper understanding of the lubrication mechanism of NADESs, the surface composition and chemical states of the wear tracks were analyzed using energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Prior to the surface analysis, the samples were cleaned with alcohol and dried. The friction chassis, made of 45# steel, is primarily composed of Fe, C, O, Cr, Si, and Mn. Figure 9 shows the scanning electron microscopy images. The wear surface lubricated with PAO40 shows severe abrasive wear, characterized by numerous abrasive wear grooves and rough indistinct boundaries. On the contrary, the wear track surface lubricated by NADESs is smooth and flat, with clear wear boundaries and significantly reduced wear, indicating favorable lubrication effect. To analyze the lubrication mechanism, the element mapping of wear marks was performed. As shown in Fig. 9 , the N element from NADESs was detected on the worn surface, indicating the formation of an adsorption film on the worn surface, while PAO40 failed to form such an adsorption film. These findings explain why, NADESs exhibit superior lubrication performance compared with the commercial lubricant PAO40. As shown in Table S3, compared to the non-friction chassis, an increase in both O and C contents was detected on the wear track surface of the samples lubricated with NADESs. This indicates that the NADESs adhere onto the 45# steel surface or form iron oxide or iron hydroxide in interaction with the steel. The increase in oxygen content is most pronounced in NADES-2. This may be due to the urea in NADES-2 breaking down the oxide protective layer on the metal surface, thereby exposing more underlying metal. In the ambient atmosphere rich in oxygen and moisture, a significant amount of iron oxide is formed. To further investigate the composition of the friction film, XPS analysis was performed. Figure 10 shows the XPS spectrum of the worn area after lubrication with NADESs. Due to the low friction coefficient of NADES-2, as shown in Fig. 9 , we focused on analyzing the XPS spectra of its worn surfaces C1s, O1s, Fe2p, and N1s.In Fig. 10 (a), the C1s spectrum of the worn surface contains four peaks, namely 283.33, 284.80, 286.66, and 288.63 eV, corresponding to C-Si, C-C/C-H, C-OH, and O-C = O groups [ 35 – 38 ] , respectively. Figure 10 (b) are that the peak values of O1s at 530.04, 531.76 and 533.7 eV belong to iron oxide,-OH and O = C-O/-CO- [ 39 – 44 ] .Among them, C-O and C = O may be due to the process of frictional oxidation reaction. This indicates that the hydroxyl groups in ChCl can adsorb onto metal surfaces through polar interactions, forming O-Fe bonds. The binding energy peaks of Fe2p near 707.23, 710.22, 711.76, and 715 eV in Fig. 10 (d) should be attributed to Fe/Fe 3 O 4 , FeCl 3 /Fe 3 O 4 /Fe 2 O 3 , FeOOH, Fe 2+ /FeO and FeCl 3 [ 45 – 47 ] , respectively. According to Fig. 10 (c), the N1s spectrum was observed at approximately 398.66, 399.65, 402 eV corresponding to -NH 2 , C-N-C [ 48 ] , N-O. These peaks are attributed to metal nitrides, indicating that ChCl-urea NADES participates in frictional chemical reactions. The spectra of the elements C, O, N, and Fe suggest that the amino group in NADES-2 undergoes chemical interactions with the worn surface via chemical adsorption, forming iron (hydrogen) oxides and nitrides with NADES during friction. The oxygen and moisture in the air also facilitate the formation of this chemical reaction layer. So, we speculate that the friction layer of NADES-2 is primarily composed of the two components: an adsorption layer formed by polar molecules at the friction interface (vertically rigid yet horizontally shearable), and a chemical reaction layer consisting of iron oxides and hydroxides. The combined effect of these layers results in a lower friction coefficient for NADES-2 compared to the other NADESs. As shown in Fig. S11, the characteristic peaks of C1s, O1s, and Fe2p in the spectra of other NADESs are similar, with their corresponding main components also including iron oxides and iron hydroxide. Based on the friction and wear results and the surface analysis findings, a lubrication mechanism for the studied NADESs is proposed. As illustrated in Fig. 11 , the friction layer lubricated by NADESs consists of two distinct layers. The bottom layer is a chemical reaction film composed of iron oxides and hydroxides, while the upper layer is a polar molecule adsorption layer formed by the polar groups of NADESs (e.g., hydroxyl groups, amino groups (R-OH, R-NH₂)). The excellent anti-friction performance of NADESs is attributed to the adsorption of these polar molecules, which form a vertically rigid yet horizontally easily shearable monolayer at the friction interface. Observation of the wear track images and wear volume data reveal that hydrogen bond strength influences the integrity of the polar molecule layer. The stronger the hydrogen bond strength, the more restricted the molecular movement within the system, resulting in a higher rigidity of the polar molecule layer in the vertical direction and a better anti-wear effect. Compared to PAO40 lubrication, this polar adsorption layer plays a crucial role in reducing wear. Furthermore, during the friction process, oxygen and moisture in the air, together with hydroxides and oxides, contribute to the formation of a chemical reaction film, further reducing the friction and wear. 5. Conclusion In this study, five natural deep eutectic solvents (NADESs) with varying hydrogen-bonding strengths and distinct hydrogen bond acceptor (HBA) structures were designed and systematically evaluated for their lubrication performance. The results highlight that both hydrogen-bonding interactions and HBA molecular architecture critically govern the tribological behavior of NADESs. Stronger hydrogen bonds restricted molecular mobility, stabilized lubricating films, and reduced friction fluctuations and wear. Structural features of HBAs further influenced wear morphology, where long-chain structures such as L-carnitine provided elastic compressibility, and polar groups such as carboxyl moieties enhanced metal coordination and anti-wear effects. 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07:17:21","extension":"png","order_by":23,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":846687,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/5fdea1fe978ebdc4aae569e8.png"},{"id":96244271,"identity":"83862e00-066c-49b2-aa27-3c2672fb46ab","added_by":"auto","created_at":"2025-11-19 07:18:02","extension":"png","order_by":24,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":2340844,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/8f860172050f652ba6d7ca0e.png"},{"id":95932138,"identity":"9cfcc355-e5f5-44dc-9225-05a57833f9ff","added_by":"auto","created_at":"2025-11-14 14:43:50","extension":"png","order_by":25,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":5144209,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/69da6cbbb46758d08b62db55.png"},{"id":96244912,"identity":"6a548a37-491b-4ea0-8935-af3ea86deb55","added_by":"auto","created_at":"2025-11-19 07:19:31","extension":"xml","order_by":26,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":114052,"visible":true,"origin":"","legend":"","description":"","filename":"6f427e0625c74e31a12955306739a8411structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/35e85736337c1893f4fe6d31.xml"},{"id":96243678,"identity":"edb1af4f-2864-48ba-b416-3b608735d67c","added_by":"auto","created_at":"2025-11-19 07:16:50","extension":"html","order_by":27,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":122916,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/c43b38515b7aff355a3282a0.html"},{"id":96243393,"identity":"ed212b67-281e-4e7e-af48-5ad12fdcde67","added_by":"auto","created_at":"2025-11-19 07:16:15","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":7610171,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Characteristics of NADES. (b) The NADESs studied in this study include NADES-1 (ChCl-glycerol), NADES-2(ChCl-urea), NADES-3(ChCl-malic acid), NADES-4 (betaine, glycerol), NADES-5(L-carnitine-glycerol).