Letter pubs.acs.org/Langmuir
Lubrication Properties of Ammonium-Based Ionic Liquids Confined between Silica Surfaces Using Resonance Shear Measurements Toshio Kamijo,† Hiroyuki Arafune,† Takashi Morinaga,† Saika Honma,† Takaya Sato,*,† Masaya Hino,‡ Masashi Mizukami,‡ and Kazue Kurihara*,‡,§ †
Department of Creative Engineering, National Institute of Technology, Tsuruoka College, Sawada, Inooka, Tsuruoka, 997-8511, Japan ‡ Institute of Multidisciplinary Research for Advanced Materials (IMRAM), and §WPI-Advanced Institute of Materials Research (WPI-AIMR), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan S Supporting Information *
ABSTRACT: To evaluate the friction properties of new lubrication systems, two types of ammonium-based ionic liquids (ILs), N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate ([DEME][BF4]) and N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl) imide ([DEME][TFSI]), were investigated by resonance shear measurements (RSM) and reciprocating type tribotests between silica (glass) surfaces. RSM revealed that an IL layer of ca. 2 nm in thickness was maintained between the silica surfaces under an applied load of 0.40 mN ∼ 1.2 mN. The relative intensity of the RMS signal indicated that the friction of the system was lower for [DEME][BF4], 0.12, than that of [DEME][TFSI], 0.18. On the other hand, the friction coefficients μk obtained from the tribotests of [DEME][BF4] were lower than that of [DEME][TFSI] for sliding velocities in the range of 5.0 × 10−4 m s−1 to 3.0 × 10−2 m s−1 under applied loads of 196−980 mN. The friction coefficients obtained by the tribotest are discussed with reference to the RSM results.
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high loads of several gigapascals.14−16 These layers have been considered to contribute to reduce the friction of the systems. On the other hand, the properties of ILs confined in a nanometer-sized (“nanoscale”) space have also been studied based on the interest in applying ILs as lubricants. Atomic force microscopy (AFM)17−19 and the surface force apparatus (SFA)20,21 have been employed. Oscillating forces observed both by AFM and SFA demonstrated the layered structure of ILs in narrow gaps. Resonance shear measurement (RSM) revealed that imidazolium-based ILs having different anions confined between silica surfaces showed marked differences in their tribological behavior.21 1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide ([C4mim][TFSI]), exhibited larger friction than 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]). The effective viscosity values, ηeff, of these ILs confined in nanospaces were 1 to 3 orders of magnitude larger than the bulk values. Furthermore, the ηeff of [C4mim][TFSI] was larger than that of [C4mim][BF4] when they were confined in the nanospace, although the bulk viscosity of [C4mim][BF4] is higher than that of [C4mim][TFSI]. These experiments showed that the lubrication properties of ILs depend on the nature of the anions, as [C4mim][BF4] exhibited better lubrication than [C4mim][TFSI] in nanospace. Molecular dynamics simulation revealed
INTRODUCTION
Ionic liquids (ILs) are organic salts wholly consisting of anions and cations that exist as liquids at room temperature. The ILs exhibit many attractive properties such as nonflammability, high thermal stability, low melting point, negligible volatility, and relatively high ionic conductivity. These properties can be varied in a controlled fashion through systematic changes in the molecular structure of their constituent ions. ILs are expected to be a promising source for new lubricants.1−5 In particular, their stability under severe conditions such as ultrahigh vacuum6,7 and extreme temperatures8 have attracted increasing interest. In order to choose suitable ILs as lubricants, it is important to understand the characteristics of the target materials. However, at present, the details of the lubrication mechanism of ILs is still not clearly understood. The tribological properties of ILs have been studied using a macroscopic tribotester. Most research focused on the lubricating behavior of ILs under boundary lubrication regimes at high loads, such as several gigapascals, and their tribochemical reactions with solid surfaces.9−13 The formation of tribo-chemical layers on metal surfaces from ILs containing a halogen such as fluorine under sliding conditions has been studied by X-ray photoelectron spectroscopy (XPS),9−12 scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS),9−11 and time-of-flight secondary ion mass spectrometry (TOF-SIMS).12 Therefore, ILs have also been used as an additive that forms a tribochemical layer under © XXXX American Chemical Society
