Superlubricity Achieved with Mixtures of Polyhydroxy Alcohols and

Apr 3, 2013 - State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, People's Republic of China. Langmuir , 2013, 29 (17), pp 5239–...
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Superlubricity Achieved with Mixtures of Polyhydroxy Alcohols and Acids Jinjin Li, Chenhui Zhang, and Jianbin Luo* State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: In the present work, we show that the superlubricity can be achieved when the polyhydroxy alcohol solutions are mixed with acid solutions. The lowest friction coefficients between 0.003 and 0.006 are obtained on a traditional tribometer with a high pressure under the lubrication of these mixtures. Experimental results indicate that the superlubricity mechanism is in accordance with that under the lubrication of the mixture of glycerol and acid solutions in the study by Li et al. (Li, J. J.; Zhang, C. H.; Ma, L. R.; Liu, Y. H.; Luo, J. B. Superlubricity achieved with mixtures of acids and glycerol. Langmuir 2013, 29, 271−275). It is also found that the superlubricity is closely dependent upon the concentration of polyhydroxy alcohol and the number of hydroxyl groups in the molecular structure of polyhydroxy alcohol. However, the number of carbon atoms and the arrangement of hydroxyl groups in the molecular structure almost have no effect on superlubricity.

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INTRODUCTION With the increasingly polluted environment and shortage of energy, the demand for machine performances is becoming stronger, which requires that the lubricant should have excellent properties with an ultralow friction. The term “superlubricity” is used to describe the case in which the ultralow friction coefficient of the lubricant is less than 0.01.1,2 At present, there are several kinds of solid lubricants found having superlubricity property, such as highly oriented pyrolytic graphite,3,4 diamond-like carbon film,5−7 molybdenum disulfide,8,9 and CNx films.10,11 The superlubricity of them is generally attributed to incommensurate contact, coulomb repulsion, or weak interfacial interaction in the condition of high vacuum or nitrogen protection.12−14 In addition to this, there are also some liquid lubricants with superlubricity property, such as polymer brushes with water,15,16 ceramic materials with water,17,18 and some kinds of polysaccharide mucilage from plants.19−21 However, to the best of our knowledge, studies on the liquid superlubricity at the macroscale are still scarce. Recently, the liquid superlubricity at the macroscale with the lubrication of phosphoric acid solution and the mixture of glycerol and acid solutions were found by our group.22,23 Both of them have the same superlubricity mechanism, that is, forming the hydrogenbond network between the two positively charged surfaces induced by hydrogen ions.24 On the basis of this superlubricity mechanism, it can be inferred that the superlubricity would probably be obtained as long as there are sufficient hydrogen ions and adequate hydroxyl groups in the solution. A direct method of meeting the two conditions above simultaneously is mixing polyhydroxy alcohol solutions with acid solutions. © 2013 American Chemical Society

Therefore, to confirm whether the superlubricity can be obtained in this lubrication system, studies on the friction properties of polyhydroxy alcohol solutions mixed with acid solutions are presented in this work. Among polyhydroxy alcohols, the following were chosen to be tested: 1-propanol, 2propanol, 1,2-ethanediol, 1,2-propanediol, 1,3-propanediol, propanetriol, 1,4-butanediol, 1,5-pentanediol, diethylene glycol, triethylene glycol, tetraethylene glycol, and pentaethylene glycol. Structures of these alcohols are shown in Figure 1. Four kinds of acid solutions are chosen to mix with these alcohols; they are sulfuric acid (H2SO4), hydrochloric acid (HCl), oxalic acid (H2C2O4), and sulfamic acid (H3NO3S). The relationship between the concentration of polyhydroxy alcohols and superlubricity and the effect of the number of carbon atoms and hydroxyl groups on superlubricity are investigated.

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MATERIALS AND METHODS

The polyhydroxy alcohols were commercial products (J&K) with a purity of greater than 99%. 1,2-Ethanediol, 1,3-propanediol, 1,4butanediol, and 1,5-pentanediol were diluted by deionized water with different concentrations (10, 20, 30, 40, 50, 60, 70, 80, 90, and 100%, v/v). The other polyhydroxy alcohols, including 1-propanol, 2propanol, 1,2-propanediol, propanetriol, diethylene glycol, triethylene glycol, tetraethylene glycol, and pentaethylene glycol, were diluted by deionized water with a concentration of 20% (v/v). The acids were also commercial products with a purity of greater than 99%. Four kinds of acid solutions (H2SO4, HCl, H2C2O4, and H3NO3S) were Received: March 4, 2013 Revised: April 2, 2013 Published: April 3, 2013 5239

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MPa), and the rotation speed of the glass plate was 180 rpm with a track radius of 4 mm (corresponding to a linear speed of 0.075 m/s), which is the same as that in ref 23. To obtain a more accurate result of the friction coefficient, the measuring errors were eliminated by adjusting the levelness of the tribometer to obtain the same friction coefficients in two reverse sliding directions before the test (see more details in ref 25). All tests were performed at the temperature of 25 °C and the relative humidity of 20−40%.