\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/24df8908c83d737f84b153c2.png"},{"id":95932103,"identity":"9fca6a85-9f46-4ec9-bb9f-73f7bca8df33","added_by":"auto","created_at":"2025-11-14 14:43:50","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1491205,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR spectra of (a) ChCl, glycerol, NADES-1 and (b) ChCl, urea, NADES-2 and (c) ChCl, malic acid, NADES-3 and (d) betaine, glycerol, NADES-4; (e) L-carnitine, glycerol, NADES-5.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/0498b7323dace5b12faeba99.png"},{"id":95932105,"identity":"f359bbe1-07da-4ba5-aa05-f94bb7f877ad","added_by":"auto","created_at":"2025-11-14 14:43:50","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":8705090,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Sketch of the system of tribological tests and (b) average COF and (c, d) COF with the load of 50 N.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/8cc7ff88dcc1afd4f4c1b41f.png"},{"id":95932107,"identity":"769defee-78e4-45ef-8a8b-d1e780c80b5a","added_by":"auto","created_at":"2025-11-14 14:43:50","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":5855448,"visible":true,"origin":"","legend":"\u003cp\u003eOptical micrograph of wear tracks under 50 N.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/c6760fa9447c09ad2b836362.png"},{"id":96243236,"identity":"683a032c-38ee-4883-aaa0-80a03cadfafe","added_by":"auto","created_at":"2025-11-19 07:15:53","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":5169460,"visible":true,"origin":"","legend":"\u003cp\u003e(a) 3D morphology, (b) Cross section profiles, (c) size of wear track, (d) wear volume and wear rate of NADESs and PAO40 under 50 N.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/362a81d2adddead9b13ab387.png"},{"id":96244313,"identity":"01d512b1-4b17-4f6a-8a0e-0ca561532545","added_by":"auto","created_at":"2025-11-19 07:18:06","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":6571277,"visible":true,"origin":"","legend":"\u003cp\u003eFriction coefficient with the load of 100 N and 150 N.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/c89801b240eeafadebb01c4c.png"},{"id":96243608,"identity":"626933d8-97f9-42f7-b407-9dc0ebd99ce2","added_by":"auto","created_at":"2025-11-19 07:16:43","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":2963450,"visible":true,"origin":"","legend":"\u003cp\u003eAverage COF with the load of 100 N and 150 N.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/e58afc9178f8779399f36184.png"},{"id":95932118,"identity":"8053ecff-21d1-4b1c-ab12-2a55f9e23a45","added_by":"auto","created_at":"2025-11-14 14:43:50","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":12535331,"visible":true,"origin":"","legend":"\u003cp\u003eCross section profiles, size of wear track and wear volume under (a, b, c) 100 N and (d, e, f) 150 N.\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/ce1af2175229d33052154945.png"},{"id":95932114,"identity":"6a7ff8e6-1aaa-4901-8039-0268d68e4cd7","added_by":"auto","created_at":"2025-11-14 14:43:50","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":9214285,"visible":true,"origin":"","legend":"\u003cp\u003eEDS images of surface scratches on 45-steel (a) NADES-1, (b) NADES-2, (c) NADES-3, (d) NADES-4, (e) NADES-5 and (f) PAO40 under 50 N.\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/cdc65ec77e4d9949590a7d81.png"},{"id":95932109,"identity":"16fd843a-28b6-4728-9faf-cee8d5acba53","added_by":"auto","created_at":"2025-11-14 14:43:50","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":2117838,"visible":true,"origin":"","legend":"\u003cp\u003eXPS spectra of worn surfaces (a) C1s, (b) O1s, (c) N1s and (d) Fe2p under the action of NADES-2.\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/094c70314c6986b661030d2d.png"},{"id":96244085,"identity":"808a9867-23a1-4dac-b675-34d47008ccaf","added_by":"auto","created_at":"2025-11-19 07:17:41","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":6758006,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of NADES frictional mechanism\u003c/p\u003e","description":"","filename":"floatimage11.png","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/8e8fdd865fa0fe7537c7a3a3.png"},{"id":96916448,"identity":"bad48f78-10f7-4b43-8439-c1ce8baed0cc","added_by":"auto","created_at":"2025-11-27 14:08:36","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":69710140,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/3be7652c-3714-433a-af90-c62a128613ce.pdf"},{"id":95932134,"identity":"e28cd6d5-d092-486c-b5ee-942babfafd9a","added_by":"auto","created_at":"2025-11-14 14:43:50","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":19414177,"visible":true,"origin":"","legend":"","description":"","filename":"Supportinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-7635266/v1/a2874a58ba32bf835ec7c982.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Hydrogen Bond-Driven Tribological Properties of Natural Deep Eutectic Solvents for Green Lubrication","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAs pervasive phenomena, friction and wear arise at the interfaces of relatively moving surfaces in both industrial systems and everyday mechanical operations\u003csup\u003e[\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e. These interactions tend to result in low energy efficiency, increased maintenance costs, and economic burdens. Lubricants exert a crucial role in mitigating these issues by reducing friction and wear at the interface\u003csup\u003e[\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e. However, the production and application of most traditional lubricants are often plagued by drawbacks including high energy consumption, intricate processing, inadequate biodegradability, and the volatilization of toxic compounds. Over recent years, deep eutectic solvents (DESs) have emerged as a novel category of green solvents and functional fluids. Consisting of a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), DESs display melting points substantially lower than those of their individual components due to the formation of extensive hydrogen-bonding networks \u003csup\u003e[\u003cspan additionalcitationids=\"CR9 CR10 CR11\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e. DESs possess a series of attractive properties such as facile and scalable synthesis\u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e, low vapor pressure\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e, non-flammability, tunable polarity\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e, and excellent thermal\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e and chemical stability\u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e. These attributes have facilitated their application in diverse fields, including electrochemistry\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e, catalysis\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e, biomass valorization\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e, and separation science\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eIn addition, DESs have also demonstrated great potential as sustainable lubricants. In 2010, Lawes et al. first proposed choline chloride(ChCl)-urea DES and ChCl-ethylene glycol DESs for surface contact lubrication of steel\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e, finding that their coefficient of friction (COF) was comparable to that of SAE 5W-30 engine oil. Subsequent to this pioneering study, researchers commenced exploring the tribological behavior of various DESs under different conditions. For example, Shi et al\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e. investigated the lubrication performance of ChCl\u0026ndash;urea, ChCl-ethylene glycol, and ChCl\u0026ndash;1,2-propanediol DESs for carbon fiber-filled PTFE composites. Their results demonstrated that DESs outperformed both water and hydraulic oil, reducing the COF and wear by approximately 60% and 50%, respectively, compared to dry friction. Hallett et al.