Effect of Molecular Orientation Angle of Imidazolium Ring on Frictional

Article pubs.acs.org/Langmuir

Effect of Molecular Orientation Angle of Imidazolium Ring on Frictional Properties of Imidazolium-Based Ionic Liquid S. Watanabe,*,† M. Nakano,‡ K. Miyake,‡ R. Tsuboi,†,§ and S. Sasaki† †

Department of Mechanical Engineering, Tokyo University of Science, 6-3-1 Niijuku, Katsushika-ku, Tokyo 125-8585, Japan Advanced Manufacturing Research Institute (AMRI), National Institute of Advanced Industrial Science and Technology (AIST), 1-2-1 Namiki, Tsukuba, Ibaraki 305-8564, Japan

‡

S Supporting Information *

ABSTRACT: Ionic liquids have significant potential as lubricants, and it is known that ionic liquids exhibit characteristic behavior at solid−liquid interfaces. Although it is believed that the structure of ionic liquids at the interface contributes to the tribological properties in the region of boundary-mixed lubrication, this contribution has not been clarified because such analysis is difficult. In this research, we clarify the lubrication mechanism of an imidazolium-based ionic liquid by comparing the results of friction tests with interfacial molecular orientation analysis using sum frequency generation spectroscopy. Consequently, we clarify that the tilt angle of the imidazolium ring affects the friction coefficient of the ionic liquid; that is, the larger tilt angle, the lower the friction coefficient.

1. INTRODUCTION Ionic liquids based on imidazolium cations have been extensively investigated over the past 20 years in the field of electrochemistry,1−3 and they are being applied to more wideranging areas of technology. Ionic liquids have properties such as high thermal stability, low volatility, nonflammability, low melting point, and a broad liquid range, which make them applicable as superior lubricants and additives.4−8 In addition, it is known that ionic liquids exhibit characteristic behavior at solid−liquid interfaces.9−13 In terms of the relation between the solid−liquid interface and friction, earlier research has been performed by Hardy14 and Bowden and Tabor.15 They noted that a molecular adsorbed film on a solid surface prevents direct contact between the solid surfaces and reduces the friction coefficient in the region of boundary/mixed lubrication. Thus, it is believed that the structure of ionic liquids at the interface contributes to the tribological properties in the region of boundary-mixed lubrication. There are some reports regarding the contribution of the interfacial structure of ionic liquids. Liu et al. performed friction tests of an aluminum-on-steel system and analyzed the worn Al surface by X-ray photoelectron spectroscopy (XPS). They noted that an ionic liquid forms a bilayer induced by the positive charge of the worn metal surface and that this bilayer structure reduces the friction coefficient in a manner similar to that of graphite or molybdenum disulfide.16 Atkin et al. investigated the tribological properties of ionic liquids at a charged surface using atomic force microscopy (AFM) and scanning tunneling microscopy (STM). They noted that the smooth and well-defined surface formed by an ionic liquid improves lubricity and that this smooth and well-defined © 2014 American Chemical Society

surface can be controlled by varying the electrical charge and the ionic liquid components.17 However, the effect of the interfacial molecular structure such as the molecular orientation and conformation of the ionic liquid on the tribological properties has not yet been clarified because of the difficulty in analyzing these properties. The purpose of this study, therefore, is to clarify the effect of the interfacial structure of an ionic liquid on its frictional properties. In this research, the interfacial molecular structure of an ionic liquid was analyzed using sum frequency generation (SFG) spectroscopy, which can selectively probe the surface and interface. However, ionic liquids have been found to undergo complex tribochemical reactions4,18−21 and corrosive wear. In our previous research at the surface of a practical ferrous material, it was found that the ionic liquid caused a chemical reaction with the ferrous material surface, creating a complex interface.22 Therefore, in order to minimize such complex chemical reactions, the interface between the ionic liquid and a self-assembled monolayer (SAM) formed on a Au substrate was analyzed. In addition, to investigate the effect of molecular conformation at the surface on the frictional properties, we performed friction tests using ionic liquids and SAM surfaces.