Received: September 6, 2015 Revised: November 23, 2015
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DOI: 10.1021/acs.langmuir.5b03354 Langmuir XXXX, XXX, XXX−XXX
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Langmuir that different organizations of ion pairs on the silica surfaces may contribute to such large difference in tribological behavior of ILs.22 However, the relationship of these nanoscale properties and macroscale lubrication properties are still not clear for the same surface and ILs. In this study, we chose aliphatic quaternary ammonium-type cation [DEME] (N,N-diethyl-N-methyl-N-2-methoxyethyl) ionic liquids, namely, [DEME] [TFSI] and [DEME] [BF4], all of which have a spherical structure. This choice reflected the fact that, we have developed a robust lubrication system using [DEME] [TFSI] and ionic liquid polymer brushes23 but have not reported its lubrication properties when confined in nanometer scale spaces. We examined the nanoscale lubrication properties of these ILs between molecularly smooth silica surfaces using RSM while controlling the surface separation (D) at 0.1 nm resolution. The obtained results were compared with the macroscale properties of ILs between glass surfaces monitored by a tribotester. We aimed to evaluate the correlation between macro- and nanolubrication properties of ILs, and the lubrication properties of ILs with different anions. This study revealed that ILs could both reduce the friction between silica surfaces under various applied loads at 298 K, and that the macroscopic lubrication behaviors of the ILs studied were correlated with their nanoscopic behaviors.
The water contents of [DEME][BF4] and [DEME][TFSI] determined by Karl Fischer titration (Kyoto electronics manufacturing Co. Ltd., MKC-610-DT) were 282 and 55 ppm, respectively. The viscosity of the ILs measured with a programmable rheometer (Brookfield, DV-III ULTRA) were 400 mPa s and 77 mPa s for [DEME][BF4] and [DEME][TFSI], respectively. RSM was performed using a home-built resonance shear system based on a SFA.25 A schematic illustration is shown in Figure 2a. The experimental setup and procedures for RSM were described in detail in a previous publication.25 Silica sheets used as samples were prepared following the procedure reported by Horn et al.26 The root-mean-square (RMS) roughness value measured by AFM (Toyo Corporation, Agilent 5100 AFM/SPM Microscope) for the silica sheets was 0.31 nm over an area of 5 × 5 μm. Using RSM, the resonance curve between molecularly smooth silica sheets was measured across IL films at a surface separation, D, with a resolution of 0.1 nm. The D value was determined by a fringes of equal chromatic order (FECO) analysis. In brief, the two back-silvered silica sheets (thickness of ca. 2−4 μm) were glued onto cylindrical quartz lenses (with a radius of curvature (R) of ca. 20 mm) and mounted onto the RSM system. The RSM system was composed of the upper surface unit hanging by a pair of vertical leaf springs and the lower surface unit mounted on a horizontal leaf spring. The upper surface unit was connected to a four-sectored piezo tube. In this instance, it can be laterally oscillated with various frequencies (ω) by applying a sinusoidal input voltage (Uin). The deflection (Δx) of the leaf spring was detected as an output voltage (Uout) by a capacitance probe (Microsense 4830, Japan ADE Ltd.). Then resonance curves, normalized amplitude (Uout/Uin) as a function of the frequency ω, were recorded at various D values. RSM was conducted at room temperature (295 ± 0.5 K) and at a humidity less than 25% maintained by placing silicagel in the sample chamber. Friction measurements were carried out on a conventional reciprocating tribotester, TRIBOGEAR TYPE 38 (Shinto Scientific Co. Ltd., Tokyo), using a glass ball of 10 mmφ(Toshinriko Co. Ltd.) and a glass plate (Matsunami Glass Ind., Ltd.). The glass ball and glass plate were treated in fresh nitric acid at 373 K for 75 min to obtain a clean surface. The RMS roughness values measured by AFM for the glass ball and plate were 9.9 and 1.2 nm over an area of 5 × 5 μm,
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EXPERIMENTAL SECTION N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate ([DEME][BF4]) and N,N-diethyl-N-methyl-N(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl) imide ([DEME][TFSI]) were purchased from Kanto Kagaku Co. Ltd. and used as received. The chemical structures are shown in Figure 1. The ion pair diameters of the IL’s, 0.79 nm
Figure 1. Chemical structures of the ionic liquids (a) [DEME][TFSI] and (b) [DEME][BF4].
