Fracture of the Intermolecular Hydrogen Bond Network Structure of

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Fracture of the Intermolecular Hydrogen Bond Network Structure of Glycerol Modified by Carbon Nanotubes Yanchao Yin, Liran Ma,* Shizhu Wen, and Jianbin Luo

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State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China ABSTRACT: An intermolecular hydrogen bond network structure is usually formed in the liquid state and affects physicochemical properties, such as melting point, boiling point, and viscosity. The intermolecular hydrogen bond network structure plays an important role in the viscosity of lubricating oil; that is, the broken bond decreases the viscosity. To determine the effect of intermolecular hydrogen bond network structure on viscosity, this study used glycerol, which contains a large amount of intermolecular hydrogen bonds, as a research object. Single- and double-walled carbon nanotubes (SWNTs and DWNTs, respectively) were used as modifiers. The glycerol mixture and the carbon nanotubes were characterized by rheology, Raman spectroscopy, transmission electron microscopy (TEM), 1H NMR, and computer modeling. Results showed that the carbon nanotube modified the intermolecular hydrogen bond network, leading to reduced glycerol viscosity. The two types of nanotubes exhibited varied effects on glycerol viscosity. The SWNTs and DWNTs decreased the viscosity by up to 2.97 and 1.81%, respectively. The Raman, 1H NMR, and TEM results indicated that the intermolecular hydrogen bond network structure was destructed because of the capillary action of the carbon nanotube. Computer simulation also showed that the carbon nanotube had space-limiting function, which could separate the new glycerol molecule clusters from one another to terminate hydrogen bonding in the body phase. decreased by heating,12,13 adding strong polar solvents,14,15 and using ultrasonic processes.16−18 It is widely used because of its excellent mechanical properties,19−22 electromagnetic performance,23,24 and chemical properties.25−29 In this study, we added a series of carbon nanotubes to break the intermolecular hydrogen bond network structure of glycerol. The relationship between the intermolecular hydrogen bond network structure and the viscosity of glycerol was investigated. Liquid paraffin without an intermolecular hydrogen bond was used for comparison.

1. INTRODUCTION Hydrogen bonds include molecular and intermolecular types; the latter is commonly studied and is formed by binding H atoms to a highly electronegative atom X and a highly electronegative atom Y in another molecule. Intermolecular hydrogen bonds are weak, with energy of 25−40 kJ/mol.1,2 The presence of intermolecular hydrogen bonds greatly affects the physical and chemical properties of liquids. These bonds increase the melting point,3 boiling point,4 solubility,5,6 and viscosity7,8 of liquid materials. Ma et al.9 studied the relationship between the molecular structure and the viscosity of four pyridinium ionic liquids; the hydrogen bond formation was relative to the substituted position of pyridinium and affected the viscosity of the liquids. By contrast, the length of the alkyl chain did not affect the viscosity. Yalkowsky10 suggested that the hydrogen bond also affected the melting point of materials, that is, the hydrogen bond strength determines the melting point. In high-viscosity glycerol, each carbon atom of the glycerol molecule contains a hydroxyl group, leading to the easy formation of hydrogen bonds.11 The number of hydrogen bonds increases with increasing number of hydroxyl groups, forming a larger hydrogen bond network structure. The length of the hydrogen bond in short-chain alcohol is shorter than that of long-chain alcohol; as such, the former also has higher bond strength. The hydrogen bond network structure in glycerol is stable and viscous. Decreasing the liquid viscosity is important in industrial production. Liquid viscosity can be © XXXX American Chemical Society

2. EXPERIMENTS In brief, 0.01 g of single-walled carbon nanotubes (SWNTs; Nanjing XFNANO Materials Tech Co., Ltd) was mixed with a certain mass of glycerol through 80 W ultrasound method for 2 h at room temperature. The mixture was set for 16−24 h. The mass ratio of the glycerol and the carbon nanotube was set as 750, 1000, 1250, and 1500. The mixture of glycerol and double-walled carbon nanotubes (DWNTs) was also prepared using the same method. The rheology of the mixture of glycerol and nanotubes was evaluated on Aaton Paar Physica MCR301/302 under airbearing torque of 0.002 μNm to 200 mNm at −150−500 °C. Received: May 24, 2018 Revised: August 8, 2018 Published: August 8, 2018 A

DOI: 10.1021/acs.jpcc.8b04941 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Rheological curves of samples. (a) SWNTs and glycerol, (b) DWNTs and glycerol, and (c) liquid paraffin and SWNTs (or DWNTs).

