Microstructure and Tribological Properties of Lamellar Liquid Crystals

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Microstructure and Tribological Properties of Lamellar Liquid Crystals Formed by Ionic Liquids as Cosurfactants Liping Chen, Lingling Ge, Lei Fan, and Rong Guo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04144 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Microstructure and Tribological Properties of Lamellar Liquid Crystals Formed by Ionic Liquids as Cosurfactants

Liping Chena, b, Lingling Gea, Lei Fana,* and Rong Guoa,*

aSchool

of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, P. R. China.

E-mail: [email protected]; [email protected] bDepartment

of Chemical Engineering, Yangzhou Polytechnic Institute, Yangzhou 225127, P. R. China.

KEYWORDS: lamellar liquid crystals, cosurfactant, molecular order, friction-reducing, anti-wear

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ABSTRACT：The lamellar liquid crystals (LLCs) have been shown lubrication properties in many documents due to their bilayers structure. Ionic liquids are often used as additive or surfactant in LLCs. However, ionic liquids used as cosurfactants, which lead to a transition from the hexagonal liquid crystals to LLCs, are relatively rare. Herein, the microstructure of Triton X-100/CnmimNTf2/H2O LLCs formed by using 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid (CnmimNTf2 n = 8, 12, 16) as cosurfactant has been determined by polarized light microscopy, small angle X-ray scattering and 2H NMR, and their rheological and tribological properties were investigated. These LLCs show good friction-reducing and anti-wear performance. The correlation between the microstructure of the LLCs and their lubricating mechanism is established. The increase of the concentration of CnmimNTf2 and the length of alkyl chain in the LLCs can lead to an obvious reduction in friction coefficients and wear volumes, which are attributed to the higher order of amphiphilic molecules, thicker of the amphiphilic bilayer and the smaller cross-sectional area of the polar head group at the hydrophilic and hydrophobic interface. The protective film formed by the physical adsorption of ionic liquid LLCs on the surface of friction disk pair and the tribochemical reaction have effectively promoted the lubrication effect. The good lubricating property and anti-wear capability indicates their promising and potential applications in water lubrication and biological lubrication.

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INTRODUCTION Lamellar liquid crystals (LLCs) are usually composed of three components, viz. surfactant, cosurfactant and water.1 The cosurfactant often comprises aliphatic alcohol with a carbon chain of medium length.2 The cosurfactant can influence the curvature radius and bending direction of surfactant molecular aggregates by changing the apparent structural parameters of the surfactants, such as the cross-section area of polar groups and the length and volume of hydrophobic groups.3 For example, both of anionic surfactant sodium dodecyl sulfonate (SDS) and cationic surfactant cetyl trimethyl ammonium bromide (CTAB) can form hexagonal liquid crystals in water. It could be changed to LLCs when the cosurfactant n-C10H21OH was added.4,5 Similarly non-ionic surfactant Triton X-100 and water can also produce hexagonal liquid crystals, and the addition of n-C10H21OH leads to a transition from the hexagonal liquid crystals to LLCs.6 Therefore, the cosurfactant plays a key role in the formation and stability of the LLCs, and thus, it is important to select an appropriate cosurfactant in the preparation and application of LLCs. LLCs composed by self-assembled surfactant bilayers with molten alkyl chains have periodic bilayer structure showing alternative amphiphile bilayer and water layer. These LLCs have great potential applications in the preparation of nanomaterials,7-9 drug carrier,10-13 and lubrication materials.14-16 Because of long order, these LLCs have a superior compressive performance and can be used not only to reduce friction and prevent direct contact between the two contact surfaces, but also reduce shear resistance due to relatively lower viscosity in the sliding shear direction. Therefore, LLCs can be used as a lubricating material with friction-reducing and anti-wear properties.17 An ionic liquid is a molten salt at room temperature with excellent chemical and physical properties, such as non-flammability, negligible vapor pressure, high thermal stability and electrochemical properties.18 Compared with traditional organic solvents, ionic liquids are considered as green solvents with advantages of being less volatile, pollution-free, easy to separate from products, and recyclable.19, 20 In 2001, Ye et al. discovered that the 3 / 27


