Ionic Liquids Confined in Hydrophilic Nanocontacts: Structure and

Article pubs.acs.org/JPCC

Ionic Liquids Confined in Hydrophilic Nanocontacts: Structure and Lubricity in the Presence of Water R. M. Espinosa-Marzal,†,‡ A. Arcifa,† A. Rossi,†,§ and N. D. Spencer†,* †

Laboratory for Surface Science and Technology, Department of Materials, ETH Zürich, CH-8093 Zürich, Switzerland Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, 09042 Cagliari, Italy

§

S Supporting Information *

ABSTRACT: We have investigated the influence of ambient humidity on the nanoconfined structure and response to shear of ionic liquids. Three ionic liquids (ILs) were selected, namely, 1-ethyl-3-methyl imidazolium ethylsulfate ([EMIM][EtSO4]), 1-ethyl-3-methyl imidazolium tris(pentafluoroethyl)trifluorophosphate ([EMIM][FAP]), and 1-hexyl-3methyl imidazolium tris(pentafluoroethyl)trifluorophosphate ([HMIM][FAP]), to investigate the influence of hygroscopic and hydrophobic anions, as well as different alkyl chain lengths. We employed an extended surface forces apparatus (eSFA) to ascertain the structure of the confined films, whereas colloidal-probe lateral force microscopy (CPM) was used to measure shear forces in the nanosized contact between mica and a silica sphere. The presence of water, the anion, and the alkyl chain length of the imidazolium cation were found to influence the equilibrium structure of the nanoconfined film, as well as its dynamic properties. Adsorbed water appears to change both the ion-pair orientation and the slip condition for film-thickness transitions, that is, the resistance of the IL layers to being squeezed out from the contact. Three lubrication regimes have been identified: a boundary-film lubrication regime with the lowest friction, an intermediate lubrication regime that is highly dependent on the IL anion, and an isoviscous rigid hydrodynamic lubrication regime (with Newtonian fluidfilm behavior). It is shown how IL composition and water both influence speed and load dependence of shear forces at the nanoscale. Understanding the response to shear provides further insight into the properties of nanoconfined IL films.

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INTRODUCTION Ionic liquids (ILs) consist of salts of organic/inorganic anions and cations that are both large and asymmetric, thereby inhibiting crystallization over a broad range of temperatures. ILs display high electrochemical and thermal stability, are nonflammable and nonvolatile,1−3 and can be designed to interact with specific chemical groups, making them attractive in many applications including self-assembly media4 and electrolytes for energy-storage systems.1 Over the past 10 years, ILs have also attracted substantial research interest as potential lubricants.5−12 As with some conventional high-end lubricants such as perfluoropolyethers, ILs can be employed for applications involving extreme operating conditions on account of their high temperature stability and low vapor pressure. The nanoscale properties of ionic liquids are also favorable for lubrication. Several studies have demonstrated the layered structure of ILs in nanoconfinement13−22 and how this structure can aid in reducing friction: The confined ions resist being “squeezed out” when surfaces are compressed, with the result that an IL film remains between the surfaces up to high pressures, thus preventing direct contact. For this reason, ILs are also of great interest in the field of micro- and nanoelectromechanical (MEMS/NEMS) device lubrication.23 In such systems, tribological effects, including friction, adhesion, and wear,6 are crucial to performance, because of © 2014 American Chemical Society

the high surface-to-volume ratio. In contrast to many other lubricants, the physical properties of ILs, as well as their physical and chemical interactions with surfaces, can be tuned through systematic changes in the structure of the ions, which potentially make ILs ideal lubricants for many applications. However, remarkably little is known about the correlation between molecular structure and interactions responsible for the lubrication mechanism. Several properties of ionic liquids (e.g., density, viscosity, polarity, and conductivity) can be changed dramatically by the presence of small amounts of other substances.24 Traces of water are ubiquitous in ILseven in those considered to be hydrophobic, such as ILs containing the tris(pentafluoroethyl)trifluorophosphate anion (see, e.g., refs 25 and 26 and Table 1). Herein, we discuss results obtained from a study of imidazolium-based ILs with different alkyl chain lengths, namely, 1-hexyl-3-methyl imidazolium (HMIM) and 1-ethyl3-methyl imidazolium (EMIM), with the two anions ethylsulfate (EtSO4) and tris(pentafluoroethyl)trifluorophosphate (FAP). We have investigated the influence of waterabsorbed from the surrounding atmosphere at 37% RHon the Received: January 1, 2014 Revised: February 28, 2014 Published: March 4, 2014 6491

dx.doi.org/10.1021/jp5000123 | J. Phys. Chem. C 2014, 118, 6491−6503

The Journal of Physical Chemistry C

Article

Table 1. Viscosities (25 °C) and Densities of the Bulk ILs (from the Manufacturer), Calculated Ion-Pair Volumes, and Measured Water Uptakes at 37% RH Following 7 Days of Equilibration IL (manufacturer) [EMIM][EtSO4] (Iolitec, Heilbronn, Germany) [EMIM][FAP] (Merck, Darmstadt, Germany) [HMIM][FAP] (Merck, Darmstadt, Germany)

viscosity (mPa s)

water uptake (%, w/w)

density (g/cm3)

ion-pair volume (nm3)

115 (17.5a)