[DEME][TFSI] for and 0.69 nm for [DEME][BF4], are determined from their densities, assuming a cubic packing geometry, according to the method described by Horn et al.24
Figure 2. Schematic drawing of the resonance shear measurement system (a) and a reciprocating type tribotester (b). The surfaces in panel a are in the crossed-cylinder geometry. B
DOI: 10.1021/acs.langmuir.5b03354 Langmuir XXXX, XXX, XXX−XXX
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IL, the parameters of the system (mass, damping parameter, and spring constant for the apparatus) determine the AS and SC resonance curves. In the presence of [DEME][TFSI], the resonance curve at D = 228.1 nm showed a peak at a frequency of 185 rad s−1, which was almost the same as the AS peak. The peak intensity Uout /U in was lower than that of the AS peak, which corresponded to energy dissipation due to the bulk viscosity of the IL. With decreasing D value, the amplitude of the resonance peaks started to decrease at 10.2 nm, and disappeared at D = 4.9 nm. In the range from D = 4.9 to 2.5 nm, broad resonance curves were observed at intermediate frequencies between the AS and the SC frequencies and the peak frequency started to shift toward the SC peak because of the weak coupling of the upper and lower surfaces mediated by the confined [DEME][TFSI].21 When the applied load (N) was further increased, the surface separation did not change and remained at 2.5 nm (see Supporting Information, Figure S1); however, the peak frequency shifted further toward the SC peak, and the amplitude increased. This means that [DEME][TFSI] remained between the silica surfaces at a 2.5 nm separation and the coupling of the upper and the lower surfaces became stronger with the increased applied load. The resonance shear behavior of [DEME][BF4] (see Figure 3a) was similar to that of [DEME][TFSI], except at the small D region below 2.5 nm (see Figure 3b). The resonance curves at 186 rad s−1 did not change in the range from 220.6 to 95.5 nm, and their amplitudes started to decrease at 20.8 nm. The resonance frequency of these curves were almost the same as the AS peak. The amplitude of the resonance peaks gradually decreased with decreasing D value from 20.8 to 11.4 nm and disappeared at D = 9.4 nm, which was longer than the disappearance distance of D = 4.9 nm for [DEME][TFSI]. In the range from D = 9.4 to 2.1 nm, broad resonance curves were observed at intermediate frequencies, and the peak frequency started to shift toward the SC peak because of the weak coupling of the upper and lower surfaces mediated by the confined [DEME][BF4]. When the applied load (N) was further increased, the surface separation remained at 2.1 nm (see Supporting Information, Figure S1), indicating that [DEME][BF4] remained trapped between the silica surfaces. Figure 5a plots the relative intensity (Ir) of the resonance peak, that is, the ratio of the peak intensity for confined ILs divided by the SC peak intensity, versus the applied load. This intensity ratio is a measure for evaluating the lubrication behavior of the confined ILs, where smaller values indicated better lubrication behavior. The Ir values of [DEME][TFSI] clearly increased from ca. 0.09 to ca. 0.16 with an increase of the applied load from 0.21 mN to 0.33 mN, and reached plateaus when the load was increased to 0.40 mN. The Ir value of [DEME][BF4] was similar to that of [DEME][TFSI]. It gradually increased from ca. 0.07 to ca. 0.12 as the applied load increased from 0.19 mN to 0.50 mN, and reached plateaus when the load was increased to 0.70 mN. The major noteworthy difference was that the Ir value for [DEME][TFSI], 0.18, was larger than those for [DEME][BF4], 0.12, at the higher loads (>0.4 mN). These results indicated that under higher loads (>0.4 mN), [DEME][BF4] should be a better lubricant than [DEME][TFSI]. On the other hand, RSM revealed that an IL layer with ca.2 nm in thickness was maintained between the silica surfaces, even under an applied loads (>0.4 mN). The plateau of the relative intensity under an applied loads (>0.4 mN) indicated that the IL layer confined between silica surfaces maintained the lubricating properties.