The detection temperature was 25 °C. Raman spectroscopy was carried out on Horiba LabRAM HR using spectrometer focal length of 800 mm and a large 1024 pixel CCD chip in fast imaging mode. 1H NMR was carried out on JEOL JNMECA600 using dimethyl sulfoxide-d6 as a solvent and configuring 0.5 mol/L detection solution concentration. The microstructure of the infiltrated carbon nanotube was determined by JEM-2100 LaB6 high-resolution transmission electron microscopy (TEM). The highest acceleration voltage was 200 kV. The maximum slant angle was ±25°. Terahertz (THz) nondestructive evaluation was conducted on CIPABCD terahertz time-domain spectrometer with 0.1−10 THz bandwidth in transmission mode using a Petri dish with 0.1 mm diameter. Computer modeling was performed using the adsorption locator module with materials studio software.

DWNTs were 1000 times larger, as shown in Figure 1b. The reduction in the viscosity of the mixture with SWNTs (2.97%) was higher than that of the mixture with DWNTs (1.81%). The capacity of the SWNTs to modulate the intermolecular hydrogen bond network structure was higher than that of DWNTs. This finding could be attributed to the structure of the carbon nanotubes. The smaller volume of SWNT made its dispersion in glycerol more uniform. The newly formed small glycerol clusters were fully isolated from each other, which hindered the formation of intermolecular hydrogen bonds between glycerol clusters. It could maximize its space-limiting action and was better than that of DWNT. The modulation capacity was detected at a certain proportion. By contrast, scholars have reported that the viscosity of lubricating grease30,31 increases with the addition of carbon nanotubes. As shown in Figure 1c, in the absence of the intermolecular hydrogen bond network, the viscosity of glycerol was larger than that of pure liquid paraffin after adding the carbon nanotubes. The Raman spectra of the carbon nanotubes in the mixture were investigated to study the mechanism of these nanocomposites in regulating the intermolecular hydrogen bond network structure. A certain interaction was found between the carbon nanotubes and the adsorbed glycerol molecule. This interaction allowed the glycerol molecule to be adsorbed stably on the internal and external surfaces of the carbon nanotube. The initial glycerol cluster was destroyed, and the intermolecular hydrogen bond network structure was broken further.

3. RESULTS AND DISCUSSION The addition of the carbon nanotubes destroyed the intermolecular hydrogen bond network structure of glycerol and reduced its viscosity. Figure 1a,b shows the rheological curves of the two mixtures. The modulation capacity of the intermolecular hydrogen bond network structure differed because of the different structures and qualities of the two types of the carbon nanotubes, resulting in the reduced viscosity of the mixture at 25 °C. As shown in Figure 1a, the viscosity of the mixture began to decrease when the ratio of glycerol to SWNT was 750:1. The B


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Figure 2. Raman spectra of SWNTs and DWNTs in glycerol.

Figure 3. 1H NMR of samples. (a) 1H NMR of mixtures by adding SWNTs, (b) 1H NMR of mixtures by adding DWNTs.

Figure 2 shows the Raman spectra of SWNTs and DWNTs in

The basic rationale of NMR was closely related to the energy-level transition of nuclear nuclei, and it could reflect the physical properties of matter from a microscopic perspective. Glycerol and mixtures were detected by 1H NMR, as shown in Figure 3. The peak of the intramolecular hydroxyl hydrogen

glycerol. The peak positions of both nanotubes red shifted. This finding further suggested that the glycerol molecules were adsorbed by the carbon nanotubes. C


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Figure 4. TEM of infiltrated carbon nanotubes.

Figure 5. THz of samples. (a) Absorption coefficient, (b) transmissivity.

was near 4.4 ppm by the 1H NMR standard spectrum of glycerol. As shown in Figure 3a, the peak of hydroxyl hydrogen was shifted after adding SWNTs. Compared with that of pure glycerol, the chemical shift of hydroxyl hydrogen moved in a small direction. It was shown that the spin motion of hydroxyl hydrogen was weakened after adding SWNTs. There must be an interaction between the glycerol intermolecular hydrogen bonds and SWNTs. As shown in Figure 3b, the peak of hydroxyl hydrogen was also shifted after adding DWNTs. The peak shape was not the same as that of SWNTs. That was due to the double-layer structure of DWNTs. The spin motion of hydroxyl hydrogen was influenced by varying degrees, which made it have three peaks. The spin motion of hydroxyl hydrogen was also weakened after adding DWNTs. SWNTs and DWNTs in glycerol were analyzed by TEM. The carbon nanotubes mainly modulated the intermolecular