2. EXPERIMENTAL SECTION 2.1. Preparation of Ionic Liquids and SAM. Three 1-butyl-3methylimidazolium [BMIM]-based ionic liquids were used. One was Received: March 28, 2014 Revised: June 18, 2014 Published: June 18, 2014 8078

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Figure 1. Structures of the ionic liquids and SAM components. trifluoromethanesulfonate ([BMIM]OTf), which is miscible with water, the second was hexafluorophosphate ([BMIM]PF6), which is water-insoluble, and the third was tricyanomethide ([BMIM]TCC), which is water-insoluble and halogen-free. In addition, in order to examine the effect of alkyl chain length which is attached with imiadzolium, 1-hexyl-3-methylimidazolium trifluoromethanesulfonate ([HMIM]OTf) was used. High-purity grade (>99%) [BMIM]OTf, [HMIM]OTf, and [BMIM]PF6 were purchased from Kanto Reagent (Tokyo, Japan). High-purity grade (>99%) [BMIM]TCC was purchased from Merck (Darmstadt, Germany). These four ionic liquids were used without further purification. Their molecular structures are shown in Figure 1. Figure 1 also includes the molecular conformation of the selected SAM, mercaptohexadecanoic acid (MHDA), which forms a hydrophilic surface. High-purity grade (>99%) MHDA was purchased from Sigma-Aldrich (St. Louis, MO). SAM substrates thinly spin coated with the respective ionic liquids were used as the samples. 2.2. Sum Frequency Generation Spectroscopy. In this study, we performed analysis of the surface structure between the ionic liquids and the SAM surfaces using SFG spectroscopy. The basics of SFG are as follows. Two pulsed laser beams, one of mode-rocked visible frequency (ωvis) and the other variable wavelength IR frequency (ωIR), are overlapped at a surface, and the nonlinear optical effect of SFG results in the emission of light at ωSFG = ωvis + ωIR, and this SFG light is detected. The intensity of the SFG light (ISFG) is proportional to |χ(2)|2, where χ(2) is the second-order nonlinear susceptibility. Because χ(2) is zero in centrosymmetric environments, SFG spectroscopy can selectively probe the surface and interface.23,24 In addition, SFG spectroscopy can be used to determine the molecular orientation from the SFG intensity. The SFG measurement in the current study used a Nd:YAG laser, which generates a mode-rocked 1064 nm fundamental pulse beam with 30 ps and 10 Hz. Fixed visible (532 nm) and tunable infrared (2.5−10 μm) beams are then generated from an optical parametric generator/optical parametric amplifier (OPG/OPA) system. The visible and infrared beams were overlapped at the sample surface with incident angles of 62° and 53°, respectively, from the surface normal. Representative values of the input energy of the visible and infrared beams were set to ∼550 and ∼260 μJ/pulse, respectively. The following function was used to fit the SFG spectrum:12,25,26

Figure 2. Definition of imidazolium orientation in the laboratory coordinate system.

I

SFG

(ω) ∝

(2) |χNR

+

χR(2) |2

= χ0 e

iΦ

+

∑ q

2

Aq ω2 − ωq + i Γq

(1)

χ(2) NR

is the nonresonant susceptibility, which is not dependent on where the infrared frequency, and χ(2) R is the resonant susceptibility attributed to the interface molecules. ω2 is the infrared frequency, q the qth resonant vibrational mode absorbance, Aq, ωq, and Γq the amplitude, frequency, and damping constant at the qth resonant vibrational mode absorbance, respectively, and χ0 and Φ are the nonresonant contribution and phase factor, respectively. From the results of the fitting, the SFG intensity at the vth vibrational mode, Iv, can be estimated from the relation between Av and Γv.27−29 ⎛ A ⎞2 Iv = ⎜ v ⎟ ⎝ Γv ⎠

(2) −1

The SFG measurement was performed at 3200−3050 cm , which includes the C−H stretching mode peak of the imidazolium ring, and we used the polarization combination ppp (SFG, visible, infrared). We considered that the orientation angle of the imidazolium ring largely affects the frictional property of the ionic liquid. The ppp polarization of an azimuthally isotropic surface probes four nonlinear susceptibility 8079