11.2

1.237

0.317

75

0.07

1.720

0.537

116

0.08

1.560

0.652

as determined by coulometric Karl Fischer titration after 7 days of equilibration at 37 ± 2% RH, and the viscosities (η) at 25 °C of the selected ILs {the value in parentheses corresponds to the viscosity of [EMIM][EtSO4] with a water content of 11.2% (w/ w)29}. Prior to the tests, vials containing ∼1 cm3 of each IL were placed into an aluminum vessel that was sealed and connected to a rotary pump. Pre-evacuation at ambient temperature was first performed for ∼5 h. Then, the temperature was increased to 50 °C, the vessel was evacuated to ∼0.03 mbar, and the vacuum was held for 3 days. The sealed vessel was then transferred to a glovebox (water content < 10 ppm), in which the vials were sealed and stored. After the samples had been dried, the water content in the ILs was below 100 ppm. Extended Surface Forces Apparatus. Surface-force isotherms30−32 were obtained using an extended surface forces apparatus (eSFA)a modified version of the Mk 3 SFA (Surforce, Santa Barbara, CA), with attachments to improve the accuracy, resolution, mechanical drift, thermal stability, imaging, and essential automation of the instrument; these modifications are described in detail in the literature.33,34 The transmitted interference spectrum that results from multiple partial reflections between the silver mirrors consists of fringes of equal chromatic order that are analyzed by fast-spectralcorrelation interferometry33 to evaluate the gap distance and the refractive index simultaneously. The accuracy of the distance measurements is typically ±30 pm. Thin mica sheets were prepared by manually cleaving ruby mica of optical quality grade #1 (S&J Trading, Inc., Floral Park, NY) in a class-100 laminar-flow cabinet. Uniformly thick (2− 5.5-μm) mica sheets with a size of ∼8 mm × 8 mm were cut using surgical scissors, to avoid possible contamination with nanoparticles.35 A 40-nm silver film was thermally evaporated onto the mica sheets in a vacuum (2 × 10−6 mbar), while the opposite mica surface was protected by a clean mica support layer. The silver-coated mica sheets were glued onto cylindrical lenses (radius = 20 mm) with a pure epoxy resin (EPON 1004, Shell Chemicals) and heated to 140 °C in a horizontal laminar flow, to minimize exposure to resin vapors. However, exposure of the mica surface to air (e.g., during cleaving or gluing)

Value reported in ref 29 for the viscosity η of [EMIM][EtSO4] with a water content of 11.2% (w/w).

a

structure of the nanoconfined IL films, as resolved with an extended surface forces apparatus (eSFA). We correlate these results with the IL-mediated lubrication of nanocontacts between hydrophilic surfaces by means of colloidal-probe lateral force microscopy (CPM) and propose a model for nanoscale lubrication. Wear and the formation of tribochemical layers can be ruled out in the performed experiments, but will be the focus of a separate study combining XPS with macroand microtribology under higher contact pressures. The present study provides a better understanding of the equilibrium and dynamic properties of ILs under nanoconfinement and ion−ion interactions at the shear plane.

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EXPERIMENTAL METHODS Materials. Two imidazolium-based cations with different alkyl chain lengths, namely, HMIM and EMIM, and two different anions, EtSO4 and FAP, were selected. According to the supplier data, the meridional isomer of FAP was provided for this work. Figure 1 shows the four ions, and the size of each ion calculated by molecular mechanics (Avogadro software27) with the MMFF94s force field. These ionic liquids differ strongly in their hygroscopic behavior (FAP is considered to be hydrophobic28). Table 1 lists the water uptakes by the bulk ILs,

Figure 1. Optimized geometries and sizes of HMIM+, EMIM+, EtSO4−, and FAP− calculated by the software Avogadro by molecular mechanics (force field = MMFF94s). 6492



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Figure 2. (a) Force−distance curves across [EMIM][EtSO4] under dry conditions (red, solid circles) and at ambient RH (blue, open symbols). The inset shows schematically the main features of the eSFA, indicating the two orthogonal springs with spring constants kn and kf whose bending measures normal and shear forces between the surfaces as they are moved normal or laterally parallel (xL), respectively, with respect to each other. (b) Sizes of layers and corresponding loads under dry conditions (red, solid circles) and at ambient RH (blue, solid diamonds). The presence of water leads to an increase in the thickness of the layers that are more strongly bound to the surface.

water uptake, if it takes place at all, is small. Continuous purging with N2 can also help in reducing the content of adsorbed water. The force measurements were made under complete immersion of tip and sample in the IL, to avoid capillary effects. In the dry experiments, the mica surface was exposed to air for less than 1 min, and therefore, hydrocarbon adsorption is expected to be less pronounced than in the dry eSFA experiments. Tipless, gold-coated cantilevers (CSC12, MikroMasch, Tallinn, Estonia) with a normal stiffness of 0.08−0.15 N/m were used to measure lateral forces. Silica microspheres (Kormasil, EKA Chemicals AB, Bohus, Sweden) with diameters ranging from 10 to 22 μm were attached to the tipless cantilevers using two different glues, Araldite and NOA 81 (Norland, Cranbury, NJ). XPS measurements showed no dissolution of the glues in the ILs. The colloid-modified atomic force microscopy (AFM) cantilever was UV-ozone-treated for 30 min prior to use. Sliding velocities between 0.03 and 300 μm/s were obtained by varying the stroke length and the scan frequency of the friction loop. The friction force was assessed from the measured friction loops (minimum of 10 loops) by calculating the loop width and averaging to obtain both mean and standard deviation using IGOR Pro 6.2 Software. At least three experiments with different mica surfaces were performed, and at least five different positions on each surface were explored. The normal and lateral signals were converted to forces by means of appropriate calibration methods. The normal springconstant calibration of the cantilever was carried out using the thermal-noise method39 before the attachment of the colloidal probe. The torsional spring constant was estimated using Sader’s method40 with the help of Sader’s online calibration applet (available at http://www.ampc.ms.unimelb.edu.au/afm/ calibration.html). The lateral sensitivity for the cantilever (probe cantilever) was estimated using the test-probe method, as described by Cannara et al.41 for rectangular cantilevers. Here, a silicon wafer was cut along the ⟨100⟩ crystal plane and glued to a glass slide such that the smooth edge of the wafer was used as a wall to twist the cantilever laterally. A test

cannot be avoided, and some adsorption of airborne hydrocarbons is expected to occur.36,37 The samples were then immediately inserted into the sealed eSFA, the fluid cell was purged with dry N2, and the mica thickness was determined by means of thin-film interferometry in mica−mica contact. The surfaces were separated to around 10 μm, and a 5 μL droplet of IL was placed between them with a syringe from a side window. The point of closest approach (PCA) was readjusted with a precision of ±1 μm in the lateral direction. Normal surface forces were measured at constant approach and separation speeds (ranging from 0.05 to 0.30 nm s−1) and constant temperature (295.0 ± 0.1 K), either upon continuous N2 purging to avoid water uptake or at ambient RH (∼37 ± 2%). A spring constant of kn = 1921 ± 88 N m−1 was used for the normal-force measurements. For the friction experiments, the lower surface was moved by means of a piezoelectric bimorph with an amplitude of xL = 10 μm and a constant speed of V = 0.1 μm/s. The bimorph was mounted on a spring with a spring constant of kn = 526 N/m. The shear forces acting on the upper surface were measured with a strain gauge mounted on a measuring spring with a spring constant of kf = 2300 ± 100 N m−1 (for technical details, see ref 38) at a measuring rate of 300 Hz. The inset in Figure 2a shows schematically the main features of the eSFA. Approach and retraction measurements were carried out by continuous motor travel, in part simultaneously with the friction measurements. Colloidal-Probe Lateral Force Microscopy. Lateral force measurements were carried out with an atomic force microscope (MFP-3D, Asylum Research, Santa Barbara, CA) using a freshly cleaved mica surface and a silica colloid probe as the countersurface, either under continuous purging with N2 or at 37% RH. For the dry experiments, the mica surface was maintained dry with a N2 stream as the IL was poured into the measuring cell. During this process, the uptake of water cannot be completely ruled out, especially by the strongly hygroscopic [EMIM][EtSO4]. The different behaviors measured reproducibly under dry and ambient conditions in at least three different experiments (on different days) demonstrate that the 6493