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RESULTS AND DISCUSSION The friction coefficient with time, lubricated by the four kinds of polyhydroxy alcohol solutions (1,2-ethanediol, 1,3-propanediol, 1,4-butanediol, and 1,5-pentanediol, 20%, v/v) was first studied, as shown in panels a−d of Figure 2. It is found that the four kinds of alcohol solutions can all lead to the friction coefficient reducing to less than 0.1 (between 0.05 and 0.07) after a running-in period. The reduction of friction is attributed to the viscosity increasing with free water evaporating from the solution as well as the contact pressure reducing with the wear region increasing,24 which leads to the formation of an elastohydrodynamic film finally between two surfaces. However, the friction coefficient hardly reduces anymore when it reaches this low value. In other words, the superlubricity (μ < 0.01) cannot be achieved with the lubrication of these polyhydroxy alcohol solutions. Next, these polyhydroxy alcohols were mixed with H2SO4 solution (pH 1), and the friction properties of them were measured again, as shown in panels a−d of Figure 2. It is observed that a stable superlubricity state (μ < 0.01) can be obtained under the lubrication of these mixtures. At the early stage of the test, the friction coefficient reduces to less than 0.1 very fast and then it decreases slowly to about 0.004. After reduction to the lowest value, the friction coefficient can remain at this low value (μ = 0.004) stably for at least 400 s. In comparison to these results, it is obvious that these polyhydroxy

Figure 1. Structure of studied polyhydroxy alcohols: (a) 1-propanol, (b) 2-propanol, (c) 1,2-ethanediol, (d) 1,2-propanediol, (e) 1,3propanediol, (f) propanetriol, (g) 1,4-butanediol, (h) 1,5-pentanediol, (i) diethylene glycol, (j) triethylene glycol, (k) tetraethylene glycol, and (l) pentaethylene glycol. prepared by diluting the pure acids with deionized water to a pH value of 1.0. The mixtures were obtained by mixing the polyhydroxy alcohol solutions with the acid solutions in a volume ratio of 10:1 (the pH value of these mixtures is approximately equal to 2). The friction tests were carried on a Universal Micro-Tribotester (UMT-3, Bruker, Billerica, MA) with a rotation mode of ball-on-disk. The friction pairs were a Si3N4 ball with a diameter of 4 mm (obtained from Shanghai Research Institute of Materials with a manufacturing process of gas-protecting sintering and hot isostatic pressing) and a glass plate with a surface roughness of 5 nm (a part of the common glass slide for a microscope). The lubricants were introduced between the ball and the disk with a volume of about 20 μL before tests. The load applied was 3 N (corresponding to a max contact pressure of 700

Figure 2. Friction coefficient with the lubrication of polyhydroxy alcohols (20%, v/v) and the mixtures of them and H2SO4 solution (pH 1): (a) 1,2ethanediol, (b) 1,3-propanediol, (c) 1,4-butanediol, and (d) 1,5-pentanediol. The volume ratio of these polyhydroxy alcohols to H2SO4 solution (pH 1) is 10:1. 5240

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alcohols can present more excellent lubricating properties once they are mixed with H2SO4 solution. To investigate whether the other acid solutions added in these polyhydroxy alcohols have the same effect on superlubricity as the H2SO4 solution, the three other kinds of acid solutions (HCl, H2C2O4, and H3NO3S, at pH 1), mixed with these polyhydroxy alcohols, were tested as lubricants between the Si3N4 ball and the glass plate. It is found that the friction behavior lubricated by these mixtures is similar to that lubricated by the mixtures of these polyhydroxy alcohols and H2SO4 solution. The results of the final friction coefficient are shown in Table 1. It is seen that all of these mixtures Table 1. Final Friction Coefficient with the Lubrication of Mixtures of Four Kinds of Polyhydroxy Alcohols (1,2Ethanediol, 1,3-Propanediol, 1,4-Butanediol, and 1,5Pentanediol) and Three Kinds of Acid Solutions ((HCl, H2C2O4, and H3NO3S, at pH 1) with the Volume Ratio of 10:1 1,2-ethanediol 1,3-propanediol 1,4-butanediol 1,5-pentanediol