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ionic liquids could work as a multi-purpose lubricant with excellent lubrication performance.21 In our previous work, we combined ionic liquid CnmimPF6 as an additive with the LLCs of Triton X-100/C10H21OH/H2O and studied the influence of CnmimPF6 (its content and alkyl chain length) on the structure and lubrication performance of Triton X-100/n-C10H21OH/H2O LLCs.22 The results show the improved lubricating property of the CnmimPF6 related LLCs, which may be attributed to larger interlayer thickness d. Although ionic liquids as a surfactant have been extensively studied because their molecular structures are similar to traditional surfactants.23-26 However, it is still rare to study LLCs using ionic liquids as cosurfactants and their lubrication applications, in which the ionic liquids leads to a transition from the hexagonal liquid crystals to LLCs and contain the lubrication properties. In this study, we report the LLCs prepared using CnmimNTf2 as the cosurfactant in the Triton X-100/CnmimNTf2/H2O system. The effects of the content of CnmimNTf2 and its alkyl chain length on the structure and improved lubricating property of the LLCs are studied. The lubrication mechanism of the LLCs is further discussed on the basis of experimental results obtained from physical adsorption using 2H NMR technique and surface composition analysis using XPS. EXPERIMENTAL SECTION Materials. Triton X - 100 (Aldrich, 99%), CnmimNTf2 (n = 8, 12, 16) (Shanghai chengjie ionic liquid co., LTD., 98%), and Heavy water (Sigma, 99%) were used in our experiments. Figure S1 shows the molecular structure of CnmimNTf2 (n = 8, 12, 16). All materials stated above were used without further purification. Deionized water was used through the experiment. Sample preparation. Samples were prepared by mixing Triton X-100, CnmimNTf2 (n = 8, 12, 16) and H2O with designed compositions (in weight percent, wt%, thereinafter). These mixtures were fully homogenized by stirring and centrifugation for a few times. Then, they were kept in an incubation bath with constant temperature at 25 ± 0.1 ◦C

for more than 2 weeks.

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Characterizations. Photographs of sample birefringence were taken by using a Leica DMLP microscope with a DFC320 CCD camera and cross polarizers. Small angle X-ray scattering (SAXS) measurements were performed at 25 ◦C using an Anton Paar SAXSess system with a Ni-filtered Cu Ka radiation (0.1542 nm) operated at 50 kV and 40 mA. The distance between the sample and detector was set at 264.5 mm. Deuterium nuclear magnetic resonance (2H NMR) experiments were conducted with a Bruker AV-600 NMR spectrometer equipped with a pulsed-field gradient module (z axis). D2O instead of H2O was used in the 2H NMR experiments. The quadrupole splitting Δν was measured as the peak to peak distance and reported in Hertz.27 The rheological properties were measured on a Haake RS600 rheometer equipped with a DC50 temperature controller (water circulating bath). A core-plate sensor was used with a diameter of 35 mm and a cone angle of 1°. The distance between sensor and cone plate was adjusted to 0.054 mm for all measurements. The linear viscoelastic region was first determined from stress sweep measurements, where the stress (τ) was variable, but the frequency was kept at 1.0 Hz. And 1 Pa was then chosen as stress amplitude for frequency sweep measurements to achieve the least disturbance of the internal structures. The frequency varied from 0.01 rad/s to 100 rad/s. The temperature was kept at 25 ± 0.1 °C. The chemical composition of the worn scars was analyzed in a X-ray photoelectron spectroscope (XPS, ESCALAB250Xi, ThermoFisher Scientific) using Al-Kα irradiation as the exciting source and the X-ray spot size of 650 μm. To determine the binding energies of the target elements, contaminated carbon with the binding energy of 284.8 eV (C1s) was used as reference in the XPS experiment with the passing energy of 29.35 eV and the resolution of approximately ± 0.3 eV. Tribology tests. The tribological properties of LLCs formed in Triton X-100/CnmimNTf2/H2O system as lubricants were conducted at room temperature using a UMT-2 tribometer which applies steel/steel contacts with a reciprocating ball-on-disk configuration. The AISI 52,100 steel ball has a diameter of 5 mm, the hardness of approximately 61-66 HRC and the roughness of 0.02 mm. The steel disk is 0Cr17Ni12Mo2 with the diameter of 18 5 / 27