respectively. A schematic illustration of the tribotester is shown in Figure 2b. The measurements were performed at a movement distance of 10 mm, with a sliding velocity in the range of 5.0 × 10−4 m s−1 to 3.0 × 10−2 m s−1 under an applied load of 196 mN−980 mN at room temperature (295 ± 0.5 K). The friction force was measured by an all-in-one load converter from a gauge attached to the sample holder and was recorded as a function of time. The friction coefficient (μ) was given by the friction force divided by the normal load. The friction coefficients obtained for at least five trials were stored and averaged.
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RESULTS AND DISCUSSION Nanoscale Lubrication Properties. First, we studied the nanoscale lubrication properties of ILs between smooth silica surfaces using RSM. Figure 3a,b show resonance curves for [DEME][TFSI] and [DEME][BF4] confined between silica surfaces at various separation distances. Two reference states, the resonance curves for the separation in air (AS) and for silica−silica contact (SC), were measured before the resonance shear measurement of the ILs (Figure 4). In the absence of the
Figure 3. Resonance curves for (a) [DEME][TFSI] and (b) [DEME][BF4] confined between silica surfaces at various separation distances under an applied load N. Resonance curves for separated in air (AS) and connected by silica−silica contact (SC) are also shown. Solid lines denote the best fit curves on the basis of a physical model.27 C
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Figure 4. Schematic illustration of (a) the separation in air (AS), (b) the silica−silica contact (SC), and (c) the IL between silica surfaces.
We suppose that the rapidly increasing point of the effective viscosity in Figure 5 corresponds to the distance at which the solid-like structure formation begins by the confinement. An increase in the viscosity of the [DEME][BF4] has begun at a clearly shorter distance than for [DEME][TFSI]. This means that this ammonium salt more readily restructures than does its TFSI salt, namely the BF4 salt is more easily crystallized. This consideration has also been supported from the crystal temperature measurement by differential thermal analysis (DSC).28 Although [DEME][BF4], for which the viscosity increases at a relatively long distance, has a distinct crystallization temperature, the TFSI salt, however, has only a glass transition temperature, and no crystallization temperature, as indicated by the DSC measurement. The same trend occurs in the case of aromatic ionic liquids. The viscosity of [C4mim][TFSI] with a specific crystallization temperature goes up at a large distance, whereas the viscosity of [C4mim][BF4] without a crystallization temperature increases at a short distance.29 One can plot the ηeff versus the applied load (see Supporting Information, Figure S2). The ηeff value of [DEME][TFSI] and [DEME][BF4] increased with the increasing applied load, and reached plateaus when the load was increased 0.4 mN. The ηeff for [DEME][TFSI], ca. 13000, was larger than those for [DEME][BF4], ca. 3000, at the higher loads (>0.4 mN). In the RSM, facing silica surfaces were completely separated by their molecular smoothness in order to avoid partial contact and to allow the analysis of the boundary lubrication property of the confined liquid at a certain separation distance. In this configuration, an increase of the viscosity of the lubricant layer under confinement directly leads to an increase of the friction at the sliding interface. The results obtained from RSM showed that the ηeff of [DEME][BF 4] is lower than that of [DEME][TFSI] at a similar separation distance (ca. 2 nm), which means that the lubrication property of [DEME][BF4] is better than that of [DEME][TFSI] under such