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(2) (2) (2) components: χ(2) xxz, χxzx, χzxx, and χzzz. On a metal surface, the z components of the IR electric field at the surface are dominant rather (2) than the x or y components. Thus, only two components, χ(2) zzz and χxxz (2) = χyyz , are independent. In addition, the complex refractive indices of a metal substrate could disturb the generation of unique fitting parameters in the case of ppp polarization.30 2.3. Pin-on-Plate Reciprocating Tribometer. To investigate the effect of the interfacial molecular orientation of the imidazolium ring on the frictional properties, friction tests were performed using a pinon-plate reciprocating tribometer. MHDA formed on a gold substrate was used as the plate, and borosilicate glass with a tip diameter of 3 mm was used as the pin. 1 μL of ionic liquid was applied to the plate surface, and the tests were performed under ambient temperature and relative humidity (25 °C, RH 30%), which was controlled by an air conditioner. A sliding test was conducted at a reciprocating frequency of 0.1 Hz, an amplitude of 10 mm, a sliding speed of 1 mm/s, and a load of 10 mN. The test duration was set for 180 min, and the friction test was performed approximately four times for each ionic liquid. Figure 3 shows a schematic of the test apparatus.

Figure 3. Schematic of pin-on-plate reciprocating tribometer.

3. RESULTS 3.1. SFG Spectrum of Interface between MHDA and Ionic Liquid. Figure 4 shows the SFG spectrum of each sample in the range of 3200−3050 cm−1, which includes the C−H stretching mode peaks. From all four spectra, dip peaks were observed in the range of 3200−3050 cm−1. The peaks at ∼3127 cm−1 [νC(2)H], ∼3153 cm−1 [νASHC(4)−C(5)H], and ∼3174 cm−1 [νSSHC(4)−C(5)H] were from the C−H normal mode vibrations of the hydrogen attached directly to the imidazolium ring.28,34−36 3.2. Calculation of Molecular Orientation of Imidazolium Ring. To investigate the molecular orientation from the results of the SFG spectra, the SFG spectra were fitted in eq 1. Figure 5 shows the peak fitting of the SFG spectrum in the case of [BMIM]OTf (Figure 4a). To constrain the variability of some parameters, we performed peak fitting with the assumption that Γq remains constant for similar molecular groups.37 In the case of ppp polarization, the phase of the nonresonant term for the Au substrate is nearly π/2.38 The specific peak fitting results are described in the Supporting Information. To analyze the orientation angle (θ, φ, χ) of the imidazolium ring, an SFG intensity ratio of νSSHC(4)−C(5)H to νASHC(4)−C(5)H is needed. The molecular orientation angle is defined as follows. A tilt angle θ of 0° is defined as the surface normal and that of 90° as parallel to the surface plane. A twist angle φ of 0° is defined as parallel to the surface and that of 90° as perpendicular to the surface. The azimuthal angle χ is within the plane of the surface. In this calculation, the average of the Euler angles can be performed for the rotationally isotropic system in the interface plane, that is, with no azimuthal angle χ dependence. In this study, we took into account the molecular orientation of the imidazolium ring. A

Figure 4. SFG spectra in the range of 3200−3050 cm−1, which includes the C−H stretching mode peaks derived from cations.

schematic of the angle definition for imidazolium is shown in Figure 2. The specific calculation is described in the Supporting Information. The SFG intensity of each ionic liquid was obtained in eq 2, and an SFG intensity ratio of νSSHC(4)− C(5)H to νASHC(4)−C(5)H was calculated for each ionic liquid. According to the calculation results, the SFG intensity ratio of νSSHC(4)−C(5)H to νASHC(4)−C(5)H was 1.9 for [BMIM]TCC, 0.80 for [BMIM]OTf, 0.34 for [BMIM]PF6, and 0.29 for [HMIM]OTf. Figure 6 shows the relation between the SFG intensity ratio of νSSHC(4)−C(5)H to νASHC(4)−C(5)H and the molecular orientation of the imidazolium ring. Baldelli et al. noted that the twist angle φ of an imidazolium ring ranges from 0° to 30° because of molecular steric hindrance.27 Thus, we calculated the twist angle φ in this range. In addition, Figure 6 includes the results of the SFG intensity ratio calculated for each ionic liquid. From Figure 6, the imidazolium tilt angle θ of each sample is 30°−33° in the case of [BMIM]TCC, 40°−45° in the 8080

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Figure 7. Time variation of friction coefficient of each ionic liquid.