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mm. The normal load was set to 10N and the reciprocating frequency was 5 Hz for all tests. After the friction test, the wear volumes of the lower disks were characterized and calculated by using a GT-K 3D noncontact surface mapping profiler. RESULTS AND DISCUSSION Partial Phase Diagram with LLCs. Figures 1a-c show the partial phase diagrams of Triton X-100/CnmimNTf2/H2O systems with n=8, 12 and 16, respectively. In the diagrams, the black thick lines on the Triton X-100/H2O line are the areas of forming hexagonal liquid crystals. The areas of solid lines containing the sample points correspond to the single-phase of Triton X-100/CnmimNTf2/H2O LLCs. These LLCs were observed either as linear oil streaks or in a cross petal shape under a polarized light microscope. Figures 1d-f show the polarization microscopy photographs (POM) of these LLCs, all of which have a Maltese cross petal shape, indicating the formation of LLCs.28, 29 It can be clearly seen from the phase diagram that CnmimNTf2 has a very low solubility (1wt%) in the hexagonal crystals in Triton X-100/H2O system. However, when CnmimNTf2 is used as cosurfactant, a transition from hexagonal liquid crystal phase to LLCs phase could be realized. The LLCs region in the phase diagram of Triton X-100/CnmimNTf2/H2O systems increases significantly from the C8mimNTf2 LLCs to C12mimNTf2 LLCs, while the change is not obvious when the alkyl chain length increase from 12 to 16.

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(a)

(b)

(c)

Figure 1. (a-c) Partial phase diagrams of Triton X-100/CnmimNTf2/H2O system at 25 o C. The areas of solid lines containing sample points correspond to the single-phase of Triton X-100/CnmimNTf2/H2O LLCs. The POM graphs of Triton X-100/CnmimNTf2/H2O LLCs, where (d) n=8, (e) n=12, (f) n=16. SAXS Measurements. Figure 2a shows the structure diagram of the LLCs formed in polyoxyethylene surfactant system, where interlayer spacing d includes amphiphilic bilayer thickness d0 and solvent layer thickness dw. The amphiphilic bilayer is mainly composed of the surfactant Triton X-100 (in blue) and cosurfactant CnmimNTf2 (in red). The solvent water could exist in three locations of the LLCs system. It may remain in the solvent layer of LLCs. It may also be located at the interface between amphiphilic bilayer and the solvent layer, or permeate into the amphiphilic bilayer.

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(a)

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(b)

Triton X- 100

CnmimNTf2

Figure 2. (a) Structural diagram of the Triton X-100/CnmimNTf2/H2O LLCs with key parameters and (b) SAXS patterns of Triton X-100/C12mimNTf2/H2O LLCs with different H2O concentrations (curve a- 29, b- 32, c -35, d -38 wt%) at a fixed weight ratio (0.3) of C12mimNTf2 to Triton X-100. The solvent water penetrating into the bilayer can affect the interlayer thickness d. And the relationship between the interlayer thickness d and the penetrated water follows this equation: 30, 31 d = d0 [1 + 1(1 − α)R]

(1)

Here R means the volume ratio of water to the other components, α is the fraction of solvent water penetrating into the bilayer. d can be measured by the the SAXS method, and d0 can be obtained by extrapolation. The term α can be calculated from the following equation: 30, 31 α = 1 − (∂d/∂R)/d0

(2)

SAXS technique was used to reveal the internal microstructure of the LLCs and determine a series of structural parameters. Figure 2b displays typical SAXS patterns of Triton X-100/C12mimNTf2/H2O systems with different H2O concentrations and a fixed weight ratio 0.3 of C12mimNTf2 to Triton X-100. Each pattern has only two scattering peaks. For the sample with H2O concentration at 29 wt%, the two scattering peaks have been detected with the scattering vectors (q) located at 0.130 Å-1 and 0.259 Å-1, respectively. The ratio of q1 to q2 is calculated to be 1: 2, which means that q1 and q2 are the scattering vectors of the 1st order and 2nd order scattering peaks 8 / 27