\u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e further showed that ChCl\u0026ndash;ethylene glycol DES (1:2 molar ratio) could form a stable friction layer on mica surfaces, enhancing lubrication. Li et al. \u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e investigated two hydrophobic DESs-tetrabutylammonium chloride\u0026ndash;decanoic acid (C\u003csub\u003e4\u003c/sub\u003e-DES) and methyl trialkylmethylammonium chloride-decanoic acid(C\u003csub\u003e8\u003c/sub\u003e-DES)and found that compared to ester-based oils, these DESs significantly reduced COF (by 29% and 36%) and wear (by 91% and 94%, respectively). Mechanistic studies indicated that DESs exert their anti-friction effects primarily via polar group adsorption and the formation of ultrathin tribo-chemical films. Despite these promising advancement, tribological studies on DESs remain fragmented. Most studies to date have focused on one or two specific DES systems, with limited efforts devoted to systematically investigate how variations in HBA and HBD components modulate lubrication behavior. Furthermore, natural deep eutectic solvents (NADESs), a subclass of DESs derived from renewable and biocompatible sources such as sugars, amino acids, organic acids, and choline or betaine\u0026mdash;have received limited attention in lubrication applications. Compared with synthetic DESs, NADESs offer enhanced environmental sustainability, low toxicity, and superior biodegradability, rendering them particularly appealing as green lubricants.\u003c/p\u003e\u003cp\u003eAs illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, this study provides the first systematic investigation of how hydrogen bond strength, regulated by different hydrogen bond donors (HBDs) and acceptors (HBAs), governs the tribological performance of natural deep eutectic solvents (NADESs). Five representative NADESs were synthesized, covering a broad spectrum of hydrogen-bonding interactions and physicochemical properties. Their viscosity, contact angle, and key thermodynamic parameters were comprehensively characterized, followed by tribological tests using a friction pair of 45# steel (AISI 1045) and GCr15 bearing steel (AISI 52100). By correlating molecular composition with friction-reducing and anti-wear properties, the results elucidate how hydrogen-bonding networks influence film formation, surface interactions, and lubrication mechanisms. This work not only provides a scientific basis for the rational design of high-performance, environmentally benign NADES-based lubricants but also opens new opportunities for extending their application in tribology and sustainable material engineering.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"2. Materials and experimental methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Materials\u003c/h2\u003e\u003cp\u003eCholine chloride, urea, betaine, malic acid, and anhydrous ethanol employed for cleaning were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Betaine and L-carnitine were purchased from Shanghai McLean Biochemical Technology Co., Ltd., with a purity greater than 99%. All reagents were used as received without further purification. A commercial base oil, specifically poly alpha olefin (PAO40; supplied by Tiancheng Meijia, China), was selected as a reference. Its frictional performance was comparatively analyzed against that of the DESs.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Synthesis of NADESs\u003c/h2\u003e\u003cp\u003eAs shown in Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, the corresponding masses of ChCl and glycerol were weighed in a molar ratio of 1:2 and introduced into a sealed glass container. The mixture was heated to 80 ˚C and stirred at 400 rpm for 1.5 hours until a homogeneous transparent liquid formed, denoted as NADES-1, NDES-2, NADES-3, NADES-4, NADES-5. For brevity, all DESs mentioned in the following text are replaced with abbreviations (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Characterization of NADESs\u003c/h2\u003e\u003cp\u003eThermogravimetric analysis (TGA) was carried out using a thermal analysis instrument (TG-DSC 3+, Mettler Toledo, Switzerland) with a temperature range of room temperature to 600 ˚C, a heating rate of 10 ˚C/min in a nitrogen atmosphere. Differential Scanning Calorimetry (DSC) testing was conducted using a thermal analysis instrument (TG-DSC 3+, Mettler Toledo, Switzerland) with a temperature range of -80 ˚C to 100 ˚C, a heating rate of 10 ˚C/min. Dissolve approximately 30 mg of the test substance in 1 mL of DMSO (99.9%) in a 5 mm diameter nuclear magnetic resonance tube. Nuclear magnetic resonance hydrogen spectra (\u003csup\u003e1\u003c/sup\u003eH-NMR) were obtained on a Bruker Avance III 400 MHz spectrometer equipped with a 5 mm BBFOZ gradient intelligent probe. All 1H-NMR data were analyzed using MestReNova software (Mestrelab Research). Contact angles of the NADESs samples on 45# steel substrates were measured using the German KRUSS optical contact angle measuring instrument DSA30S. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FIR) analysis of the NADESs sample was performed using a Fourier transform infrared spectrometer (Spectrum 3, PerkinElmer, USA). The scanning range was set from 650 to 4000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, with a scanning rate of 20 scans per minute. The dynamic viscosities of the 5 NADESs were measured at 25\u0026deg;C using a rotational rheometer (MCR302, Anton Paar, measuring head model CP20/MRD).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4 Calculation of intermolecular interaction energy\u003c/h2\u003e\u003cp\u003eQuantum mechanical calculations were carried out to explore the intermolecular interactions. All calculations were completed using the Gaussian 09 software package. The M06-2x\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e functional and 6\u0026ndash;31\u0026thinsp;+\u0026thinsp;g(d) basis set were employed for structure optimization and single - point energy calculations. The Multiwfn 3.8 program was utilized to analyze the distribution of electrostatic potential (ESP) on the molecular surface. The interaction energy was calculated according to the following formula:\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/p\u003e\u003c/p\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003c/span\u003e \u003cspan class=\"InlineEquation\"\u003e\u003c/span\u003e where E\u003csub\u003eint\u003c/sub\u003e represents the interaction energy, A and B represent individual molecules, A-B represents the complex formed following the interaction between A and B, and E\u003csub\u003eBSSE\u003c/sub\u003e represents the basis set superposition error.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.5 Stability testing\u003c/h2\u003e\u003cp\u003eTo evaluate the corrosion behavior of the prepared NADESs, the 45# steel used in this experiment was immersed in NADESs for testing. The polished and cleaned 45# steel specimens were completely submerged in NADESs at room temperature (25\u0026deg;C) for 7 days, and photos were taken daily to record the surface changes of the 45# steel. Additionally, the FTIR spectra of the NADESs before and after friction were compared to investigate whether the hydrogen - bond interactions and components of NADESs changed after lubrication.