Figure 5. Fitted SFG spectrum of BMIM]OTf. The red line shows the original spectrum, and the green line shows the fitting curve.

Figure 8. Relation between the tilt angle of the imidazolium and the friction coefficient.

Figure 8 shows the relation between the tilt angle of the imidazolium ring and the friction coefficient, which is a mean value of four tests. From this result, we found that the larger the tilt angle of the imidazolium, the lower the friction coefficient. The large tilt angle of the imidazolium ring means that the imidazolium ring is oriented parallel to the surface. Let us discuss the reason why the parallel-oriented imidazolium ring exhibits a low friction coefficient. First, let us focus on the interfacial molecular structure. The imidazolium ring regularly exists at the surface with some orientation angle, which is likely related to the tilt angle of the imidazolium ring of each ionic liquid because peaks derived from the imidazolium ring were observed in the SFG spectrum. Therefore, we may predict that hydrogen bonding between C(2)H of the imidazolium ring and MHDA binds the imidazolium ring to the MHDA surface, which results in the formation of a regular orientation. To explain the differences in the tilt angle between each ionic liquid, we think that the anion size affects the interfacial structure. To confirm this, we estimated the radius of the anion of each ionic liquid. In the computation, the maximum radius was calculated for nonspherical anions. The radius of PF6 (1.57 Å) was the smallest of the three ionic liquids, with OTf and TCC placing second and third at 1.94 and 2.57 Å, respectively. In a comparison with the tilt angle of the imidazolium ring, the smaller anion led to a larger tilt angle. On an uncharged surface, it is known that ionic liquids form a mixed layer as an adsorbed layer.10,39−43 On the other hand, cations aggregate with each other owing to solvophobic interactions of the alkyl chains in liquid bulk.44−46 In our present research, we focused on the solid−liquid interface. Consequently, we considered that ionic liquids will form conformations different from aggregation structures found in bulk. At an uncharged surface, recent molecular dynamics (MD) simulation research indicates that anions and cations form alternate arrangements at the surface.40 Therefore, alternate arrangements at the surface are considered reasonable and proper in our case. In addition, the stable structure of an

Figure 6. Relation between the SFG intensity ratio of νSSHC(4)− C(5)H to νASHC(4)−C(5)H and the molecular orientation of the imidazolium ring.

case of [BMIM]OTf, 61°−65° in the case of [BMIM]PF6, and 69°−71° in the case of [HMIM]OTf. To confirm the accuracy of our peak fitting, we performed peak fitting under other assumptions and took into account other parameter sets. Although the other parameter sets showed some deviation in terms of the magnitude of the SFG intensity ratio, the magnitude relation of SFG intensity ratio for each ionic liquid was unchanged. Thus, we can discuss the relation between the molecular orientation angle and the frictional property qualitatively, even though there is variability in the magnitude of the SFG intensity ratio and the molecular orientation angle. 3.3. Friction Test Results. To investigate the effect of the interfacial molecular orientation of the imidazolium ring on the frictional properties, friction tests were performed using a pinon-plate reciprocating tribometer. Figure 7 shows representative results of time variations of the friction coefficient of each ionic liquid. At 180 min, comparing ionic liquids with the same cation, [BMIM]PF6 exhibited the lowest friction coefficient of the three ionic liquids, and [BMIM]OTf and [BMIM]TCC placed second and third, respectively. In addition, [HMIM]OTf, which has a long alkyl chain in its cation, showed the lowest friction coefficient among the four ionic liquids.

4. DISCUSSION Here we consider the results of the friction test from the viewpoint of the molecular orientation of the imidazolium ring. 8081

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ring is approximately 1.12 Å, and this is almost the same size as the PF6 anion (1.57 Å). This could lead to the formation of a smoother and better-defined shear plane.17 This smoother surface limits the energy loss by rotation of the imidazolium ring and by extending the lattice spacing of the surface layer of the ionic liquid induced by shear. As a result, friction would be reduced. On the other hand, in the case of OTf and TCC, the difference in size between the imidazolium ring and the anion might make the surface rough. Under a shear field, energy is needed for the rotation of the imidazolium ring and extension of the lattice spacing of the surface layer of the ionic liquid in order to smooth the rough surface. In this present study, the interfacial molecular measurements and friction tests were independent. Thus, many unclear points still remain about the molecular behavior of an ionic liquid at the interface under lubricating conditions and its contribution to the frictional properties. To clarify these points, in situ interfacial molecular observation using SFG spectroscopy is needed.