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respectively, indicating the formation of LLCs.32-34 The interlayer spacing d can be calculated using the first-order scattering peak q1 and the Bragg’s equation (d=2π/q1). Figures 3a-c show the linear relationship between the interlayer spacing d of Triton X-100/CnmimNTf2/H2O LLCs (n=8, 12 and 16) and solvent (water) content R for different weight ratios (0.2, 0.25 and 0.3) of CnmimNTf2 to Triton X-100. When the straight lines are extrapolated to R=0 or zero water content, the thicknesses of the amphiphilic bilayer d0 can be obtained from the figures according to formula (1). The d0 values are summarized in Table 1. The thickness of the solvent layer dw can be calculated by using the equation dw＝d-d0, and the relationships between solvent layer thickness dw and the solvent (water) content R are further plotted in Figure 4, where the linear relationships remain, but all lines meet at the origin, that is, when R=0, dw=0. The slopes of the linear lines in Figures 3a-c may be used to extract the values of the water permeability α in the amphiphilic bilayer according to equation (2) and these values are shown in Table 1. From Figures 3 and 4, we may see how different factors, such as water content and the ratio of CnmimNTf2 to Triton X-100, play their roles in structure parameters of the LLCs. For a specific CnmimNTf2 and a fixed mass ratio of CnmimNTf2 to Triton X-100, both the interlayer spacing d (Figures 3a-c) and the thickness of the solvent layer dw (Figures 4a-c) increase with water content. Therefore, the increase of the interlayer spacing d is mainly due to the increase of solvent layer thickness dw. This means that the LLCs system conforms to one-dimensional swelling mechanism,35 that is, water affects the LLCs lattice parameters by changing the thickness of the water layer. For water content fixed, the increase of the mass ratio of CnmimNTf2 to Triton X-100 by increasing the content of CnmimNTf2 may result in the increase of d0 (Table 1) and the decrease of dw (Figures 4a-c). With the higher content of CnmimNTf2, the order degree of Triton X-100 molecules becomes higher, leading to larger effective molecule length which is about (1/2) d0. This result is different from our previous research result of the Triton X-100/C10H21OH/CnmimPF6/H2O system, where the ionic liquid CnmimPF6 was used an additive added to the LLCs of Triton X-100/C10H21OH/H2O. The d0 value decreases with CnmimPF6 concentration is that CnmimPF6 9 / 27


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increases the bending and entanglement of the Triton X-100 molecules and leads to the order degree of Triton X-100 molecules becoming lower. Meanwhile, the solvent permeability fraction becomes higher (Table 1), leading to the slightly smaller solvent layer thickness dw (Figures 4a-c). As the d0 value changes more significantly than the dw value, so the interlayer spacing d, i.e. the sum of d0 and dw , increases with the CnmimNTf2 content (Figures 3a-c). As for a specific water content and a fixed mass ratio of CnmimNTf2/Triton X-100, the dw value (Figures 4d-f) becomes smaller as the n or the length of the alkyl chain carbon chain of ionic liquid increases. This is due to the increase of the permeability of water molecules in the amphiphilic bilayer (Table 1). Meanwhile, the thickness of amphiphilic bilayer d0 increases (Table 1) with n. As discussed above, the increase of d0 value plays a major role in the change of interlayer spacing, so the interlayer spacing d increases with the ionic liquid alkyl chain length.

Figure 3. Relationship between the interlayer thickness d and the volume ratio of solvent to the other components R in Triton X-100/CnmimNTf2/H2O LLCs with constant weight ratio of CnmimNTf2 to Triton X-100.

Table 1. Interrelated Parameters of Lamellar Liquid Crystals in Triton X-100/CnmimNTf2/H2O System CnmimNTf2/ Triton X-100 (wt/wt)

C8mimNTf2

C12mimNTf2

C16mimNTf2

d0(nm)

α(%)

d0(nm)

α (%)

d0(nm)

α(%)

0.20

3.51

22.8

3.85

36.4

3.96

37.6

0.25

3.66

28.9

3.94

37.6

4.13

45.1

0.30

3.74

31.8

4.01

39.2

4.28

50.2

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Figure 4. Relationship between solvent layer thickness dw and the volume ratio of solvent to the other components R in Triton X-100/CnmimNTf2/H2O LLCs with constant weight ratio of CnmimNTf2 to Triton X-100. The molecular cross-section area A of polar head group of the Triton X-100, which is associated with the hydrophilic and hydrophobic interface of the LLCs, can be calculated using following equation .36, 37 A=2×1024M/ρNd

（Å2/molecule）

（3）

Where, M is the molar mass of the surfactant, ρ denotes the density of surfactant, N is the Avogadro constant， and d is interlayer spacing. It's easy to calculate the value of A using the value of d. Considering the relationship between d value and the concentration of ionic liquid CnmimNTf2, we can deduce the relationship between A and the concentration of ionic liquid CnmimNTf2. Figure 5a shows cross-section area A which varies with the concentration of the ionic liquid CnmimNTf2. The apparent molecular cross-sectional area A of Triton X-100 polar head group which ends at the hydrophilic and hydrophobic interface decreases as the content of CnmimNTf2 increases. It also decreases with the increase of n, the alkyl chain length of the ionic liquid CnmimNTf2. A possible explanation for this result is that CnmimNTf2 may reduce the bending and entanglement of Triton X-100 molecules 11 / 27


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and improve the molecular orderliness, which leads to a decrease in the apparent cross-sectional area A of the Triton X-100 molecule.