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e2.6 Friction Testing\u003c/h2\u003e\u003cp\u003eThe friction performance of NADESs was evaluated using the rotary module of a ball disc friction and wear tester (UMT-2, Bruker, USA). The friction ball (upper sample) is made of GCr15 bearing steel (φ\u0026thinsp;=\u0026thinsp;6.35 mm, Ra\u0026thinsp;=\u0026thinsp;30 nm), while material of the friction disc (lower sample) is 45# steel (φ\u0026thinsp;=\u0026thinsp;40 mm). Bearing steel and 45# steel are widely used in various mechanical parts\u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e. The surface of 45# steel was polished with sandpaper of 100 mesh, 500 mesh, 1000 mesh, and 2000 mesh, respectively. After polishing, the disc was cleaned with ethanol, and its surface roughness was measured using a white light interferometer, with the roughness controlled it within 20 nm. Before testing, the steel balls and discs were cleaned with ethanol using ultrasonic waves. The test parameters were set as follows: the rotation radius of 4 mm, rotation speed of 60 r/min, and friction duration of 30 minutes under different loads, with the COF recorded throughout. The loads used in this study were 50 N (2.32 GPa), 100 N (2.93 GPa), and 150 N (3.35 GPa), respectively. The calculation formula for Hertz contact stress is \u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/p\u003e\u003c/p\u003e\u003cp\u003ewhere ρ is the comprehensive curvature radius and E is the comprehensive elastic modulus. The experimental temperature is 20\u0026deg;C and the relative humidity is 50%. Each experiment should be repeated at least 3 times to eliminate accidental errors and ensure the accuracy of experimental data.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e2.7 Analysis of worn surfaces\u003c/h2\u003e\u003cp\u003eUpon completion of the friction tests, the width and depth of the wear marks were measured for 3 times using a white light interferometer (Verified XL, ZYGO, USA), and the wear volume and wear rate were subsequently calculated. To elucidate the lubrication mechanism of the NADESs, the morphology and chemical composition of the worn surface of the chassis after friction were analyzed. Firstly, images of the wear tracks were taken using an optical microscope (Smart Zoom 5, Zeiss, Germany). The elemental composition was then characterized using a Gemini300 thermal field emission scanning electron microscope. Then, XPS (K-Alpha, Thermo Fisher Scientific, USA) was used to analyze the chemical composition of the friction surface and the possible formation of friction film.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Characterization of NADESs\u003c/h2\u003e\u003cp\u003eThe formation of hydrogen bonds and the successful synthesis of the five NADESs were confirmed by FTIR spectroscopy, as evidenced by characteristic peak shifts associated with functional group interactions\u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a) shows the FTIR spectra of ChCl, glycerol and NADES-1 respectively. The characteristic peaks at 3286 cm⁻\u0026sup1; and 1029 cm⁻\u0026sup1; are attributed to the -OH and C-O groups of glycerol respectively. In the FTIR spectrum of NADES-1, we found that the hydroxyl group in glycerol shifted to a higher wavenumber, appearing at 3305 cm⁻\u0026sup1;. Also, the C-O peak of glycerol also exhibited a hypsochromic shift, moving from 1029 cm⁻\u0026sup1; to 1036 cm⁻\u0026sup1;. These observations demonstrate that hydrogen bonds were formed between ChCl and glycerol, indicating the successful synthesis of NADES-1. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b) shows the FTIR spectra of ChCl, urea, and NADES-2 respectively. From the figure, the peaks at 1675 cm⁻\u0026sup1; and 1589 cm⁻\u0026sup1; in urea correspond to the C\u0026thinsp;=\u0026thinsp;O and -NH groups. After the formation of NADES, they shifted to different wavenumbers, moving to 1660 cm⁻\u0026sup1; and 1605 cm⁻\u0026sup1; respectively, which provides evidence for the formation of hydrogen bonds between ChCl and urea. Similarly, for NADES-3, the -OH and C\u0026thinsp;=\u0026thinsp;O peaks of malic acid shifted from 2873 cm⁻\u0026sup1; and 1691 cm⁻\u0026sup1; to 2915 cm⁻\u0026sup1; and 1722 cm⁻\u0026sup1; respectively in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(c). For NADES-4, the C\u0026thinsp;=\u0026thinsp;O peak of betaine and the -OH and C-O peaks of glycerol shifted from 1612 cm⁻\u0026sup1;, 3288 cm⁻\u0026sup1;, and 1029 cm⁻\u0026sup1; to 1624 cm⁻\u0026sup1;, 3300 cm⁻\u0026sup1;, and 1038 cm⁻\u0026sup1; respectively in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(d). In the case of NADES-5, the COO- peak of L-carnitine and the C-O peak of glycerol shifted from 1373 cm⁻\u0026sup1; and 1029 cm⁻\u0026sup1; to 1391 cm⁻\u0026sup1; and 1039 cm⁻\u0026sup1; respectively in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(e).Collectively, the observed shifts in these characteristic peaks across all five NADESs confirm the formation of hydrogen bond interactions between the respective HBA and HBD components, thereby validating the successful synthesis of the targeted NADESs.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e presents the \u0026sup1;H-NMR spectra of the five synthesized NADESs along with their corresponding individual components. In the \u0026sup1;H-NMR spectra, the formation of hydrogen bonds reduces the electron shielding of active hydrogen atoms, resulting in a downfield shift (higher chemical shift) in their resonance signals\u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e. As shown in Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, the active hydrogen signals of the HBDs in all NADESs exhibit such downfield shifts due to hydrogen bond formation with the HBAs. The specific chemical shift values are summarized in Table S2. Based on the FTIR and \u0026sup1;H-NMR characterizations, we also calculated the interaction energies ΔE of the five NADESs using quantum mechanical calculations (QC). As shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Fig. S2, the interaction energies ΔE of the NADESs are 22.45, 22.95, 24.04, 23.24, and 25.71 kcal/mol respectively. It was noteworthy that the value of the interaction energy ΔE mentioned above is calculated based on NADESs with a molar ratio of 1:1, because NADESs with a molar ratio of 1:1 have a lower solidification point, rendering them unsuitable for lubrication applications. However, the above calculation results can still represent the relative size of NADESs with a molar ratio of 1:2. Therefore, when ChCl is used as the HBA, the hydrogen-bond strength follows the sequence: NADES-3\u0026thinsp;\u0026gt;\u0026thinsp;NADES-2\u0026thinsp;\u0026gt;\u0026thinsp;NADES-1. When glycerol acts used as the HBD, the hydrogen bond strength follows the order: NADES-5\u0026thinsp;\u0026gt;\u0026thinsp;NADES-4\u0026thinsp;\u0026gt;\u0026thinsp;NADES-1. These results are consistent with the trend observed in the \u0026sup1;H-NMR spectra.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003ePhysico-chemical properties of NADESs.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSample\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eInteraction energies ΔE\u003c/p\u003e\u003cp\u003e(kcal/mol)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMelting\u003c/p\u003e\u003cp\u003etemperature(\u0026deg;C)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDecomposition temperature (\u0026deg;C)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eViscosity\u003c/p\u003e\u003cp\u003e(MPa\u0026middot;s)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eContact\u003c/p\u003e\u003cp\u003eangle on 45# steel (\u0026deg;)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNADES-1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e22.45\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-77.