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standard deviations (in parentheses in Table 2) for the fitting parameters, especially under ambient conditions. Furthermore, the variation of the dielectric constant with the water content and with the distance to the interface43 was neglected, and therefore, the fitting parameters provide only a rough estimate of these physical quantities. It should be noted that, until the publication of ref 21, a high degree of ionic dissociation was assumed for ILs, which led to estimated Debye lengths that were on the order of 0.1 Å, ∼1−2 orders of magnitude smaller than the molecular dimensions of even the smallest ionic-liquid cations and anions. This discrepancy is typically provided as evidence that ionic liquids completely electrostatically screen charged surfaces within a few bound ion layers and that the diffuse layer does not exist. In contrast, ref 21 proposed ILs to behave as highly dilute electrolyte solutions, based on the measured long-range doublelayer force between gold and mica at different potentials and the correspondingly low effective ion dissociation degree. These results have been the subject of recent controversial discussions. In our eSFA measurements, we also note the action of a nonnegligible (but small) long-range repulsive force that scales exponentially with the surface separation for D > 7 nm (see Figure 2 and Figure 1a,b in ref 42). The calculated surface potential is in agreement with values expected for mica in aqueous electrolyte solution, but there is still no direct evidence that demonstrates the origin of this force. Hydrogen bonds have a significant effect on the interaction strength between water and the anions. EtSO4 anions are better hydrogen-bond acceptors than FAP anions, which explains the larger amount of absorbed water at ambient RH {11.24% (w/ w) for [EMIM][EtSO4] vs 0.08% and 0.07% (w/w) for [HMIM][FAP] and [EMIM][FAP], respectively; Table 1}. The influence of adsorbed water at ambient RH on the doublelayer repulsion is small for the FAP ILs but very significant for [EMIM][EtSO4], for which the Debye length decreases from ∼9 to ∼2 nm upon exposure to ambient RH. Thus, water favors ion dissociation according to the double-layer model proposed in ref 21. At D < 5 nm, a short-range repulsion, resulting from the contribution of ion−surface binding forces, forces induced by ion structuring, and van der Waals forces, dominates and reveals film-thickness transitions that are characteristic of a layer of ions being collectively squeezed13−21 out from the contact. Thus, the nanoconfined films appear to be ordered in layers. For [EMIM][EtSO4], the first layer under dry conditions is resolved at a surface separation of ∼7 nm (Figure 2a). Figure 2b shows the measured thickness of the layers (Δ) under dry conditions and at ambient RH as a function of the applied load (Δ is strictly the change of the film thickness D that is measured when a layer is squeezed out and considered to be the layer thickness). The thickness of the layers under dry conditions is Δ = 5.3 (0.6) Å, as obtained from four force measurements with different pairs of mica surfaces. A further transition was measured at higher loads (∼70 mN/m) but is not shown in Figure 2a: Owing to the mica deformation and to the increased contact radius, it is well-known that loads above 10 mN/m are overestimated and there is an error in the absolute distance measurement (±0.5 nm) but not in the thickness of the film transitions. Figure 2b shows that the thickness of the layer that is most strongly bound to the surface under ambient RH is larger than that of the other layers [Δ = 8.3 (0.4) Å instead of Δ = 5.4

cantilever to which a colloidal sphere had been attached (with sphere diameter larger than the width of the cantilever) was twisted against the silicon wall, and the lateral sensitivity was determined from the slope of the lateral deflection plotted as a function of the piezo distance. The lateral sensitivity of the target cantilever was estimated from the measured lateral sensitivity of the test cantilever, as described in detail in ref 41. Prior to the measurements, the colloidal sphere was slid for ∼15 min over the freshly cleaved (atomically smooth) mica in the corresponding IL at the maximum applied load (∼50 nN), to remove possible asperities from the silica sphere.

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RESULTS Equilibrium Structure of Nanoconfined IL Films. Isothermal force−distance curves were measured with an eSFA at various speeds. We discuss here the results obtained at approach/separation speeds of ∼0.08 nm/s, that is, in a quasi-equilibrium state. Figure 2 shows representative results from [EMIM][EtSO4] at 0% and 37% RH. We recently reported quasi-equilibrium surface forces across the selected FAP ILs.42 A small, long-range repulsion that extends out to separations of ∼50 nm is characteristic for the selected ILs. Hydrodynamic drag does not account for the measured long-range repulsion (D > 7 nm) assuming constant viscosity: The hydrodynamic drag is below 0.1 mN/m at an effective approach speed of ∼1 Å/s, which is higher than the speed at which the surfaces are approached in our experiments. The force scales exponentially with the surface separation D (equal to the film thickness at the point of closest approach), in agreement with a double-layer repulsion, as found in recent measurements with other ILs.21 Table 2 lists the surface potentials, Debye lengths, and ion dissociation degrees obtained by fitting the Derjaguin− Laudau−Verwey−Overbeek (DLVO) equation42 to the longrange repulsion. The measured repulsion is small and has a large variability between experiments, which leads to the large Table 2. Surface Potentials, Debye Lengths, and Ion Dissociation Degrees Obtained from Fits of the DLVO Equation42 to the Measured Long-Range Repulsion between Mica Surfaces in [EMIM][EtSO4], [EMIM][FAP], and [HMIM][FAP] under Dry Conditions and at Ambient RHa surface potential (mV) [EMIM][EtSO4] (dry) [EMIM][EtSO4] (37% RH) [EMIM][FAP] (dry) [EMIM][FAP] (37% RH) [HMIM][FAP] (dry) [HMIM][FAP] (37% RH)