800

700

600

500

400

300

200

100

0

-100

15.0 ppm (t1)

10.0

5.0

0.0

-5.0

Figure 5. (a) The area per polar group at the hydrophilic and hydrophobic interface A and (b) the quadrupole splitting Δν as a function of CnmimNTf2 concentrations at the constant weight ratio 1.25 of Triton X-100 to H2O in Triton X-100/CnmimNTf2/H2O LLCs. Insert in (b) is the typical

2H

NMR spectrum of Triton

X-100/C16mimNTf2/H2O LLCs. 2H

NMR Measurements. 2H NMR measurements may be used to measure the order degree S of the LLCs. The

type of the quadrupole splitting can be learnt from the distance between the two absorption peaks in 2H NMR graphs. The value Δν of the 2H NMR quadrupole splitting of LLCs can be expressed as38, 39 Δν = (3/4) QS

(4)

Here, the Δν is the frequency difference between two neighboring peaks in 2H NMR graph, Q is a quadrupole coupling constant of the deuterium nucleus in the OD bond in D2O (220 kHz). S is an order parameter of the bound water molecule, given by S =1/2(3cos2–1), where θ is the time-average angle between the symmetry axis of the mesophase and the electric-field gradient. The greater the value of S, the greater the order of molecule in the LLCs. Usually the value of Δν can be directly used to reflect the ordered state of the LLCs molecules because it is sensitive to phase anisotropy.40-44 The insert of Figure 5b is the

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2H

NMR spectrum of Triton

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X-100/C12mimNTf2/H2O system. The red line in Figure 5b shows the Δν values vs the concentrations of ionic liquid C12mimNTf2 at a fixed mass ratio of Triton X-100 and H2O. The higher the Δν values, the higher the molecular order in the LLCs of Triton X-100/C12mimNTf2/H2O system. So it is concluded that the order degree of Triton X-100 and C12mimNTf2 molecules in the amphiphilic bilayer of the LLCs increases with the concentration of C12mimNTf2. For other systems, the same conclusions can be drawn from Figure 5b as well. For the same concentration of ionic liquids, Δν values increase with the increase of the alkyl chain length of the ionic liquids, which means that the order of molecules in the amphiphilic bilayer of the liquid crystal increase with the alkyl chain length of the ionic liquids. This result confirms that the d0 value of Triton X-100/C12mimNTf2/H2O LLCs system in Table 1 is related to the molecular order degree in the amphiphilic bilayer, and the ionic liquid content and alkyl chain length are two major factors to affect the d0 value. Rheological Properties. The rheological properties of the Triton X-100/CnmimNTf2/H2O LLCs were measured for different concentrations of the ionic liquid CnmimNTf2. These LLCs are proved to be viscoelastic materials and their properties are closely related to the content and the alkyl chain length of CnmimNTf2. The frequency sweep measurement results of elastic modulus (G΄), viscous modulus (G˝) and complex viscosity (|η*|) of these LLCs are displayed in Figures 6a-c. One can clearly see that G΄ is almost independent of the applied angular frequency (ω) and there exists a minimum point in the G˝ curve. The complex viscosity (|η*|) decreases linearly within the whole measured ω range, which is the typical behaviour of LLCs.45, 46 The curves in Figures 6a-c indicate that values of the the physical quantity G΄, increase with the content and alkyl chain length of CnmimNTf2, which means the mechanical strength of the LLCs become stronger. This is ascribed to the decrease in apparent molecular cross-sectional area A of Triton X-100 polar head group ended at the hydrophilic and hydrophobic interface (Figure 5a) and is consistent with what Mohamed Ahmed Siddig47 pointed out, i.e. the smaller the cross-sectional area of the amphiphilic molecules, the stronger the structural strength of the liquid 13 / 27


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crystal.