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e263.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e296\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e\u003cp\u003e85.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNADES-2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e22.95\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e21.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e242.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e611\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e\u003cp\u003e80.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNADES-3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e24.04\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-37.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e241.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e54619\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e\u003cp\u003e45.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNADES-4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e23.24\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-71.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e266.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e1809\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e\u003cp\u003e71.4\u0026thinsp;\u0026plusmn;\u0026thinsp;1.6\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNADES-5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e25.71\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e-55.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e200.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e6445\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e\u003cp\u003e69.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe melting temperature and decomposition temperature are key characteristics that define the temperature range within which NADESs remain in a liquid state \u003csup\u003e[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e. The melting temperatures of the five NADESs were analyzed using differential scanning calorimetry (DSC). As shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Fig. S3, the melting temperatures of NADES-1, NADES-2, NADES-3, NADES-4, and NADES-5 are \u0026minus;\u0026thinsp;77.6\u0026deg;C, 21.8\u0026deg;C, -37.3\u0026deg;C, -71.1\u0026deg;C, and \u0026minus;\u0026thinsp;55.6\u0026deg;C, respectively. When compared to the melting temperatures of the raw materials used for synthesis (glycerol: 17.8\u0026deg;C, ChCl: 302\u0026deg;C, urea: 133\u0026deg;C, malic acid: 101\u0026deg;C, betaine: 293\u0026deg;C, and L-carnitine: 200\u0026deg;C), the synthesized NADESs exhibit substantially lower melting temperatures, which is one of the most distinct features of NADESs. Additionally, we investigated the thermal stability of the five synthesized NADESs using thermogravimetric (TG) and differential thermogravimetric (DTG) analyses. The TG curves and corresponding DTG curves are shown in Fig. S4. The results indicate that all five NADESs undergo initial decomposition above 200\u0026deg;C and complete decomposition at approximately 300\u0026deg;C. Clearly, they all exhibit strong thermal stability, remaining stable under relatively high-temperature conditions. Both the DSC and TG characterizations confirm that NADESs possess a broad operating temperature range.\u003c/p\u003e\u003cp\u003eViscosity reflects the internal resistance of molecules during fluid flow, affecting their ability to form a lubricating film, and is the most important performance indicator of lubricating oils\u003csup\u003e[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e. The dynamic viscosities of the five NADESs were measured using a rheometer. As shown in Fig. S5(a) and Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, at a shear rate of 100 1/s, the viscosities are 0.296, 0.611, 54.619, 1.809, and 6.445 Pa\u0026middot;s, respectively. It can be observed that when the hydrogen bond acceptor (HBA) is the same (NADES-1, NADES-2, NADES-3), adjusting the hydrogen bond strength by modifying the hydrogen bond donor (HBD) leads to a positive correlation between viscosity and hydrogen bond strength in NADES-1, NADES-2, and NADES-3. Similarly, when the HBD is the same (NADES-1, NADES-4, NADES-5), the same pattern is observed. In addition, we also measured the viscosity of commercial lubricating oil PAO40, which was 0.5 Pa\u0026middot;s.\u003c/p\u003e\u003cp\u003eThe affinity of lubricants to sample surfaces is evaluated by their wettability, which also affects their lubrication performance\u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e. To assess this, we measured the contact angles of the five NADESs on 45# steel at 25\u0026deg;C, as shown in Fig. S5(b) and Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The results indicate that the contact angles of NADES-1, NADES-2, NADES-3, NADES-4, NADES-5 and PAO40 are 85.2\u0026deg;, 80.4\u0026deg;, 45.2\u0026deg;, 71.4\u0026deg;, 69.9\u0026deg; and 30.2\u0026deg; respectively. For NADESs, the contact angle and viscosity of these NADESs do not correspond. Among NADES-1, NADES-2, and NADES-3, NADES-3 exhibited the smallest contact angle. Typically, liquids with higher viscosities tend to have larger contact angles, however, this is not the case here. As shown in the electrostatic diagram in Fig. S6, the red area represents positive electrostatic potential, while the blue area represents negative electrostatic potential. The smaller contact angle of NADES-3 may be due to the strong electronegativity of the carboxyl group, which promotes coordination with the metal surface. When comparing NADES-1 and NADES-2, it is evident that NADES-2 has a smaller contact angle, despite the lower electronegativity of urea compared to glycerol. This suggests that the amino group in urea is more likely to form coordination with metals, further reducing the contact angle. Additionally, in NADES-1, NADES-4, and NADES-5, the contact angle is in the following order: NADES-5\u0026thinsp;\u0026gt;\u0026thinsp;NADES-4\u0026thinsp;\u0026gt;\u0026thinsp;NADES-1. This may be attributed to the higher electronegativity of L-carnitine compared to betaine, and betaine\u0026rsquo;s higher electronegativity than ChCl. Moreover, the chloride ions in ChCl may be encapsulated by glycerol, limiting their interaction with the metal surface, resulting in a relatively large contact angle for NADES-1. In conclusion, both the electronegativity of the NADES components and their coordination strength with metals play a crucial role in determining the contact angle.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.2. Stability of steel in NADESs\u003c/h2\u003e\u003cp\u003eTo investigate the tribological properties of NADESs with varying hydrogen bond strengths, the five types of NADESs were divided into two groups based (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e) on variations in hydrogen bond donors (HBD) or hydrogen bond acceptors (HBA). The first group consists of NADES-1, NADES-2, and NADES-3, which share the same HBA but differ in HBD. The second group consists of NADES-1, NADES-4, and NADES-5, characterized by the same HBD but distinct HBAs. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) shows a schematic diagram of the friction experiments using GCr15-45# steel friction pair.