confinement. We concluded that [DEME][BF4] under the higher load (>0.4 mN) should be a better lubricant than [DEME][TFSI]. Macroscopic Tribological Properties. We next studied the boundary lubrication properties of ionic liquids between a glass ball and a glass plate using a macroscopic tribotester. Figure 6 shows plots of the friction coefficients between a glass ball and a glass plate in [DEME][TFSI] or [DEME][BF4] versus a parameter consisting of sliding velocity (V)/Load (L) (m s−1 N1−). The measurements were conducted by sliding a glass plate over a distance range of 10 mm at various sliding velocities from 5.0 × 10−4 m s−1 to 3.0 × 10−2 m s−1 and under various applied loads from 196 mN to 980 mN at 298 K. The friction coefficient for [DEME][BF4] increased from 0.05 to 0.08 on decreasing the parameter V/L from 1.5 × 10−1−5.1 × 10−3 and was nearly constant, μ ≈ 0.08 over the V/L range of 5.1 × 10−3−1.0 × 10−3. The friction coefficient results for
Figure 5. (a) Relative intensity (peak intensity confined ILs/the SC peak intensity), (b) effective viscosity ηeff versus the separation distance for [DEME][TFSI] (●) and [DEME][BF4] (□) .
We analyzed the resonance curves using a physical model27 to obtain quantitative properties of the confined ILs. The details of the analytical procedure were described in previous papers.21,27 Figure 5b plots the effective viscosity (ηeff) thus obtained for the ILs versus the separation distance between the silica surfaces. The ηeff values at larger separations were constant, and were essentially identical to the viscosity values of the bulk ILs, 400 mPa s for [DEME][BF4] and 77 mPa s for [DEME][TFSI]. However, the ηeff value sharply increased with decreasing D below 11.4 nm for [DEME][TFSI] and below 10.4 nm for [DEME][BF4]. The ηeff value for [DEME][TFSI] confined in the nanospace (D ≤ 4.9 nm) was 1 to 2 orders of magnitude higher than that at larger separations (D ≥ 10.2), and the ηeff value for [DEME][BF4] confined in the nanospace (D ≤ 9.4 nm) was about 1 order of magnitude higher than that at larger separations (D ≥ 11.4). We may note that the ηeff value of [DEME][TFSI] was higher than that of [DEME][BF4] in the nanospace, whereas the viscosity of [DEME][BF4] is higher than that of [DEME][TFSI] in the bulk. This behavior is similar to the trend shown by the ionic liquids [C4mim][TFSI] and [C4mim][BF4] confined between silica surfaces, although the cations are different between this study and those used in ref 21. D
DOI: 10.1021/acs.langmuir.5b03354 Langmuir XXXX, XXX, XXX−XXX
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RSM showed that ILs were maintained between the silica surfaces at a surface separation of 2.5 nm for [DEME][TFSI] and 2.1 nm for [DEME][BF4], which are usually referred to as the hard wall thickness. The diameters of the ion pairs were 0.79 nm for [DEME][TFSI] and 0.69 nm for [DEME][BF4] determined from the density, assuming a cubic packing geometry following the method by Horn et al.24 From these values, we determined that, in both systems, three layers of ILs were trapped between the silica surfaces. The dramatic reduction of the friction coefficient in “the boundary lubrication region” with ILs was presumably due to the presence of the three layers of ILs between silica surfaces under an applied load of 196 mN. These results indicated that the tribotester macroscopic tribological properties correspond to the nanoscale lubrication properties obtained from on nanoscopic RSM.
Figure 6. Friction coefficient between a glass ball and a glass plate in [DEME][TFSI] (●), in [DEME][BF4] (□), by sliding the glass plate over a distance range of 10 mm at sliding velocity of 5.0 × 10−4 m s−1−3.0 × 10−2 m s−1 and under an applied load of 196 mN−980 mN at 298 K. The dashed lines are guides to the eyes.