Debye length (nm)

ion dissociation degree (%)

−42 (2)

9 (2)

0.0010 (0.0006)

−108 (22)

1.8 (0.2)

0.20 (0.13)

−118 (5) −105 (12)

8.9 (0.6) 6 (2)

0.006 (0.001) 0.018 (0.011)

−128 (44)

5 (2)

0.032 (0.020)

−122 (11)

6 (1)

0.018 (0.005)

a

Dielectric constants for [EMIM][FAP] and [HMIM][FAP] were assumed to be 12.3 and 11.4, respectively (see ref 44 for similar imidazolium-based ILs). Dielectric constant of bulk [EMIM][EtSO4] is 27.9.44 Refractive index measured with the eSFA ≈ 1.375, 1.387, and 1.474 and calculated Hamaker constant = 1.304 × 1020, 1.163 × 10−20, and 5.173 × 10−21 for [EMIM][EtSO4], [EMIM][FAP], and [HMIM][FAP], respectively. Influence of water on the Hamaker constant was neglected. 6494



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alkyl chains are combed as the surfaces pass each other, similarly to the alignment of closely packed self-assembled monolayers during shear observed in simulations47 and experiments with straight-chain and branched hydrocarbons.48 The layer size at ambient RH on the sliding is Δ=10.0 (1.8) Å. When the load was further increased to 200 mN/m (Hertzian pressure ≈ 22 MPa), no further film-thickness transitions could be detected in the selected ILs, only a compression of the boundary film superposed on the mica compression. Dynamics of Film-Thickness Transitions. Figure 3 shows the time-dependent film-thickness transitions in [EMIM]-

(0.6) Å]. There is an increase in pull-off force with increasing RH, from 1.0 (0.5) to 4.5 (0.5) mN/m. Because the viscosity decreases by a factor of ∼7 upon water incorporation (see ref 29), the hydrodynamic drag must decrease upon water uptake and cannot explain the measured increase in pull-off force. The pull-off force is related to the adhesion, that is, to the interaction energy between the surfaces across the thin film that remains in the contact upon approach. Accordingly, the increase in pull-off force results from the exposure of the system to 37% RH, and in this case, it is related to the increase in the water content of the IL. It should be noted that an oscillatory force between mica surfaces across [EMIM][EtSO4] was previously measured by SFA,17 but the reported results differ from ours. The resolution in ref 17 allowed only two layers to be resolved in films of D < 2 nm, whereas we have resolved at least three very pronounced layers below the same applied load (∼100 mN/m, Hertzian pressure ≈ 17.6 MPa). The resolution and scale of the y axis in ref 17 do not allow for the identification of long-range repulsion. Moreover, the size of the two layers shown in ref 17 (Δ1 ≈ 5 Å and Δ2 ≈ 8 Å) is in better agreement with our results at ambient RH. We note that P2O5 was used as a drying agent in ref 17, instead of N2 as in our experiments under dry conditions. The normal-force measurements obtained for [EMIM][FAP] and [HMIM][FAP] were recently published elsewhere;42 here, we summarize the most important results, which will be useful for the understanding of the subsequent discussion. The average thickness of the [EMIM][FAP] layers under dry conditions is Δ=5.4 (0.5) Å, while the last resolved layer is characteristically smaller, Δ = 4.2 (0.2) Å. At ambient RH, the average layer thickness is clearly larger, Δ=7.1 (0.2) Å, and more transitions are resolved. Moreover, the IL film that remains between the two mica surfaces at the highest load that it was possible to apply during the experiment, indicated as a boundary film in the subsequent discusson, has a thickness that depends on the presence of water [D0 = 1.8 (0.3) nm under dry conditions vs D0 = 1.0 (0.3) nm at 37% RH]. Furthermore, the adhesion increases by a factor of 2, implying a change in the surface energy of the system. This behavior is completely reversible after drying. Similar measurements were carried out with [HMIM][FAP], but they did not provide evidence of a clear layered structure for the nanoconfined IL,42 in strong contrast to the two ILs with the [EMIM] cation. The surface-adsorbed layer is enriched in cations, for both the [EMIM] or [HMIM] ILs, attracted by the negative surface charge,22 whereas other layers or sublayers consist of ion pairs to preserve electroneutrality. [EMIM] cations can readily incorporate the ethyl group into the first layer above the mica surface.45 In contrast, the hexyl chains presumably adopt an upright orientation46 because of attractive lateral interactions between the hydrocarbon chains, promoting interlayer interdigitation22 and disrupting the layered structure. In repeated force measurements between gold and a silica sphere22 measured by AFM, layers of [HMIM][FAP] could be resolved, and the layers of [EMIM][FAP] had a characteristic size of ∼9 Å (larger than if confined between mica surfaces), where all of this indicates the significant influence of the surface−IL interactions and confinement geometry on the film structure. As the mica surfaces are brought together during sliding, the layered structure of [HMIM][FAP] becomes more pronounced in the structural force,42 indicating a shear-driven layering: The

Figure 3. Dynamics of film-thickness transitions of [EMIM][EtSO4] under dry conditions (red and orange circles) and at ambient RH (blue triangles) at applied loads equal to (a) 0.4 (0.1) mN/m and (b) 70 (5) mN/m (dry, red), 10 mN/m (dry, orange), and 15 (5) mN/m (37% RH, blue). Transitions were measured with different pairs of mica surfaces.