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Tribological test analysis\u003c/h2\u003e\u003cp\u003eTo investigate the tribological properties of NADESs with different hydrogen bond strengths, five types of NADESs were divided into two groups based (Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e) on the changes in hydrogen bond donors (HBD) or hydrogen bond acceptors (HBA). The first group consists of NADES-1, NADES-2, and NADES-3, sharing the same HBA but with different HBD. The second group consists of NADES-1, NADES-4, and NADES-5, with the same HBD but different HBAs. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) shows a schematic diagram of friction experiments using GCr15-45# steel friction pair.\u003c/p\u003e\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c), when ChCl serves as the HBA, the friction coefficient of NADES-1 exhibits multiple significant fluctuations throughout the lubrication process, with numerous peaks and valleys evident in the friction curve, and an average friction coefficient of 0.1153. This indicates that NADES-1 exhibits lubrication instability throughout the entire friction stage. In contrast, NADES-2 displays relatively minor fluctuations, with no obvious peaks or valleys in its friction curve. Instead, it shows a stable decreasing trend, with an average friction coefficient of 0.0969. This improvement can be attributed to the higher hydrogen bond strength in NADES-2, which renders the intramolecular hydrogen bonds more stable and less prone to cleavage, thereby limiting molecular mobility. Compared to NADES-1, NADES-2 exhibits higher viscosity, enhanced stability, and a lower friction coefficient. In addition, as shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the amino groups in urea can coordinate with metals, which strengthens the affinity of NADES-2 for metal surfaces and reduces friction. Although NADES-3 exhibits the strongest hydrogen bonding interactions and contains carboxyl groups that form strong coordination with metals, theoretically it should have a lower friction coefficient due to its smaller contact angle. However, the actual results are exactly the opposite. Specifically, the friction coefficient of NADES-3 is higher than those of NADES-1 and NADES-2, with an average of 0.1333. This phenomenon can be attributed to the exceptionally high viscosity of NADES-3, which slows mass transfer and hinders the recombination of hydrogen bonds, thereby resulting in higher friction. During the experiment, NADES-3 exhibited turbidity during friction, which dissipated upon standing. The infrared characterization in Fig. S8 verifies that the composition of NADES-3 remains unchanged before and after friction. Despite its high coefficient, NADES-3 demonstrates greater stability, with an almost linear friction curve. This may be due to its high viscosity and strong intermolecular hydrogen bonding interactions, which facilitate the maintenance of a stable equilibrium state even when the hydrogen bonds are cleaved. Therefore, it can be inferred that stronger hydrogen bonding interactions enhance the lubrication stability of NADES.\u003c/p\u003e\u003cp\u003eIn the second group of NADESs, there is no significant difference in the coefficient of friction (COF) among the three NADESs in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(d). However, the fluctuation of NADES-1 is still the largest, while NADES-4 shows good stability with an average friction coefficient of 0.125. NADES-5 maintains high stability throughout the entire friction process, with an average friction coefficient of 0.123. As the hydrogen bond strength increases (Fig. S2), the stability of NADES is enhanced. In addition, compared with ChCl in NADES-1, betaine in NADES-4 and L-carnitine in NADES-5 constrain molecular mobility after forming hydrogen bonds with glycerol. In contrast, the chloride ions in ChCl exhibit relatively high mobility, which accounts for the superior stability of NADES-4 and NADES-5. Furthermore, we also chose commercial lubricant PAO40 as a comparison. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the average friction coefficient of PAO40 is 0.1437, which is considerably higher than that of NADESs. Compared with NADES-2, the friction coefficient of PAO40 is reduced by 32.6%, highlighting the excellent lubrication performance of NADESs.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e presents the optical micrographs of the wear tracks. Due to its strong hydrogen-bond interactions, high viscosity, and metal surface interaction, NADES-3 exhibits a distinct boundary after wear, with extremely faint wear tracks, indicating minimal wear. In the second group of NADESs, a clear trend is observed: as the hydrogen-bond strength increases, the boundaries of the wear tracks become increasingly distinct, smooth, and uniform. Among these, NADES-5 displays the most regular and distinct boundary, with the most uniform wear and the worn surface retaining its metallic luster. This phenomenon can be attributed to the strong hydrogen-bond strength of NADES-5, which enhances the restricted movement of molecules, leading to a stable and high-strength molecular adsorption film that effectively reduces wear. In comparison, PAO40, being non-polar, cannot form an anti-wear molecular layer. Its wear track has a rough and unclear boundary, with uneven wear.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe wear volume is also a crucial indicator for evaluating the lubrication effect. 3D topography images of the wear tracks are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a), from which the width and depth of the wear track were obtained. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b) and (c), among the NADESs in the first group, NADES-2 exhibits the largest wear dimensions (both in terms of width and depth). Its wear width is 440 \u0026micro;m, and the depth is 6.3 \u0026micro;m. This may be due to the presence of urea in NADES-2, which may break down the iron oxide protective layer, leading to greater wear. In contrast, NADES-3 shows the smallest wear dimensions, with a width of only 218 \u0026micro;m and a depth of 1.2 \u0026micro;m. This can be attributed to its strong hydrogen-bond strength, where the carboxyl group forms a robust coordination and reaction with iron, generating an iron carboxylate protective layer that effectively reduces wear under load. In the second group of NADESs, a comparison reveals that NADES-1, with its lower hydrogen-bond strength, experiences the most severe wear. As the hydrogen-bond strength increases, the wear dimensions decrease to varying degrees. Furthermore, the structural features of HBAs play a crucial role in wear morphology: longer chains like those in L-carnitine, provide elastic compressibility, thereby reducing wear depth. The carboxyl groups in both betaine and L-carnitine can form coordination bonds with the metal, contributing to a reduction in wear track dimensions. Compared with NADESs, the commercial lubricant PAO40 exhibits a significantly higher wear depth of 10.0 \u0026micro;m which is 88% higher than that of NADES-2, indicating the poor lubrication performance of PAO40.\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(d) shows the wear amount of the wear marks. Compared with PAO40, NADES-1, NADES-3, NADES-4, NADES-5, the wear volume of NADES decreased by 41.3%, 89.1%, 64.8% and 76.2% respectively, indicating that the anti-wear effect of NADESs is significant. Due to the presence of urea in its composition, NADES-2 damages the protective layer of iron oxide, resulting in relatively severe wear that is comparable to that of PAO40. The wear reduction behavior is related to the interactions between polar functional groups (hydroxyl (R-OH), amino (R-NH), etc.) on NADES and surfaces. According to the model proposed by Hardy and Doubleday\u003csup\u003e[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]\u003c/sup\u003e, polar molecules are adsorbed onto the surface to form a closely - packed, vertically - oriented monolayer. In contrast, PAO40 lacks such polar effects, resulting in a significantly larger wear volume compared to the NADESs.