Figure 6. (a)-(c) Elastic modulus (G΄), viscous modulus (G˝) and complex viscosity (|η*|) of Triton X-100/CnmimNTf2/H2O LLCs measured as a function of angular frequency ω for different concentrations (10, 12, 14 and 16 wt%) of CnmimNTf2 at the constant weight ratio 1.25 of Triton X-100 to H2O, where (a) n=8, (b) n=12, (c) n=16. (d) Viscosity hysteresis loops of Triton X-100/CnmimNTf2/H2O LLCs measured when a steady shear was applied. The mechanical strength of the LLCs can also be evaluated by the yield stress (τ) and storage modulus (G΄) at τ obtained from stress sweep measurements. The yield stress refers to the critical shear stress value at which the LLCs structure is destroyed. As clearly seen from Figure S2, the increase of either concentration or alkyl chain length of CnmimNTf2 result in the high mechanical strength, which are ascribed to the stiffer bilayers with high heterogeneity and long range order. To check whether the viscoelasticity of the samples can be recovered after the shear strain is removed, the shear 14 / 27


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rate was set in a cycle variation mode by first increasing the rate to 1000 and then decreasing the rate to 0.001, and the thixotropy of the LLCs was observed as shown in Figure 6d. Within the test time (about 30 minutes), the viscoelasticity of all the samples was almost completely restored. The reversibility of the viscoelastic and shear thinning state in the shear process, a characteristic property of a lubricant,48 indicate that the Triton X-100/CnmimNTf2/H2O LLCs have a good lubrication performance. Tribological Properties. The ionic liquid LLCs with lamellar structure and thixotropy are promising lubricating materials. Tribological tests for steel/steel disk contacts using Triton X-100/CnmimNTf2/H2O LLCs as lubricants were carried out at 25 ℃ under a constant load of 10 N and a frequency of 4 Hz. As a reference, tribological tests were first conducted on CnmimNTf2/H2O and Triton X-100/H2O samples separately under the same conditions. Curves a and b in Figure 7a show the time dependent friction coefficients of the sliding disks lubricated with CnmimNTf2/H2O and Triton X-100 /H2O, respectively, both of the friction coefficients are relatively high with significant fluctuation during the friction process. In contrast, the friction coefficient curves (curves c-e) associated with the Triton X-100/CnmimNTf2/H2O LLCs are relatively stable. When Triton X-100/CnmimNTf2/H2O LLCs have the weight ratio of 49:12:39, the friction coefficients may vary between 0.13 and 0.15 (curves c-e), which are much smaller than those of CnmimNTf2/H2O (0.3-0.4) and Triton X-100 /H2O (0.16-0.18) systems. For each Triton X-100/CnmimNTf2/H2O system, where n=8, 12 and 16, respectively, the increase of CnmimNTf2 ionic liquid concentration may monotonically reduce the LLCs related friction coefficients (Figure 7b). Moreover, for each fixed concentration of the ionic liquids, the friction coefficients decrease with the length of alkyl carbon chain of the ionic liquids, that is, C12mimNTf2 LLCs have smaller friction coefficient than the C8mimNTf2 LLCs and C16mimNTf2 LLCs have the smallest friction coefficient (0.136) when the concentration reaches the highest one (16 wt%) in our experiments (Figure 7b). In summary, the LLCs related friction coefficients decrease with the concentration of CnmimNTf2 ionic liquids and the length of the alkyl carbon chain (n) of CnmimNTf2. 15 / 27


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Figure 7. (a) Friction coefficients of steel discs lubricated by a-CnmimNTf2 + H2O, b-Triton X-100 + H2O, c-LLCs of Triton X-100/C8mimNTf2/H2O, d-LLCs of Triton X-100/C12mimNTf2/H2O and e-LLCs of Triton X-100/C16mimNTf2/H2O. (b) Friction coefficients of steel discs lubricated by Triton X-100/CnmimNTf2/H2O LLCs at a load of 10 N and a frequency of 4 Hz. CnmimNTf2 concentrations are respectively 10, 12, 14 and 16 wt% at the constant weight ratio 1.25 of Triton X-100 to H2O in the LLCs. The improvement of the friction-reducing performance with the increase of ionic liquid concentration and the length of alkyl carbon chain can be correlated with LLCs microstructures. As indicated above, the increase of ionic liquid concentration and the length of alkyl carbon chain can result in the larger thickness of the amphiphilic bilayer d0 (Figure 3), improve the order degree S of the amphiphilic bilayer molecules in the LLCs, and reduce the cross-sectional area A of Triton X-100 polar head group at the hydrophilic and hydrophobic interface. The improvement of molecular order in LLCs may be the fundamental reason for the improvement of friction-reducing performance. Meanwhile, the increase of ionic liquid concentration and the length of alkyl carbon chain may also enhance solvent permeability α and reduce the solvent layer thickness dw, which is also helpful for the improvement of lubrication performance.15, 16 Therefore, in Triton X-100/CnmimNTf2/H2O systems, the increase of d0 value of bilateral bilayer thickness and the decrease of dw of solvent layer thickness play a synergistic role in friction-reducing performance enhancements. 16 / 27


