Conservative and Dissipative Interactions of Ionic Liquids in

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Conservative and Dissipative Interactions of Ionic Liquids in Nanoconfinement James R. T. Seddon* Nanoionics, MESA + Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands ABSTRACT: By applying a small-amplitude (∼200 pm) oscillation to an atomic force microscopy probe during force−distance spectroscopy we are able to separate the “resistance to squeeze” of an ionic liquid nanoconfined between the probe and a mica sheet into its conservative and dissipative components. The interaction stiffness of the ionic liquid increases to a solid-like value as the mica is approached, while the “effective local viscosity” dramatically increases above the bulk value due to adjacent layers of ion-pairs interlocking as they slide over each other.

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INTRODUCTION Ionic liquids (ILs) are solvent-free salts that are liquid at ambient conditions. They are finding use in an increasing range of applications, such as lubrication, damping, energy storage, solvation, and catalysis.1−6 Due to their combination of low dielectric constant and high ionic strength, their Debye lengththe length scale over which ions in liquid can screen out wall chargeis typically less than a single molecular diameter. This means that historical continuum concepts of liquid electrostatics, specifically the electric double layer (EDL), are not valid: Charge screening can be accomplished through discrete ion rearrangement immediately adjacent to a charged wall. A key advantage of losing the diffuse ion region is a huge increase in capacitance. This has allowed ILs to find use in ultracapacitors for energy storage as well as inducing superconductivity in thin oxides, both of which benefit from maximal ionic packing. In fact, nontrivial localized packing constraints heavily amplify the role of electrostatics, affecting both the local ion structure and dynamic response.7−10 To date, much work has been carried out to try to uncover the local IL packing structure adjacent to a charged wall, including surface force apparatus,11−13 atomic force microscopy,10,14,15 and X-ray scattering.16,17 It is established that ILs form discrete molecular layers on interfaces, measured as resistance to squeeze in nanoconfinement. However, the separate roles of the conservative and dissipative interactions are not understood, i.e., which contribution to the resistance to squeeze comes from localized packing and which comes from molecular reorientation and localized interfacial flow. With this in mind, in this paper we detect individual molecular layers of an IL near a substrate using small-amplitude force−distance (FD) spectroscopy. By tapping the AFM cantilever at submolecular amplitudes we are able to detect © 2014 American Chemical Society

IL layering. Coupling this to quasi-statically slow wall approach allows us to map out both the conservative and dissipative components of the near-wall IL structure as a function of tip− sample separation. Our results demonstrate a layer-by-layer increase in local order (to solid-like) coupled with a dramatic increase in “local effective viscosity” as the substrate is approached.

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EXPERIMENTAL METHODS Freshly cleaved mica was used as the substrate (Bruker). The room temperature ionic liquid was 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]+[BF4]−) (>99%, Sigma-Aldrich) used as delivered by the manufacturer. We conducted cyclic voltammetry on the RTIL and found an electrochemical window of 3.8 V as compared to the “pure/dried” value of 4.3 V (Sigma-Aldrich). Furthermore, there was no evidence of water oxidation or reduction. To maintain purity, handling was conducted in an MBraun LABmaster 130 glovebox, operated under prepurified nitrogen with H2O and O2 both 3−4 nm) the force exerted on the cantilever is zero. This corresponds to the IL behaving as bulk liquid, i.e., we can detect no variation in the liquid due to the presence of the wall at these large distances. However, when the cantilever approaches within a distance of ∼3.5 nm a series of steps appears in the force curve. These 22198

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arises because of interaction with the structured interstitial IL. Although the form of Fts is unknown, we can proceed by Taylor expanding it around zero perturbation, with the first order expansion being Fts ≈ (∂Fts/∂z)z + (∂Fts/∂ż)ż. The first of these partial differentials can clearly be interpreted as an interaction stiffness, −kts, while the second is a damping term, −bts. The equation of motion for the cantilever then becomes mz̈ + btż + ktz = Fd, where kt = kc + kts and bt = bc + bts.22 The Ansatz z = Aei(ωt+ϕ) + Adeiωt readily yields the solution, where ω and ϕ are the frequency and phase of the cantilever, A is the amplitude of the free end of the cantilever, and Ad is the amplitude of the base of the cantilever (i.e., the driven end, not the free end). The reason for inclusion of the base motion is one of technical accuracy: AFMs measure motion with respect to the driven end, not with respect to the zero-amplitude position. Following the formulation above means we are able to separately extract both the conservative and dissipative components of the interaction by monitoring the variation to simple harmonic motion of the cantilever during interaction with local IL structure. The final expressions for the conservative and dissipative components of the tip−sample interaction in the presence of interstitial IL are k ts = −kc + mω 2 +

kcAd (Ad + A cos ϕ) Ad 2 + A2 + 2Ad A cos ϕ

(1)

and bts = −bc +

kcAd sin ϕ 2

ω(Ad + A2 + 2Ad A cos ϕ)