HCl

H2C2O4

H3NO3S

0.004 0.003 0.005 0.006

0.004 0.003 0.005 0.004

0.003 0.003 0.004 0.003

Figure 3. Relationship between the final friction coefficient and the concentration of polyhydroxy alcohols: (a) 1,2-ethanediol, (b) 1,3propanediol, (c) 1,4-butanediol, and (d) 1,5-pentanediol. The volume ratio of these polyhydroxy alcohols to H2SO4 solution (pH 1) is 10:1.

mentioned above can lead to the reduction of the friction coefficient to less than 0.01, despite different acid ions, which indicates that the superlubricity is independent of the acid ions. Instead, the hydrogen ions in the acid solution show an important role in achieving superlubricity. To investigate whether the concentration of polyhydroxy alcohols has an effect on superlubricity, the friction behavior under the lubrication of these polyhydroxy alcohols with different concentrations (10, 20, 30, 40, 50, 60, 70, 80, 90, and 100%, v/v) mixed with H 2SO 4 solution (pH 1) was investigated, as shown in panels a−d of Figure 3. It is observed that the final friction coefficient is less than 0.01 when the concentration of 1,2-ethanediol is lower than 40%. For the other three kinds of polyhydroxy alcohols, when the concentration is lower than 50% for 1,3-propanediol, 40% for 1,4-butanediol, and 30% for 1,5-pentanediol, respectively, the final friction coefficient can reduce to less than 0.01. It suggests that only when the concentration of these polyhydroxy alcohols is lower than the value indicated can the superlubricity be achieved after the running-in period, whereas the final friction coefficient would be greater than 0.01 if the concentration of these polyhydroxy alcohols is higher than this critical value. Evidently, the concentration of polyhydroxy alcohols has an effect on the viscosity of the mixture, which determines whether it can form an elastohydrodynamic film between two surfaces. Therefore, to analyze the relationship between the final friction coefficient and the concentration of polyhydroxy alcohols in Figure 3, the viscosity of these four kinds of polyhydroxy alcohols with different concentrations (approximately equal to the viscosity of corresponding mixtures) was measured by a standard rheometer (Physica MCR301, Anton Paar), as shown in Figure 4. It is found that the viscosity of these four kinds of polyhydroxy alcohols increases with the concentration increasing. Additionally, the viscosity of polyhydroxy alcohol increases with the number of carbon atoms in the carbon chain increasing when the concentration is kept constant (1,5pentanediol > 1,4-butanediol > 1,3-propanediol > 1,2-

Figure 4. Relationship between viscosity and concentration of polyhydroxy alcohols (1,2-ethanediol, 1,3-propanediol, 1,4-butanediol, and 1,5-pentanediol).

ethanediol). In combination with Figures 3 and 4, it is found that, when the concentration is in the range where the superlubricity can be achieved, the viscosity of these polyhydroxy alcohols is very low (less than 4 mPa s). With such low viscosity, it is hard to form an elastohydrodynamic film between the two contact surfaces according to the Hamrock−Dowson (H−D) theory.26 Therefore, it can be concluded that only when the concentration of polyhydroxy alcohols is so low that there is no elastohydrodynamic film formed between the two contact surfaces at the early stage of the test can the superlubricity be achieved. In comparison to these results and the friction coefficient lubricated by the mixture of glycerol and acid in ref 23, it is found that the features of the friction coefficient with time for these mixtures are almost the same. The friction coefficient first reduces to less than 0.1 at the early stage of the test, and then it decreases further to below 0.01 slowly. Moreover, the relationship between superlubricity and the concentration of these polyhydroxy alcohols is also similar; that is, the superlubricity can be obtained only when the concentration is 5241

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very low, corresponding to a very low viscosity (less than 4 mPa s). Therefore, it is concluded that the superlubricity mechanism of the mixtures of these polyhydroxy alcohols and acids is similar to that lubricated by the mixture of glycerol and acids. In our previous work, the superlubricity of the mixture of glycerol and acids is attributed to forming a hydrated water layer between the hydrogen-bond networks of glycerol−water on the stern layer after the running-in process.27 Therefore, it can be deduced that these four kinds of polyhydroxy alcohols can also form the hydrogen-bond network with water on the stern layer (induced by hydrogen ions) after the running-in process, which can lead to a hydrated water layer adsorbed on its surfaces to reduce friction. The possible lubrication model is illustrated in Figure 5.