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a

b

e

d

c

f

g

h

Figure 8. (a-d) Three-dimensional optical microscopy images and (e-h) corresponding cross-sectional profiles of the worn surfaces, where Triton X-100/H2O non-LLCs (a, e), Triton X-100/C8mimNTf2/H2O LLCs (b, f); Triton X-100/C12mimNTf2/H2O LLCs (c, g) and Triton X-100/C16mimNTf2/H2O LLCs (d, h) were used. In order to further explore the anti-wear performance of ionic liquid LLCs, the wear volume of the bottom fixed steel block was measured (Figures 8 and 9). After the friction tests, obvious marks on the grinding surface were observed. Figure 8 shows a group of 3D optical microscopy images (Figures 8a-d) and corresponding cross-sectional contour curves (Figures e-h) of the worn block surface tested at room temperature and lubricated with Triton X-100/H2O non-LLCs and Triton X-100/CnmimNTf2/H2O LLCs. The wear volume reaches 3.06106 m3 for the sample lubricated by the Triton X-100/H2O non-LLCs (Figure 8a) and the wear mark width is 0.49 mm (Figure 8e). Using Triton X-100/CnmimNTf2/H2O LLCs, the wear mark widths are reduced to about 0.43 mm, 0.35 mm and 0.3 mm, corresponding to Triton X-100/C8mimNTf2/H2O, Triton X-100/C12mimNTf2/H2O and Triton X-100/C16mimNTf2/H2O LLCs (Figures 8f-i) respectively. The wear volumes are reduced with the increase of alkyl chain length in ionic liquid LLCs (Figures 8b-d and Figure 9) as well. Their experimental values are in the range of 0.6-2.5106m3, as illustrated in Figure 9. Figure 9 also displays the wear volume changes with the concentration 17 / 27


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of the ionic liquid in the LLCs. Triton X-100/C16mimNTf2/H2O LLCs with 16wt% of C16mimNTf2 show the best anti-wear performance and the corresponding wear volume is about 0.6106m3 which is five times lower than the value of the non-LLC lubricant. This indicates that LLCs are physically adsorbed on the surfaces of friction disks and form an effective boundary lubrication film during the friction process. For LLCs with higher concentration of ionic liquids and larger alkyl carbon chain, their enhanced molecule order leads to their improved structural strength, which thus is helpful to reduce the wear on the friction pair surfaces lubricated with the ionic liquid LLCs.

C8mimNTf2 LLCs

C12mimNTf2 LLCs

C16mimNTf2 LLCs

Triton X-100/H2O non-LLCs

Figure 9. Wear volumes of steel discs lubricated by Triton X-100/CnmimNTf2/H2O LLCs with different CnmimNTf2 concentrations: 10, 12, 14, 16 wt% and Triton X-100/H2O non-LLCs at a load of 10 N and a frequency of 4 Hz. Worn Surface Characterization. To understand the lubrication mechanism between the steel/steel friction pairs, the worn surface lubricated with Triton X-100/CnmimNTf2/H2O LLCs was analyzed by X-ray photoelectron spectroscopy (XPS). The XPS results enable us to understand the chemical state and chemical composition of characteristic elements on the worn surface, and propose a possible lubrication mechanism. Figure 10a shows the full range XPS spectrum of the test block surface which has been worn under the UMT friction at room temperature, revealing all elements on the surface. The absorption peak at 684-685.4 eV (Figure 10b) indicates that F ions are present on the worn surface, perhaps in the form of FeF2 or FeF3. 49, 50 Another absorption peak at 689.0 eV is due to a covalent compound with C-F bond.50 In order to determine the chemical state of Fe or whether Fe and F 18 / 27