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo investigate the load-bearing capacity of NADESs, their further examined their friction performance under 100 N and 150 N loads were further examined. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a) and (b) show the friction coefficient of NADESs at 100 N. When the load increases to 100 N, the friction coefficient of NADESs remains almost unchanged compared to those under 50 N. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(a), the average friction coefficients of NADES-1 and NADES-2 are 0.1150 and 0.1094, respectively, with significant fluctuations. But with the enhancement of hydrogen bonding, the friction coefficient curve of NADES-3 shows stability consistent with 50 N, with an average friction coefficient of 0.1350. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b) shows the friction coefficient of the second group of NADESs under 100 N, with the average friction coefficients of NADES-4 and NADES-5 being 0.1201 and 0.1257, respectively. The friction coefficient curve of NADES-5 is also a straight line with almost no fluctuations, while NADES-4 and NADES-1 have significant fluctuations. The fluctuation of commercial lubricating oil PAO40 is the largest, with an average friction coefficient of 0.1335, indicating that the lubrication stability of PAO40 is significantly inferior to that of the NADESs. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(c) and (d), when the load increases to 150 N, the average friction coefficient of NADESs remains stable and almost unchanged. The friction curves of NADES-3 and NADES-5 remain stable due to their high hydrogen bond strength, with average friction coefficients of 0.1301 and 0.1233, respectively. The friction curve of NADES-1 shows multiple sudden increases in friction coefficient, with an average friction coefficient of 0.105. This may be attributed to the weakest hydrogen bond strength and insufficient load bearing capacity, as hydrogen bonds are constantly breaking and reformation due to incomplete destruction. The NADES-2 exhibits the lowest friction coefficient, at 0.0984. The average friction coefficient of NADES-4 is slightly lower than that of NADES-5, at 0.12. Commercial lubricants exhibit the worst lubrication effect, with an average friction coefficient of 0.128.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e, with increasing the load, the friction coefficient of NADESs remained nearly stable at higher loads (100 N and 150 N). The load dependent pattern of the COF may not be typical. However, this phenomenon has been observed in numerous studies on dry and lubricated sliding contacts of various bulk materials\u003csup\u003e[\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]\u003c/sup\u003e. In conclusion, the friction coefficients of the two NADESs remained relatively stable throughout the entire test process and did not exhibit a sudden increase in friction coefficient, indicating that NADESs have excellent load-bearing capacity.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e compares the wear dimensions and wear volume of NADES and PAO40 under loads of 100 N and 150 N. The corresponding optical photos and 3D morphology images are shown in Fig. S9 and S10. In the second group of NADESs, the long chains in L-carnitine provide elastic compressibility, resulting in its minimum depth. Compared with PAO40, the depth of wear marks after NADESs lubrication decreased significantly, which is speculated to be related to the adsorption lubrication film formed by polar molecules in NADESs, effectively reducing vertical wear. Unlike the changing trend of COF, the wear volume of NADESs increases with the increase of load, consistent with the trend predicted by the classical Archard wear law. Compared with commercial lubricant PAO40, NADESs exhibit excellent wear resistance, with NADES-3 showing the best performance. Specifically, compared with PAO40, the wear volume was reduced by 87.6% and 74.3% under loads of 100 N and 150 N, respectively.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Analysis of wear surface and lubricating mechanism","content":"\u003cp\u003eTo gain a deeper understanding of the lubrication mechanism of NADESs, the surface composition and chemical states of the wear tracks were analyzed using energy-dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Prior to the surface analysis, the samples were cleaned with alcohol and dried. The friction chassis, made of 45# steel, is primarily composed of Fe, C, O, Cr, Si, and Mn. Figure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e shows the scanning electron microscopy images. The wear surface lubricated with PAO40 shows severe abrasive wear, characterized by numerous abrasive wear grooves and rough indistinct boundaries. On the contrary, the wear track surface lubricated by NADESs is smooth and flat, with clear wear boundaries and significantly reduced wear, indicating favorable lubrication effect. To analyze the lubrication mechanism, the element mapping of wear marks was performed. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, the N element from NADESs was detected on the worn surface, indicating the formation of an adsorption film on the worn surface, while PAO40 failed to form such an adsorption film. These findings explain why, NADESs exhibit superior lubrication performance compared with the commercial lubricant PAO40.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAs shown in Table S3, compared to the non-friction chassis, an increase in both O and C contents was detected on the wear track surface of the samples lubricated with NADESs. This indicates that the NADESs adhere onto the 45# steel surface or form iron oxide or iron hydroxide in interaction with the steel. The increase in oxygen content is most pronounced in NADES-2. This may be due to the urea in NADES-2 breaking down the oxide protective layer on the metal surface, thereby exposing more underlying metal. In the ambient atmosphere rich in oxygen and moisture, a significant amount of iron oxide is formed.\u003c/p\u003e\u003cp\u003eTo further investigate the composition of the friction film, XPS analysis was performed. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e shows the XPS spectrum of the worn area after lubrication with NADESs. Due to the low friction coefficient of NADES-2, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, we focused on analyzing the XPS spectra of its worn surfaces C1s, O1s, Fe2p, and N1s.In Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(a), the C1s spectrum of the worn surface contains four peaks, namely 283.33, 284.80, 286.66, and 288.63 eV, corresponding to C-Si, C-C/C-H, C-OH, and O-C\u0026thinsp;=\u0026thinsp;O groups\u003csup\u003e[\u003cspan additionalcitationids=\"CR36 CR37\" citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]\u003c/sup\u003e, respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(b) are that the peak values of O1s at 530.04, 531.76 and 533.7 eV belong to iron oxide,-OH and O\u0026thinsp;=\u0026thinsp;C-O/-CO- \u003csup\u003e[\u003cspan additionalcitationids=\"CR40 CR41 CR42 CR43\" citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]\u003c/sup\u003e .Among them, C-O and C\u0026thinsp;=\u0026thinsp;O may be due to the process of frictional oxidation reaction. This indicates that the hydroxyl groups in ChCl can adsorb onto metal surfaces through polar interactions, forming O-Fe bonds. The binding energy peaks of Fe2p near 707.23, 710.22, 711.76, and 715 eV in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(d) should be attributed to Fe/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, FeCl\u003csub\u003e3\u003c/sub\u003e/Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e/Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, FeOOH, Fe\u003csup\u003e2+\u003c/sup\u003e/FeO and FeCl\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e[\u003cspan additionalcitationids=\"CR46\" citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]\u003c/sup\u003e, respectively. According to Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e(c), the N1s spectrum was observed at approximately 398.66, 399.65, 402 eV corresponding to -NH\u003csub\u003e2\u003c/sub\u003e, C-N-C \u003csup\u003e[\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]\u003c/sup\u003e, N-O. These peaks are attributed to metal nitrides, indicating that ChCl-urea NADES participates in frictional chemical reactions.