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CONCLUSION We performed RSM and reciprocating-type tribotests to evaluate the friction properties of lubrication systems consisting of the ionic liquids ([DEME][BF4], [DEME][TFSI]) between silica (glass) surfaces. The RSM results revealed that ILs of ca. 2 nm in thickness remained between the silica surfaces under applied loads of 0.40−1.2 mN, where the relative intensity (Ir) was lower for [DEME][BF4], 0.12, than [DEME][TFSI], 0.18. On the other hand, the friction coefficient μk of [DEME][BF4] obtained by the tribotests was lower than that of [DEME][TFSI] at over a range of sliding velocities from 5.0 × 10−4 m s−1 to 3.0 × 10−2 m s−1 under applied loads of 196−980 mN. Even though the RSM and tribotest measurement were made under different applied loads, the difference of the friction coefficient between [DEME][TFSI] and [DEME][BF4] at the boundary lubrication region observed by the tribotest corresponds to their behavior when confined between silica surfaces under the higher load (>0.4 mN) observed by RSM. These results indicate that the nanometer-scale space property observed by RSM can provide important insights for the study of the friction coefficients (macro-scale lubrication properties) obtained by tribotests.
[DEME][TFSI] was similar to those for [DEME][BF4]; however, the friction coefficient observed in the region of V/ L from 1.5 × 10−1−1.0 × 10−4, was significantly higher than that of [DEME][BF4]. Such behavior of the friction coefficient against V/L corresponds to the shift of the lubrication region of [DEME][TFSI] and [DEME][BF4] from mixed-lubrication to boundary lubrication. As a control, the friction coefficient was measured between the glass ball and plate without IL to be μ ≈ 0.7 at a sliding velocity of 1.0 × 10−3 m s−1 and under a normal load of 196 mN (data was not shown). The presence of ILs at the glass−glass interface dramatically reduced the friction coefficient to less than ca. 20% of the value obtained without ILs. Since the boundary lubrication of a confined IL is dominant, while the contribution of hydrodynamic lubrication by the change of lubricant thickness is not effective at this measurement condition, the difference of boundary lubrication between [DEME][BF4] and [DEME][TFSI] can be explained based on the effective viscosity obtained from the RSM measurements. The obtained RSM results showed that the ηeff of [DEME][BF4] was lower than that of [DEME][TFSI], while they were maintained at a similar separation distance (ca. 2 nm), which means that the lubrication property of [DEME][BF4] is better than that of [DEME][TFSI] under such a confined condition. The inversion of the order of the viscosities of ILs under confinement compared with bulk values appears to be due to the differences in the layering structure of the ILs based on the differences of their anion structure. Canova et al. reported the layering structure of ILs ([C4mim][TFSI] and [C4min][BF4]) confined to a silica contact plane studied by molecular dynamics simulations. In the case of [C4mim][TFSI], ion pairs alternately orient to form a checkerboard structure where each type of ion is forced to contact the same-charged ions during shearing.22 Repulsive force between same-charged ions destabilizes the crystal structure of the IL and triggers overall restructuring of inner molecular layers to form a more stable configuration, which results in high friction. In contrast, [C4mim][BF4] formed a layer-by-layer structure where cations and anions did not suffer from such repulsions since each ion layer is sandwiched between oppositely charged layers, which leads to smooth shearing and low friction. Therefore, we think that better boundary lubrication properties of [DEME][BF4] compared with [DEME][TFSI] are due to such differences of layering structure under confinement even though the cations used in our study are different from those studied by Canova et al.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03354. S1: Profiles of normal force normalized by surface curvature radius (F/R) as a function of surface separation D between silica surfaces in ILs. S2: Plots of the effective viscosity ηeff versus the applied load. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (T.S.). *E-mail:
[email protected] (K.K.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by the “Green Tribology Innovation Network” Advanced Environmental Materials Area, Green Networks of Excellence (GRENE) program and JSPS KAKENHI Grant Numbers 25810091, 26820034, 30399258. E
DOI: 10.1021/acs.langmuir.5b03354 Langmuir XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.langmuir.5b03354 Langmuir XXXX, XXX, XXX−XXX