[EtSO4], as measured by eSFA under dry conditions (circles) and at ambient RH (triangles). The transitions under dry conditions proceed up to ∼9 times more slowly than those at ambient RH. The viscosity of the bulk IL/water mixture decreases by a factor of ∼6.5; higher viscosities of nanoconfined fluids compared to bulk values at the same applied pressure have been previously reported49,50 (see ref 50 for ILs). The measured time dependence of the IL “squeeze-out” from the contact indicates a higher resistance to layer outflow for the nanoconfined dry ILs. Ambient RH also strongly accelerates the transition dynamics in [EMIM][FAP] (see ref 42): The layers slip up to ∼25 times faster if the mica surface is wetted by water. Load-Dependent Friction Studies: Influence of Water on Low- and High-Friction Regimes. These studies were performed at a sliding speed of 1 μm/s. The maximum applied load was 50 nN, which corresponds to a maximum Hertzian pressure of ∼82 MPa for a 10-μm-radius colloidal sphere and a contact-area radius of ∼17 nm. Under these conditions, no 6495



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friction regime was detected at any potential, but a linear relationship between lateral force and applied normal load was observed, implying a coefficient of friction (COF, defined as normal load divided by measured friction) of COF ≈ 0.12− 0.23 at a negative surface potential comparable to that of the mica substrate used in our experiments. A higher ratio between lateral force and normal load and a different ion-pair orientation (derived from the layer thicknesses) were obtained in the present work for [EMIM][FAP] confined between mica surfaces, which suggests that the two phenomena are related, as discussed below. The calibration methods differ and might also play a role in the calculated friction force. Speed-Dependent Friction Studies. Colloidal-probe lateral force microscopy was used to explore a broad range of speeds v (from 30 nm/s to 300 μm/s) at different loads L. Figure 5 shows the ratio between friction and normal load as a

wear of the mica surface or of the silica sphere takes place. The contact radius is much smaller than in the eSFA experiments (∼500 times smaller), and therefore, the relaxation times of the ions in contact during shear might be different. The load-dependent friction response of [EMIM][EtSO4] exhibits a very pronounced low-friction regime, where the friction force is negligible, and a high-friction regime (Figure 4). The low-friction regime is less pronounced at 37% RH in the selected experiments, but it will become evident in the speeddependent studies discussed later.

Figure 4. Load-dependent lateral force measured by CPM between mica and a silica colloidal sphere in dry [EMIM][EtSO4] under a dry atmosphere (sphere radius = 9 μm) (red circles and diamonds) and in equilibrated [EMIM][EtSO4] at ambient RH (sphere radius = 7 μm) (blue squares and triangles). The two sets of data were obtained at different positions on a mica surface. The onset of the high-friction regime occurs at a pressure of ∼52 MPa under dry conditions and ∼46 MPa at ambient RH (i.e., under wet conditions).

Three different friction regimes as a function of the load were observed for ethylammonium nitrate (EAN),51 but much lower friction was measured. [EMIM][EtSO4] is much more viscous than EAN, which induces higher viscous forces. The transition pressure between low- and high-friction regimes ranges from 30 to 52 MPa (the Hertzian pressure range obtained from at least four experiments with different substrates and at least five loaddependent curves per substrate at different positions) under dry conditions and from 28 to 46 MPa under wet conditions. In the high-friction regime, friction is slightly higher under ambient conditions, despite the strong decrease of the bulk viscosity. Under dry conditions, a low-friction regime is observed for [EMIM][FAP] that disappears at 37% RH with the simultaneous appearance of adhesion (see Figures S1 and S2 in the Supporting Information). At a separation speed of 100 nm/s, the pull-off force increases to 6.5 (0.9) nN (under dry conditions the pull-off force is smaller than 1 nN). Similarly, a low-friction regime is measured in dry [HMIM][FAP], which vanishes at ambient RH, while the pull-off force increases from 0.5 nN to 2.6 (0.8) nN. The increase of pull-off force with RH cannot be attributed to a change in the hydrodynamic drag, because the viscosity and the hydrodynamic drag remain constant, as the water uptake by the IL is negligible. As discussed before, we attribute the increase in pull-off force to the change in interaction energy between the two mica surfaces across the thin film of IL when exposed to 37% RH. For the hydrophobic ILs we expect the water to concentrate at the hydrophilic surface. An increase in adhesion with ambient RH was also found in the eSFA measurements.42 For gold−silica systems16 in presumably dry [EMIM][FAP] and [HMIM][FAP], no low-

Figure 5. Ratio between lateral force and applied normal load vs vη/L for [EMIM][EtSO4] under dry conditions (red symbols) and at 37% RH (blue symbols). Viscosity = 115 and 17.6 mPa s, respectively.29 Loads in the low-friction regime: 5 nN (dry, red diamonds) and 1 and 5 nN (37% RH, blue diamonds and squares, respectively). Loads in the high-friction regime: 42 and 104 nN (dry, red triangles and circles, respectively) and 43 nN (37% RH, blue circles). Radius of sphere = 9 μm.

function of vη/L for [EMIM][EtSO4]. The difference between the low- and high-friction regimes expands over the whole range of speeds at 0% and 37% RH. The lateral force was measured under noncontact conditions (separation of >5 μm between countersurface and substrate) and is referred to as the hydrodynamic lateral drag in the following. Owing to the high viscosity of [EMIM][EtSO4], the viscous forces during the lateral sliding of the cantilever tip are not negligible at speeds of ≥60 μm/s, and they increase linearly up to ∼5 nN at 250 μm/s (load L = 0). With an increase in load, the measured lateral force is higher, but it still increases linearly with speed, indicating hydrodynamic lubrication and Newtonian behavior of the IL in the low-friction regime. In the high-friction regime, friction increases logarithmically with speed, indicating shear thinning. Deviations from the linear or logarithmic trends are found at the lowest vη/L values, namely, in the boundary-film lubrication regime; the friction force in this regime is very low under dry conditions, within the resolution of the instrument (negative values are often measured and do not appear on the logarithmic scale). It should be noted that the boundary-film lubrication regime replaces the classical boundary-lubrication regime, which is 6496



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characterized by the highest friction values owing to asperity− asperity contacts. Instead, in the boundary-film lubrication regime, there is a remaining (and structured) IL film that prevents contact, and it is here that the lowest friction values are achieved. Figure 5 demonstrates that there is no agreement between the nanoscale Stribeck curves of dry and wet [EMIM][EtSO4] if the water uptake is taken into account by the decrease in viscosity of the bulk IL (17.5 instead of 115 mPa·s); a higher viscosity for the nanoconfined wet IL, 36 mPa·s instead of 17 mPa·s, leads to a superposition of the low-friction regime, indicating either that the viscosity of the nanoconfined IL differs from that of the bulk IL or that the water uptake in nanoconfinement is different. In the low-friction regime at vη/L > 1000, friction also increases linearly with speed, indicating hydrodynamic lubrication and Newtonian behavior of the wet IL. Figure 6 shows the ratio between the measured lateral and normal forces for [EMIM][FAP] under dry conditions and at

Figure 6. Ratio between lateral force and applied normal load vs vη/L for [EMIM][FAP] under dry conditions (red) and at 37% RH (blue). Viscosity = 75 mPa·s. The low-friction regime under dry conditions is shown at 1 nN (circles), whereas an applied load of 10 nN leads to the characteristic high-friction regime (diamonds). At ambient RH, the results for 10 nN (circles) and 20 nN (diamonds) overlap.