Figure 6. Friction coefficient with the lubrication of the mixture of polyhydroxy alcohols and H2SO4 solution: (a) 1-propanol, (b) 2propanol, (c) 1,2-propanediol, and (d) propanetriol. The volume ratio of these polyhydroxy alcohols to H2SO4 solution (pH 1) is 10:1.

propanol and H2SO4 cannot realize superlubricity, although there is a reduction of the friction coefficient at the early stage of the test. However, the other three kinds of polyhydroxy alcohols (1,2-propanediol, 1,3-propanediol, and propanetriol) can realize superlubricity when mixed with H2SO4. Because 1propanol and 2-propanol have only one hydroxyl group in their molecular structure and the other three kinds of polyhydroxy alcohols have two or three hydroxyl groups in their molecular structures, it can be inferred that only when the number of hydroxyl groups in their molecular structure is more than one can the superlubricity be achieved. In addition, from the results of 1,2-propanediol and 1,3-propanediol, it is also found that the arrangement of hydroxyl groups in the molecular structure almost has no effect on superlubricity. How do the hydroxyl groups influence superlubricity? Table 2 lists the viscosity of these five alcohols with the volume concentration of 20 and 100%. It is seen that the viscosities of these five alcohols are all very low (less than 4 mPa s) with the concentration of 20%. However, with the concentration of 100%, the viscosities of 1,2-propanediol, 1,3-propanediol, and propanetriol are very high (much more than 4 mPa s) but the viscosities of 1-propanol and 2-propanol are still very low (less than 4 mPa s). In fact, the viscosity of polyhydroxy alcohols is directly related to the hydrogen-bond network. As the directional intermolecular hydrogen bonds O−H···O form the network, their viscosity property differs essentially from those of the non-associated liquids.27 When there is only one hydroxyl group in their molecular structure, it is hard to form the hydrogen-bond network among hydroxyl groups to increase its viscosity, such as 1-propanol and 2-propanol. However, when there are more than one hydroxyl group in their molecular structure, it is easy to form hydrogen bonds among hydroxyl groups to increase its viscosity. We also measured the friction coefficient with the lubrication of these alcohols (20%, v/v) without acids, as shown in Figure 7. It is found that 1,2propanediol, 1,3-propanediol (shown in Figure 2b), and

Figure 5. Schematic illustration of the possible lubrication model between two friction surfaces. The red atoms represent oxygen, and the white atoms represent hydrogen.

Although the superlubricity mechanism has been discussed above, there is still a question whether all polyhydroxy alcohols mixed with acids can realize superlubricity when they are used as lubricants. In other words, what conditions polyhydroxy alcohols satisfy for superlubricity is not clear, but it is very important for us to identify more kinds of liquid superlubricity materials. For polyhydroxy alcohols, the most important factors influencing their physical and chemical properties are the number of hydroxyl groups and carbon atoms and the arrangement of hydroxyl groups in their molecular structure. Therefore, in the following text, the relationship between the three factors and superlubricity would be investigated to find out the conditions for superlubricity. To determine the effect of the number and arrangement of hydroxyl groups in the molecular structure on superlubricity, five kinds of polyhydroxy alcohols, including 1-propanol, 2propanol, 1,2-propanediol, 1,3-propanediol, and propanetriol (all of them have three carbon atoms but have different numbers and arrangements of hydroxyl groups in their molecular structures, as shown in Figure 1), mixed with H2SO4 solution (pH 1), were chosen to be tested as the lubricant. The friction result under the lubrication of the mixture of 1,3-propanediol and H2SO4 has been presented in Figure 2b. The friction results under the lubrication of the other four kinds of mixtures are shown in Figure 6. In comparison to the friction results of the five different mixtures, it is found that the mixture of 1-propanol and H2SO4 and the mixture of 25242

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Table 2. Viscosities of 1-Propanol, 2-Propanol, 1,2-Propanediol, 1,3-Propanediol, and Propanetriol with the Concentrations of 20 and 100% (v/v) viscosity (Pa s) (20%, v/v) viscosity (Pa s) (100%, v/v)

1-propanol

2-propanol

1,2-propanediol

1,3-propanediol

propanetriol

0.0011 0.0019

0.0014 0.0020

0.0014 0.0462

0.0018 0.0432

0.0019 1.20

Figure 7. Friction coefficient with the lubrication of polyhydroxy alcohols (1-propanol, 2-propanol, 1,2-propanediol, and propanetriol) with the concentration of 20%.