ion pair might affect the surface structure. Tsuzuki et al. performed ab initio molecular orbital calculations of the interactions between ions in an ionic liquid and noted that anions prefer to have close contact with the C(2)H of the imidazolium ring.47,48 In the case of large anions, there is less of a difference in energy by changing the anion−cation arrangement from a stable structure than in the case of small anions. In the case of small anions, anions prefer to locate to the stable structure of a cation−anion pair. However, we predicted that the C(2)H of the imidazolium ring would interact with the MHDA surface. Consequently, anions approached the MHDA surface and cations tilted parallel to the surface because of the electrostatic attraction between the anions and cations. On the other hand, in the case of large anions, the force of forming a stable structure does not predominantly affect the surface structure because of the minor difference in energy between the most stable structure and other structures. Thus, the electrostatic attractive force between the cations and anions is greater than the force of forming a stable structure, and thus the anions locate horizontally next to the cations. As a result, the imidazolium ring tilts with respect to the surface normal owing to the horizontally directed electrostatic attractive force. Figure 9 shows a schematic image of the surface structure.

5. CONCLUSION The interface between MHDA and ionic liquids was measured by SFG spectroscopy, and the orientation angle of the imidazolium ring was calculated. As a result, the tilt angle of the imidazolium ring was in the order of [HMIM]OTf > [BMIM]PF6 > [BMIM]OTf > [BMIM]TCC. In addition, from the results of friction tests, the friction coefficient was in the order of [BMIM]TCC > [BMIM]OTf > [BMIM]PF6 > [HMIM]OTf. Considering the results of the SFG measurements and friction tests, we clarified that the tilt angle of the imidazolium ring affects its friction coefficient; that is, the larger the tilt angle, the lower the friction coefficient.

■

Figure 9. Schematic image of the surface structure of each small and large anion.

ASSOCIATED CONTENT

S Supporting Information *

Orientation calculation of sum frequency generation spectroscopy; SFG fitting results. This material is available free of charge via the Internet at http://pubs.acs.org.

We can now discuss how the tilt angle of the imidazolium ring affects the frictional property under a shear field. We propose two possibilities to explain the tilt-angle dependence of the frictional property: the film formation ability and the energy loss by molecular motion. 4.1. Film Formation Ability. In the case of small anions, the distance between anions and cations decreases, and thus small anions form a dense film. This dense film is capable of supporting a load and preventing serious breakdown, consequently leading to friction reduction. As noted above, the molecular density of the films varies according to the anion size. To eliminate the effect of anion size on the film density, we performed experiments using [BMIM]OTf and [HMIM]OTf (OTf as a fixed anion). The results indicate that the HMIM cation takes a larger orientation angle and shows a lower friction coefficient than the BMIM cation. Hence, the density of films of [HMIM]OTf would be almost identical to that of [BMIM]OTf because the OTf anion is common to both ionic liquids. These results led us to conclude that other factors need to be taken into account in order to explain the tilt-angle dependence of the frictional property. Thus, we discuss molecular motion next. 4.2. Energy Loss by Molecular Motion. The molecular motion hypothesis is based on the supposition that the entire surface layer of the ionic liquid has sufficient ability to support a load and prevent serious breakdown. In the case of the PF6 anion, the half value of the diagonal length of the imidazolium

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (S.W.). Present Address §

Department of Mechanical Engineering, Daido University, 103 Takiharu-cho, Minami-ku, Nagoya 457-8530, Japan. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful to Dr. Takayuki Miyamae (National Institute of Advanced Industrial Science and Technology (AIST)) for many helpful discussions regarding SFG analysis. In addition, this work was supported by a Grant-in-Aid for Scientific Research (A) (No. 23246030) from the Japan Society for the Promotion of Science (JSPS).

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dx.doi.org/10.1021/la501099d | Langmuir 2014, 30, 8078−8084