\u003c/p\u003e\u003cp\u003eThe spectra of the elements C, O, N, and Fe suggest that the amino group in NADES-2 undergoes chemical interactions with the worn surface via chemical adsorption, forming iron (hydrogen) oxides and nitrides with NADES during friction. The oxygen and moisture in the air also facilitate the formation of this chemical reaction layer. So, we speculate that the friction layer of NADES-2 is primarily composed of the two components: an adsorption layer formed by polar molecules at the friction interface (vertically rigid yet horizontally shearable), and a chemical reaction layer consisting of iron oxides and hydroxides. The combined effect of these layers results in a lower friction coefficient for NADES-2 compared to the other NADESs. As shown in Fig. S11, the characteristic peaks of C1s, O1s, and Fe2p in the spectra of other NADESs are similar, with their corresponding main components also including iron oxides and iron hydroxide.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eBased on the friction and wear results and the surface analysis findings, a lubrication mechanism for the studied NADESs is proposed. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e, the friction layer lubricated by NADESs consists of two distinct layers. The bottom layer is a chemical reaction film composed of iron oxides and hydroxides, while the upper layer is a polar molecule adsorption layer formed by the polar groups of NADESs (e.g., hydroxyl groups, amino groups (R-OH, R-NH₂)). The excellent anti-friction performance of NADESs is attributed to the adsorption of these polar molecules, which form a vertically rigid yet horizontally easily shearable monolayer at the friction interface. Observation of the wear track images and wear volume data reveal that hydrogen bond strength influences the integrity of the polar molecule layer. The stronger the hydrogen bond strength, the more restricted the molecular movement within the system, resulting in a higher rigidity of the polar molecule layer in the vertical direction and a better anti-wear effect. Compared to PAO40 lubrication, this polar adsorption layer plays a crucial role in reducing wear. Furthermore, during the friction process, oxygen and moisture in the air, together with hydroxides and oxides, contribute to the formation of a chemical reaction film, further reducing the friction and wear.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eIn this study, five natural deep eutectic solvents (NADESs) with varying hydrogen-bonding strengths and distinct hydrogen bond acceptor (HBA) structures were designed and systematically evaluated for their lubrication performance. The results highlight that both hydrogen-bonding interactions and HBA molecular architecture critically govern the tribological behavior of NADESs. Stronger hydrogen bonds restricted molecular mobility, stabilized lubricating films, and reduced friction fluctuations and wear. Structural features of HBAs further influenced wear morphology, where long-chain structures such as L-carnitine provided elastic compressibility, and polar groups such as carboxyl moieties enhanced metal coordination and anti-wear effects. Compared with the commercial lubricant PAO40, optimized NADESs exhibited superior friction-reducing and wear-resistant capabilities. These findings demonstrate the synergistic role of hydrogen bond networks and molecular structure in shaping lubrication mechanisms, thereby offering theoretical guidance for the rational design of high-performance, sustainable lubricants. Overall, this work not only advances the fundamental understanding of NADES-based lubrication but also supports their broader potential as eco-friendly tribological materials.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData will be made available on request.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eAcknowledgement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was supported by the National Natural Science Foundation of China (Grant No. 52075264).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eYUE L, MENG Y, ZHOU X, et al. Tribological characteristics and degradation mechanism of typical synthetic lubricants from room temperature to 300 ℃ [J]. Tribology International, 2024, 198: 109865.\u003c/li\u003e\n\u003cli\u003eBASIRON J, ABDOLLAH M F B, ABDULLAH M I C, et al. 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Talanta, 2022, 245.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Deep eutectic solvents, Hydrogen bonding, Green lubricants, Friction, Tribo-chemical film","lastPublishedDoi":"10.21203/rs.3.rs-7635266/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7635266/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eNatural deep eutectic solvents (NADESs) have recently attracted considerable attention as sustainable lubricants due to their excellent tribological performance, biocompatibility, tunable composition, and low toxicity. Nevertheless, the relationship between their molecular structures and lubrication mechanisms remains insufficiently understood. In this work, five NADESs with distinct hydrogen-bonding strengths and hydrogen bond acceptor (HBA) structures were prepared using choline chloride, betaine, or L-carnitine combined with glycerol, urea, or malic acid. Their physicochemical and tribological properties were systematically evaluated. The results indicate that stronger hydrogen-bonding interactions restrict molecular mobility, thereby improving lubrication stability, reducing wear track dimensions, and generating smoother surface morphologies. Moreover, functional groups such as carboxyl moieties in HBAs can coordinate with metal surfaces, further enhancing anti-wear effects. Compared with the commercial lubricant PAO40, the optimized NADES achieved a 32.6% reduction in the friction coefficient and an 89.1% reduction in wear volume. These findings highlight the synergistic role of hydrogen bonding and HBA molecular design in determining lubrication behavior, and provide a theoretical basis for the rational design of high-performance, environmentally friendly lubricants based on NADESs.\u003c/p\u003e","manuscriptTitle":"Hydrogen Bond-Driven Tribological Properties of Natural Deep Eutectic Solvents for Green Lubrication","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-14 14:43:45","doi":"10.21203/rs.3.rs-7635266/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"410da6db-26c5-4648-a29f-655c2ea92128","owner":[],"postedDate":"November 14th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-11-26T03:08:43+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-14 14:43:45","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7635266","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7635266","identity":"rs-7635266","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}