hydrogen bond network structure by adsorption. As shown in Figure 4, comparing with the TEM of pure SWNTs and pure DWNTs, the glycerol molecule existed in the carbon nanotubes by capillary force after ultrasonic mixing. Few glycerol molecules were absorbed on the surface defect position of the carbon nanotubes. These nanotubes broke the original intermolecular hydrogen bond network structure and reduced the size of the glycerol molecular clusters. This process reduced the number of hydrogen bonds and the viscosity of glycerol. Liquid paraffin without an intermolecular hydrogen bond was used for comparison to confirm the ability of the carbon nanotubes in regulating the intermolecular hydrogen bond. The samples were analyzed by THz. The THz absorption coefficient and transmissivity of glycerol differed from those of liquid paraffin. The carbon nanotubes evidently modulated the intermolecular hydrogen bond. As shown in Figure 5a,b, D


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Figure 6. Adsorption model of SWNTs.

Figure 7. Adsorption model of DWNTs.

A cavity was formed between the glycerol molecule and the carbon nanotube in the body phase because of the strong hydrophobicity of the tubes. The well-distributed carbon nanotubes played a space-limiting role. The newly formed small glycerol molecular clusters were separated from one another by the carbon nanotubes. This space confinement ability destroyed the hydrogen bond network structure. Consequently, the number of the hydrogen bonds was further reduced. Computer simulation showed that the H···O distance (0.99 Å) of glycerol in samples was shorter than that (1.11 Å) of pure glycerol. The short H···O distance of glycerol decreased the binding ability of intermolecular hydrogen bonds, which was reflected by Raman analysis. The HO···O angle (88.8° by adding SWNTs and 55.4° by adding DWNTs) of glycerol in samples was less than that (100.2°) of pure glycerol. The space-limiting action of DWNTs was stronger than that of SWNTs. The HO···O angle (55.4°) of glycerol with DWNTs was less than that (88.8°) with SWNTs. The reducing effect of DWNTs on the viscosity of glycerol was greater than that of SWNTs. This finding could be due to the larger volume and more surface defect positions of DWNTs. However, in actual research, the low purity of DWNTs (60%) made it less effective in reducing glycerol viscosity than high-purity SWNTs (>95%).

glycerol with SWNTs exhibited lower absorption coefficient and higher transmissivity than glycerol alone. However, the results from liquid paraffin were opposite. Hence, the carbon nanotubes reduced the number of intermolecular hydrogen bonds in the body phase and the viscosity of glycerol. As such, glycerol reduced the absorptive capacity of the spectrum and increased the transmittance. The carbon nanotubes were considered a thickening agent for liquid paraffin without intermolecular hydrogen bonds. Liquid paraffin increased the absorptive capacity of the spectrum and reduced the transmittance. An adsorption model of the carbon nanotubes to the glycerol molecule was established to verify the modulation mechanism of an intermolecular hydrogen bond. The calculation results are shown in Figures 6 and 7. The model parameter setting of SWNT was (13, 1), which belonged to chiral carbon nanotubes. Its repeat units were 2. Its diameter was 10.59 Å, and its length was 115.26 Å. The parameter setting of DWNT was the same as that of SWNT. Its interlayer distance was 3.35 Å. The number of the glycerol molecules in this adsorption model was 200. As shown in Figure 6, a part of the glycerol molecule was absorbed into SWNTs, but the absorption on the surface defect position of SWNTs was not found because of the complete structural model. The absorption on the surface defect position of SWNTs was judged by the Raman peak shift and TEM analysis. As shown in Figure 7, the glycerol molecule existed in the inner tube but not in the cavity between two tubes. Similar to those of SWNTs, the adsorption on the surface defect position was only judged by the Raman peak shift and TEM analysis. In practical experiments, the glycerol molecule in the inner tube could be confirmed by TEM.

4. CONCLUSIONS This paper presented a new physical method for regulating intermolecular hydrogen bonds. The modulation of the intermolecular hydrogen bond reduced the viscosity of glycerol when SWNTs or DWNTs were added at a certain proportion. E


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The action mechanism of the carbon nanotubes was investigated by experiment and computer simulation. The hydrogen bond network structure of glycerol was destroyed by the adsorption of the carbon nanotubes and space confinement, resulting in reduced number of intermolecular hydrogen bonds and lower glycerol viscosity. The proposed method overcomes the limitations of physical methods in permanently reducing the viscosity of liquids. The method opens up a new research direction for permanent viscosity reduction.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Liran Ma: 0000-0002-7186-6659 Jianbin Luo: 0000-0002-5132-0712 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the National Natural Science Foundation of China (51675297, 51527901). REFERENCES

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Fracture of the Intermolecular Hydrogen Bond Network Structure of

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