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elements form FeF2 or FeF3 on the worn surface, Fe2p related XPS peaks (Figure 10c) have been studied. The absorption peaks at 711.4, 711.3 and 724 eV are associated with FeF2, FeF3, Fe(OH)O and Fe2O3,51 respectively. Figure 10e displays the absorption peaks of N element on the friction surfaces lubricated with different LLCs. These peaks are in the range of 399.7-400.2 eV and look relatively similar for the LLCs. It could be inferred that the C-N bonds or amines are generated on the worn surface.52, 53 This means that the ionic liquid LLCs are possibly physically adsorbed on the surfaces of the friction disk pair. All the absorption peaks of O appear at 532.0 eV (Figure 10d). Combining with S2p (Figure 10F) around 168.7eV, one concludes that there are iron sulfate and ferrous sulfate on the worn surface.54, 55 The curve d in Figure 10 is the element spectrum obtained from Triton X-100/H2O non-LLCs worn surface. According to peaks in Figure 10, the Fe(OH)O and iron oxide are formed. Because there is no ionic liquid, there are no F, N and S peaks and no other substances are formed. These formed protective films enhance the anti-wear effect of lubrication. Unfortunately, XPS could not provide any quantitative information about the quantitative relationship between the formed substance and ionic liquid concentration or the alkyl chain length.

Figure 10. (a) Full scan and (b-f) high resolution XPS spectra of F1s (b), Fe2p (c), O1s (d), N1s(e) and S2p(f) obtained at the worn surfaces which were lubricated by Triton X-100/C8mimNTf2/H2O LLCs (curve a), Triton X-100/C12mimNTf2/H2O LLCs (curve b), Triton X-100/C16mimNTf2/H2O LLCs (curve c), Triton X-100/H2O non-LLCs (curve d). 19 / 27


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CONCLUSIONS LLCs can be successfully prepared by using CnmimNTf as cosurfactant. As the concentration of the ionic liquid CnmimNTf2 increases, both the order degree of the LLCs molecules and the thickness of the amphiphilic bilayer d0 increase, whereas the cross-sectional area of the polar head group at the hydrophilic and hydrophobic interface decreases. As the length of the alkyl chain of the ionic liquid CnmimNTf2 increases, the thickness of the amphiphilic bilayer d0 and the degree of molecular order increase, while the cross-sectional area of the polar head group at the hydrophilic and hydrophobic interface decreases. The permeability of solvent increases and the thickness of solvent layer dw decreases with the increase of concentration of ionic liquids and the growth of the alkyl carbon chain. The tribological properties of the LLCs can be effectively improved by increasing the content of the ionic liquid and the alkyl chain length. The improved tribological properties are primarily due to the enhancement of the structural strength of the LLCs, which is related to the increase in the molecular order of the LLCs, the decrease in molecular cross-sectional area A of the polar head group at the hydrophilic and hydrophobic interface and the reduction of dw in solvent layer thickness. The protective film formed by the physical adsorption of ionic liquid LLCs on the surface of friction disk pair and the complex frictional chemical reaction have effectively promoted the lubrication effect of the LLCs.

ASSOCIATED CONTENT Supporting Information Additional figures about material. This information is available free of charge via the Internet at http://pubs.acs.org/. Molecular structures of the simulation model, Variations of elastic modulus as a function of oscillatory stress. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Lei Fan); [email protected] (Rong Guo) ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21073156, 21573191), the 20 / 27


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University Natural Science Foundation of Jiangsu Province (16KJD150004). The authors gratefully acknowledge financial support from the Priority Academic Program Development of Jiangsu Higher Education Institutions and Top-notch Academic Programs Project of Jiangsu Higher Education Institutions. We would also like to acknowledge the technical support received from the Testing Centre of Yangzhou University.

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TOC

C8mimNTf2 LLCs

C12mimNTf2 LLCs

C16mimNTf2 LLCs

non-LLCs

The increase of the concentration of CnmimNTf2 and the length of alkyl chain in the lamellar liquid crystals can lead to an obvious reduction in wear volumes, which is are attributed to the higher order of amphiphilic molecules, thicker of the amphiphilic bilayer and the smaller cross-sectional area of the polar head group at the hydrophilic and hydrophobic interface.

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Microstructure and Tribological Properties of Lamellar Liquid Crystals

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