propanetriol can lead to the friction coefficient reducing to less than 0.07 after a running-in period. It is because, with free water evaporating from the solution, the alcohol molecules begin to interact with each other or through water molecules by hydrogen bonding, which results in the increase in viscosity and an elastohydrodynamic lubrication film between two sliding surfaces. However, 1-propanol and 2-propanol cannot lead to the friction coefficient reducing to such a low value because of its very low viscosity even at a high concentration. Therefore, it can be concluded that the condition for superlubricity is that the number of hydroxyl groups in the molecular structure of polyhydroxy alcohol is more than one to form the hydrogenbond network. To determine the effect of the number of carbon atoms in the molecular structure on the superlubricity, four kinds of polyhydroxy alcohols, including diethylene glycol, triethylene glycol, tetraethylene glycol, and pentaethylene glycol (all of them have two hydroxyl groups at the two ends of the chain in their molecular structures, as shown in Figure 1), mixed with H2SO4 solution (pH 1), were chosen to be tested as the lubricant. The friction results with the four kinds of mixtures are shown in Figure 8. It is found that the superlubricity can be achieved after a running-in period with the lubrication of these mixtures despite different numbers of carbon atoms in the molecular structure (n = 4, 6, 8, and 10). Therefore, it can be concluded that the number of carbon atoms in the molecular structure probably has no effect on superlubricity. According to the superlubricity mechanism, it can be inferred that all four kinds of glycols can form the hydrogen-bond network to achieve superlubricity. Therefore, the viscosity properties of these four glycols should be the same as 1,2propanediol, 1,3-propanediol, and propanetriol. As shown in Table 3, it is seen that the viscosities of these four glycols are all very low (less than 4 mPa s) with the concentration of 20% and very high (more than 10 mPa s) with the concentration of 100%, which is similar to the viscosity properties in Table 2. Therefore, it is inferred that these glycol solutions without acids can also form an elastohydrodynamic film with the viscosity increasing with the test time. To confirm this inference, the

Figure 8. Friction coefficient with the lubrication of the mixture of polyhydroxy alcohols and H2SO4 solution: (a) diethylene glycol, (b) triethylene glycol, (c) tetraethylene glycol, and (d) pentaethylene glycol. The volume ratio of these polyhydroxy alcohols to H2SO4 solution (pH 1) is 10:1.

friction coefficient with the lubrication of these polyhydroxy alcohols (20%, v/v) without acids is measured, as shown in Figure 9. It is found that all of them can form an elastohydrodynamic film finally, leading to the friction coefficient reducing to about 0.1. According to these results, it can be found that all of these polyhydroxy alcohols that can obtain superlubricity have three main features. They are liquid without volatile, are completely soluble in water, and have more than one hydroxyl group in their molecular structure. First, the non-volatile means that the polyhydroxy alcohol molecules can be held in the contact region in the whole test, which can keep the superlubricity state stable. Second, the solubility in water means that the polyhydroxy alcohol molecules and water molecules can interact with each other, which can provide a favorable condition for forming the hydrogen-bond network. Third, the molecules with polyhydroxyl groups provide the spatial structure to from the hydrogen-bond network with water molecules. It is clear that the three features are in accordance with the superlubricity mechanism. As long as the polyhydroxy alcohols can meet the three features above simultaneously, the superlubricity can be achieved. However, there is no direct relation to the number of carbon atoms (less than 10) and the arrangement of hydroxyl groups in the molecular structure.

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CONCLUSION The work has presented that the superlubricity can be obtained with the lubrication of mixtures of polyhydroxy alcohol and 5243

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Table 3. Viscosities of Diethylene Glycol, Triethylene Glycol, Tetraethylene Glycol, and Pentaethylene Glycol with the Concentrations of 20 and 100% (v/v) viscosity (Pa s) (20%, v/v) viscosity (Pa s) (100%, v/v)

diethylene glycol

triethylene glycol

tetraethylene glycol

pentaethylene glycol

0.0015 0.0279

0.0015 0.0352

0.0017 0.0428

0.0017 0.0505

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Figure 9. Friction coefficient with the lubrication of glycols (diethylene glycol, triethylene glycol, tetraethylene glycol, and pentaethylene glycol) with the concentration of 20%.

acid solutions. The superlubricity mechanism is attributed to forming the hydrogen-bond network between polyhydroxy alcohol and water molecules on the stern layer after the running-in process. The condition for superlubricity is that the number of hydroxyl groups in the molecular structure of polyhydroxy alcohol should be more than one to form the hydrogen-bond network with water molecules. However, the number of carbon atoms and the arrangement of hydroxyl groups in the molecular structure have almost no influence on superlubricity. According to these results, a new superlubricity system based on the mixtures of polyhydroxy alcohol and acid solutions is established, which is very useful for us to search for more kinds of liquid materials with superlubricity properties for technological applications in the future.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 8610-62781385. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Key Basic Research Program of China (2013CB934200), the National Natural Science Foundation of China (NSFC) (51021064, 51222507, and 51075227), and the Academic Scholarship for Doctoral Candidates.

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REFERENCES

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