Figure 7. (a) Hydrodynamic lateral drag (no contact between surface and countersurface) in [HMIM][FAP] (circles) (dotted line is intended as a guide to the eye) and friction at an applied load of 2 nN in two different experiments (triangles and squares). (b) Ratio between lateral and normal load vs vη/L for dry [HMIM][FAP] in two different experiments (red and green symbols); radius of sphere = 7 μm (green) and 10 μm (red). Several data points at low speed in the low-friction regime (2 nN) are not shown on the logarithmic scale because they are approximately equal to or less than 0. (c) Ratio between lateral and normal load vs vη/L for equilibrated [HMIM][FAP] at ambient RH in two different experiments (blue and purple symbols). Radius of sphere = 7 and 11 μm, respectively. See superposed diagrams in Figure S4 (Supporting Information).

37% RH. Panels b and c of Figure 7 show the results for [HMIM][FAP] at 0% and 37% RH, respectively. It is reasonable to consider that the viscosity of the bulk FAP ILs is not affected by the small amount of adsorbed water [ 2000), the increase in friction is linear with speed, suggesting that the system enters the hydrodynamic lubrication regime and New6497



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Figure 8. Friction force between mica surfaces in boundary films of [EMIM][EtSO4] (brown, open diamonds), [EMIM][FAP] (blue, solid diamonds), and [HMIM][FAP] (red circles) sliding at 0.1 μm/s over a distance of 10 μm (a) under dry conditions and (b) at ambient RH, as measured by eSFA.

[HMIM][FAP] consists of sequences with initial stick and superposed microstick slips and larger slips, meaning that, at 37% RH, the critical speed vc for [HMIM][FAP] is larger than that for [EMIM][FAP] and [EMIM][EtSO4] and larger than the sliding speed, 0.1 μm/s. After the yield point is achieved, a collective slip leads to an abrupt reduction of friction (within 0.3 s) to nonmeasurable friction values. The maximum friction is 0.6 mN at the maximum load of 2.05 mN. Complex friction responses have previously been observed in SFA measurements:52,53 For squalane between mica surfaces, the kinetic friction increased steadily over repeated cycles and collapsed intermittently to the average value; individual slip cycles also consisted of a cascade of smaller slip events, indicating the presence of large domains or long-range cooperativity, compared to the molecular dimensions in the film. Thus, it should be noted that the measured stick−slip response with an SFA results from multiple cooperative processes with different relaxation times, taking place54 within a contact area of tens of square micrometers, and thus should not be confused with atomic stick−slip55 described commonly by a Tomlinson-like model. This detailed friction response was not observed in our CPM experiments (trace−retrace friction loops). This arises from the different inertial properties and compliance of the systems, as well as the different geometries of the contact and commensurability of the countersurfaces.

tonian behavior of the IL, under both humidity conditions. Boundary-Film Lubrication Regime. The lubricating performance of the ILs was also investigated by eSFA at a constant sliding speed of 100 nm/s. A maximum Hertzian pressure of ∼22 MPa was applied with the eSFA-friction setup. Under both dry and ambient conditions, the selected ILs exhibit a coefficient of friction (COF) that is smaller than 0.015, before the last detected layer is squeezed out from the contact according to eSFA experiments (D > 2 nm). Figure 8a shows that, for [EMIM][EtSO4] under dry conditions, the COF is still ≤0.015 after the last layer has been squeezed out (Hertzian pressure ≈ 17 MPa), that is, as a boundary film forms with an average thickness D0 = 1.8 (0.3) nm. In contrast, at ∼37% RH (Figure 8b), the friction across the boundary film increases up to a yield point and then remains constant (smooth sliding), leading to COF ≈ 0.1. Very different results were obtained for the FAP ILs under similar conditions.42 Figure 8a shows the nonperiodic stick− slip response of [EMIM][FAP] under dry conditions, sliding at 0.1 μm/s, which means that the sliding speed required for smooth sliding (critical speed of vc) is larger than the selected speed. The following friction sequence repeats during unidirectional sliding after the last layer of [EMIM][FAP] has been pushed out: Initially, friction increases, up to a yield point, and then eventually undergoes a slow relaxation followed by another increase in friction. Then, the friction decreases abruptly within 0.5 s (collective slip) down to values on the order of magnitude of the instrument resolution. During the increase in friction, microstick−slip events are observed (see ref 42 for more details). The maximum measured friction in the boundary film is 1.2 mN at a maximum load of 1.7 mN. In contrast, for dry [HMIM][FAP], the resistance of the boundary film to shear remains very low (COF ≤ 0.015 for a boundary film thickness of D0 = 1.5 ± 0.5 nm). At ambient RH, the friction responses of the two boundary FAP IL films differ greatly (Figure 8b). For [EMIM][FAP], sliding is smooth after reaching the yield point (friction = 1.5 mN, load = 2 mN, COF ≈ 0.75), meaning that the critical speed for stick−slip initiation48 in the presence of interfacial water is lower (vc < 0.1 μm/s) than under dry conditions. Although stick−slip vanishes, friction is high, as expected from the increase in adhesion. In contrast, the friction response of

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DISCUSSION The presented results show the remarkable effect that ambient humidity (∼37% RH) has on the equilibrium structure of nanoconfined ILs, the dynamics of film-thickness transitions, and the friction across the IL films in hydrophilic nanocontacts. These phenomena are discussed in this section. RH has a dramatic influence on the structural forces of both hygroscopic EtSO4-containing and hydrophobic FAP-containing ILs and, therefore, on the structures of the corresponding nanoconfined fluids. We show that exposure to ambient RH maintains the layered structure of the ILs. This is in agreement with the pioneering work of Horn et al.,13 who measured a pronounced layered structure for EAN/water mixtures up to 30 vol % water with an SFA, but it contrasts with findings for bis(trifluoromethylsulfonyl)imide- (TFSI-) based ILs.18 We recently resolved the layered structure of confined [BMIM]6498