(2)

respectively. Ad can be found by measuring the amplitude far from the substrate, while we calculate bc and m from Sader’s expression for added mass due to the interaction between the cantilever beam and surrounding bulk fluid.20 Hence, we are now able to separately calculate the conservative and dissipative components of the tip/IL/mica interaction. We plot both the amplitude and phase in Figure 2 as a function of tip−sample separation. The amplitude of oscillation reduces from a free amplitude of ∼200 pm far from the wall to ∼100 pm as the mica is reached. The data for our IL appear in discrete packets at each structural layer of the liquid. We further note that the amplitude of oscillation at contact must clearly be zero; the nonzero value in Figure 2a is an artifact of the measurement technique (principally from the lock-in amplifier trying to extract frequency information from a white-noise thermal background). We plot the interaction stiffness, kts, in Figure 3a. The discrete layered structure is clearly visible but now the ordinate is system nonspecific, i.e., the layer-by-layer values of kts should be universal for the [Emim]+[BF4]−/mica interface. It is very interesting that the first layer of IL appears to be almost as stiff as the solid mica wall (i.e., the first IL layer appears almost “solid-like”). In fact, we cannot distinguish between the sublayer (attributed to anions on the AFM tip) and the mica wall at all, other than the fact that the sublayer eventually succumbs to squeeze and the tip pushes through. Further from the wall the stiffness reduces. This is because more compaction and rearrangement is allowed when more layers are present and the liquid’s isotropy increases, i.e., the last four liquid layers can rearrange during squeeze, probably through the molecules changing orientation. Such compaction/ rearrangement suggests multiple stable configurations near the

Figure 2. (a) Amplitude and (b) phase of the small-amplitude simple harmonic motion of the cantilever. In the near-wall vicinity the driven harmonic motion is perturbed by the presence of the IL layers.

wall, which is also compatible with known layer hysteresis.23 As these outer layers are pushed through, the remaining layers are less susceptible to changing orientation due to their stronger interaction with the mica. The final layer is completely incapable of molecular reorientation. Returning to Figure 1a, this is represented by the fact that the FD data for both the substrate and first layer have approximately the same slope, while the less stiff layers further from the substrate have smaller gradients in the FD plot. As well as the conservative component of the interaction of each layer of IL we also have access to the dissipative component, bts (Figure 3b). While the conservative component represents the interaction potential, the dissipative component can be identified as resistance to being squeezed out (i.e., resistance to flow). We must be careful in choosing our terminology for further analysis: We measure dissipation but can readily convert this into a “local” (or “effective”) viscosity. While “viscosity” is a bulk material property (without a nanoscopic counterpart), “effective viscosity”, μeff, is a useful parameter to indicate how quickly the layers of the near-wall structure can flow past each other. Conversion from bts to μeff is done using Reynolds’ lubrication approximation to the Navier− Stokes equations, i.e., bts = 6πμeff 22199

R tip2 d

(3)

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the IL behaves solid-like, whereas the following layers allow some amount of compaction/rearrangement. Next, we converted the dissipation into an “effective local viscosity” using Reynolds’ lubrication approximation to the Navier−Stokes equations. The local viscosity of the near-wall layers was dramatically higher than the bulk viscosity value of ∼35 mPa·s. As an example, the viscosity between the first and second IL layers was ∼20× higher than the bulk value. Our results are expected to have direct application to electroactuation: Local molecular structure changes IL flow dynamics, so manipulating the structure with an electrified interface will allow flow control. We also note that characterization of IL dissipation is essential for, e.g., quartz crystal microbalance measurements.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author thanks Mark Hempenius for use of the glovebox during the preliminary experiments, Serge Lemay for general support, and the University of Twente for financial support.

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REFERENCES

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Figure 3. (a) kts (interaction stiffness) and (b) bts (dissipation) of the locally structured IL adjacent to the mica substrate.

where Rtip is the radius of the AFM tip and d is the tip−wall separation. Using this, the first layer of [Emim]+[BF4]− on mica has an effective viscosity of μeff ≈ 550 mPa·s. This is dramatically higher than the bulk value of ∼35 mPa·s, i.e., the local near-wall structuring of the IL ions leads to increased intermolecular friction which slows down the dynamics in the squeeze-film geometry. The data for the dissipative component of the interaction in Figure 3b are complementary to the recent SFA work of Smith et al.,24 where they demonstrated that the [C4C1Pyrr][NTf2]/ mica system exhibits layered friction, i.e., the frictional force resisting motion of an IL in nanoconfinement increases stepwise for the last few structured liquid layers adjacent to mica. Thus, similar to the SFA shear data, we find that our IL provides significantly more resistance (here to squeeze-out) with its last few structured layers than its bulk properties would suggest.

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CONCLUSIONS In conclusion, we measured the layered structure of the IL [Emim]+[BF4]− adjacent to a mica wall using FD spectroscopy. The IL forms distinct layers adjacent to the wall, which provide sequentially more resistance to squeeze as the mica is approached. We separated the “force resisting squeeze” into its conservative and dissipative components by applying a smallamplitude oscillation to the AFM cantilever. The first layer of 22200

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