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[TFSI] between silica surfaces at ambient humidity with an atomic force microscope (publication in preparation). The bulk structure of water-containing ILs is mainly determined by anion−water interactions. Experimental (Raman and IR) studies and simulations show that the small amount of water content in bulk hydrophobic ILs does not interact with the anions, for example, with bis(trifluoromethylsulfonyl)imide (TFSI).56 Thus, anion hydration can be ruled out in the bulk solution. Previous studies showed that, at moderate RHs, a monolayer of water was adsorbed on mica immersed in octamethylcyclotetrasiloxane,57 which is a hydrophobic fluid. Thus, adsorption of water is expected on the hydrophilic mica surface, especially when it is immersed in a hydrophobic liquid. A quantification of the surface-adsorbed water in [EMIM][FAP] is not possible in the absence of knowledge of the corresponding partition coefficient, but the increase in adhesion with RH supports the presence of water at the IL−mica interface when it is immersed in the hydrophobic IL (for [EMIM][FAP], see Figures S1 and S2 of the Supporting Information). In ref 42, a constant layer size of ∼7 Å was observed at ambient RH, significantly higher than that under dry conditions (5 Å). This is interpreted as a rearrangement of the ion pairs owing to the presence of water at the solid−liquid interface: Anions interact directly with the surface-adsorbed water, substituting the cation-rich adlayer that forms under dry conditions by a cation/anion adlayer; layers are composed of anions and cations in a checkerboardlike configuration (although the relative concentrations of anions and cations are unknown) with the largest dimension of the ions (∼7 Å for both anion and cation) oriented perpendicular to the surface. Owing to the different arrangement of the ions, a change of the interactions at the shear plane is very likely. Figure 9 shows one possible ion arrangement that would explain the discussed experimental results; it is, however, currently not possible to determine the structure unambiguously.

Bulk [EMIM][EtSO4] is strongly hygroscopic. At 61% RH, Raman and IR spectra show a very strong water−EtSO4 interaction that is dominated by hydrogen bonding, which weakens cation−anion interactions.58 MD simulations59 show that the probability of finding dissociated ions in the presence of 95.3% (w/w) water is ∼60%, as the solvation energy of the ions by water balances the loss of cation−anion interaction energy and allows cation−anion separation. At the ambient RH of our experiments, water does not influence the structure of the bulk IL network significantly; it merely expands it, although the ions still form a continuous aggregate and water forms separate chainlike domains.59 Thus, no hydrated ions are expected in bulk [EMIM][EtSO4] at 37% RH. However, confinement could affect the water uptake by the ILs, as well as the water/IL miscibility, owing to the reduction of entropy of the system and change of free energy. Unfortunately, no information on this issue is available for the nanoconfined [EMIM][EtSO4]. Our eSFA experiments show that the layer size of [EMIM][EtSO4] at 37% RH increases from Δ ≈ 5.4 (0.6) Å to Δ ≈ 8.3 (0.4) Å toward the surface (Figure 2b), which indicates a structural gradient of the wet IL film that is not present under dry conditions. Partitioning coefficients are unknown and, therefore, so is the water content across the gap (it might be higher close to the surface if surface adsorption dominates). The expansion of the layers at ambient RH is illustrated in Figure 9, as well as the possible presence of an anion-rich adlayer (assuming a high water content at the surface). If ion hydration is possible under nanoconfinement or at the interface, a significant increase in volume of the hydrated ions is obtained, which could explain the increase of the layer size with RH: The ion-pair volume is ∼0.32 nm3 (density = 1.23 g/cm3), whereas ∼0.59 nm3 is obtained for an IL/water mixture with 11.2% (w/w) water content (density = 1.21 g/cm3 from ref 29). The varying layer thickness of [EMIM][EtSO4] might be related to a gradient in water content and to the varying orientation of the ion pairs across the gap. SFA measurements cannot distinguish between the possible origins of the layer increase. Synchrotron X-ray reflectivity experiments on nanoconfined [EMIM][EtSO4] between mica surfaces exposed to different RHs have been performed to address this question, and the modeling and analysis of the results is ongoing. The measured duration and thickness of the transitions allows the speed at which the layers are squeezed from the contact to be determined. Figure 10 shows the speed of the film-thickness transitions as a function of the applied Hertzian pressure for [EMIM][EtSO4] and [EMIM][FAP], which differs between 0% RH (red circles and triangles) and ambient RH (blue circles and triangles). The results were obtained on one set of mica surfaces for each IL; each symbol gives the average of three force curves, but the standard deviation is smaller than the symbol size. The lines (a power law) are to guide the eye. The pressure dependence of the resistance to squeezing is stronger for [EMIM][FAP] than for [EMIM][EtSO4], and it is therefore IL-dependent. The transition speed is 1 order of magnitude greater for [EMIM][EtSO4] under ambient conditions than at 0% RH, whereas for [EMIM][FAP], the effect of water seems to increase by over 1 order of magnitude with applied pressure. Thus, although no viscosity change is expected for [EMIM][FAP] when exposed to ambient RH, the filmthickness transitions are significantly faster. According to the

Figure 9. Schematics of the proposed layered structures of the IL films [EMIM][EtSO4] and [EMIM][FAP] during the last measured transition under dry conditions and at ambient RH. In [EMIM][EtSO4], water hydrates the anion and expands the structure across the gap, leading to a larger layer thickness. In [EMIM][FAP], water mainly hydrates the mica surface. Under dry conditions, a cation-rich adlayer is expected. The presence of water might reduce the cation surface concentration, replacing the well-accepted, cation-rich surfaceadsorbed layer in the dry experiments by an adsorbed layer consisting of both anions and cations. 6499



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boundary-film lubrication regime. At a moderate load, for example, 10 nN, isoviscous rigid fluid-film lubrication60 leads to film thicknesses of >15 nm above 25 μm/s. The fluid-film thickness increases to 2 μm at the maximum speed of the CPM experiments (300 μm/s). For the FAP ILs the pressure− viscosity coefficient is not known. Estimations were performed using the pressure−viscosity coefficient for other FAP ILs, ∼2 times larger than that for [EMIM][EtSO4],63 and the results were found to be qualitatively similar. When exposed to ambient humidity, the viscosity of [EMIM][EtSO4] decreases, and as a result, higher speeds are required to achieve hydrodynamic lubrication. If the bulk viscosity were the only IL property determining hydrodynamic lubrication, the nanoscale Stribeck curves for the three ILs should converge into a single curve for any load. We note a good agreement for the dry ILs (see Figure S5, Supporting Information) considering the bulk viscosity but a large discrepancy for [EMIM][EtSO4] at ambient RH (Figure 5). The disagreement in the low-friction regime suggests that the viscosity and/or the water content of the bulk IL differs from the viscosity and/or water content of the confined IL. A characteristic regime in the range of 2000 > vη/L >1 is identified in the high-friction regime of the nanoscale Stribeck curves of the FAP ILs, for which friction remains roughly constant or even decreases with increasing speed over a broad range of speeds and loads (Figures 6 and 7); we denote this as an intermediate regime. In contrast, we observe a pronounced logarithmic dependence of friction on speed for [EMIM][EtSO4] in the high-friction regime, that is, shear-thinning behavior of the fluid (Figure 5). This speed dependence has been attributed to atomic stick−slip behavior (Tomlinson-like or Tomlinson−Eyring model) in ILs:16,51 At medium loads, the weak compression of the confined ion layer leads to an “ionic” roughness, resulting in a discontinuous stick−slip sliding process. This does not explain the results for FAP ILs at similar loads. However, a decrease in friction with increasing speed has also been observed for unsaturated organic friction modifiers; Langmuir−Blodgett calcium stearate monolayers;64 and weakly adhesive, surfactant-bearing surfaces.65 The Tomlinson−Eyring model does not capture the observed behavior of FAP ILs. An extension of this model was proposed by Schallamach66 to describe sliding as a result of incoherent shearing and re-formation of nanodomains or “junctions” as a surface moves against its countersurface. Each junction is stretched until it breaks due to either thermal excitation or/and external force. Accordingly, if bond formation is slower than bond rupture, at sufficiently high speeds, there is insufficient time to form bonds, and friction might either decrease or remain constant. The different behavior of [EMIM][EtSO4] and FAP ILs in the intermediate regime suggests that the nature of the anion plays a decisive role in determining the time necessary for bond rupture and formation with the imidazolium cation at the shear plane. In the boundary-film regime, vη/L 2000, in the hydrodynamic lubrication regime, friction increases linearly with speed (Newtonian behavior), and the curves for the low- and high-friction regimes converge into a single curve. The latter is not shown in the selected experiments for [EMIM][EtSO4], but it was demonstrated in other experiments with this IL at higher speeds. In the lowfriction regime, the linear dependence of friction with speed extends down to lower speeds (or vη/L values). The thickness of the IL film during load-/speed-dependent friction measurements cannot be measured by CPM. Therefore, we estimated the speed- and load-dependent film thickness using Hamrock’s model,60 being aware of the limitations of this model for nanometer-thick films (15 nm above 5 μm/s that further increase with speed. This shows that the lubrication mechanism in the low-friction regime is mainly hydrodynamic. No estimation of film thickness can be performed in the 6500



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confined films with a thickness smaller than 2 nm and, therefore, demonstrates the solidlike behavior of the boundary IL films under strong nanoconfinement. In fact, a strong dependence of yield stress on normal pressure has been observed for several nanoconfined nonpolar fluids between mica surfaces and attributed to the decrease of orientational degrees of freedom in confinement, that is, to the compressibility of the film.67 A sharp increase in the coefficient of friction with increasing applied pressure for various ILs has been also measured by others in previous SFA experiments:50,68 As the confined IL film achieves its minimum thickness (1−2 nm or approximately three layers), friction increases notably owing to more significant intralayer attractions.68 Thus, SFA results are in qualitative agreement with the existence of lowand high-friction regimes as observed in CPM experiments. For the investigated ILs, a more efficient ion and/or ion-pair packing at high compression leads to a different boundary-film structure and resulting intralayer interactions, which enhances friction significantly and leads to a different friction response (high-friction regime). The flip-flop model introduced in ref 42 describes the nonperiodic stick−slip response of the FAP ILs measured with the eSFA (Figure 8a,b). Stick−slip as resolved in SFA experiments has been classically considered to result from freezing (stick)−melting (slip) phase transitions or, more broadly, from a rearrangement of molecules under confinement.48 Melting is unlikely, because of Coulombic interactions between ions69 and also based on the measured shear-induced layered structure of [HMIM][FAP].42 Instead, we propose the slip events to be influenced by so-called “flip-flops”, which are cooperative processes in nanodomains within the contact area resulting from interlayer (perhaps intralayer) ion correlations at the shear plane. In summary, we assume the IL-boundary layer (∼1−1.5 nm) in the absence of bound water to consist of alternating cation and anion sublayers,17,70 the strongest interaction being between cation and surface, where the charge is localized. Thus, the shear plane is probably located between the cation and anion sublayers. During stick, the lower surface moves laterally at 0.1 μm/s, whereas at the shear plane, a cation sublayer Coulombically interacts with the upper anion layer; the upper layers and the surface are pulled laterally, with ions reorienting, rearranging, and realigning (the “flip” in the model), which leads to energy dissipation. As soon as the yield point in a nanodomain is reached, the upper layers flip back (“flop”), releasing kinetic energy (up to 10 times faster than the velocity of the lower surface). This can happen locally, leading to microslips, or collectively over the contact area, leading to a collective slip, but in both cases, it leads to energy dissipation with different relaxation times. Friction reduces until the cation sublayer interacts with the oncoming anion at the shear plane and they stick together again (the lower surface has moved effectively 30−50 nm at 0.1 μm/s). The flip-flop picture is consistent with Schallamach’s model,66 which describes sliding as a result of incoherent shearing and re-formation of nanodomains or junctions, as a rubber surface moves against its countersurface. Each nanodomain is stretched until it breaks, as a result of either thermal excitation or/and external force. This explanation is also in accord with Drummond et al.’s energy-dissipation model for weakly adhering surfactant-bearing surfaces.65 Furthermore, ion rearrangements require time to take place as the interactions break and form, which leads to the characteristic speed dependence of friction and a significant

influence of the IL composition: Whereas FAP anions are large and bulky, EtSO4 anions are smaller and “chainlike” and therefore easier/faster to orient or align during sliding. This chainlike structure of the EtSO4 anions favors faster rearrangement and bond rupture/formation, and therefore, it leads to the logarithmic dependence of friction on speed in the intermediate regime, as well as to smooth sliding at speeds as low as 0.1 μm/ s (critical speed for stick−slip of

Ionic Liquids Confined in Hydrophilic Nanocontacts: Structure and

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