Quantized friction across ionic liquid thin films

Nanofriction of Graphene/Ionic Liquid-Infused Block Copolymer Homoporous Membranes

We have infused graphene/ionic liquid into block copolymer homoporous membranes (HOMEs), which have highly ordered uniform cylindrical nanopores, to form compact, dense, and continuous graphene/ionic liquid (Gr/IL) lubricating layers at interfaces, enabling a reduction in the friction coefficient. Raman and XPS analyses, confirmed the parallel alignment of the cation of ILs on graphene by the π-π stacking interaction of the imidazolium ring with the graphene layer. This alignment loosens the lattice spacing of Gr in Gr/ILs, leading to a larger lattice spacing of 0.36 nm in Gr of Gr/ILs hybrids than the pristine Gr (0.33 nm). The loose graphene layers, which are caused by the coexistence of graphene and ILs, would make the sliding easier, and favor the lubrication. An increase in the friction coefficient was observed on ILs-infused block copolymer HOMEs, as compared to Gr/ILs-infused ones, due to the absence of Gr and the unstably formed ILs film. Gr/ILs-infused block copolymer HOMEs also exhibit much smaller residual indentation depth and peak indentation depth in comparison with ILs-infused ones. This indicates that the existence of stably supported Gr/ILs hybrid liquid films aids the reduction of the friction coefficient by preventing the thinning of the lubricant layer and exposure of the underlying block copolymer HOMEs.

Near-Wall Molecular Ordering of Dilute Ionic Liquids

The interfacial behavior of ionic liquids promises tunable lubrication as well as playing an integral role in ion diffusion for electron transfer. Diluting the ionic liquids optimizes bulk parameters, such as electric conductivity, and one would expect dilution to disrupt the near-wall molecular ordering. We study this ordering in the ionic liquids [Emim]⁺[NTf₂]^-, [Emim]⁺[DCA]^-, and [C₄mpyr]⁺[NTf₂]^-, diluted in the solvent dimethyl sulfoxide. We found a structural crossover from well-ordered ionic liquids to a well-ordered solvent with increasing dilution, but this occurs nonlinearly, with solvent molecules initially space-filling and solvating and later disrupting the ionic layers. This is of key importance for ionic liquids as optimized tunable nanolubricants.

Controlling turbulent drag across electrolytes using electric fields

Reversible in operando control of friction is an unsolved challenge that is crucial to industrial tribology. Recent studies show that at low sliding velocities, this control can be achieved by applying an electric field across electrolyte lubricants. However, the phenomenology at high sliding velocities is yet unknown. In this paper, we investigate the hydrodynamic friction across electrolytes under shear beyond the transition to turbulence. We develop a novel, highly parallelised numerical method for solving the coupled Navier-Stokes Poisson-Nernst-Planck equation. Our results show that turbulent drag cannot be controlled across dilute electrolytes using static electric fields alone. The limitations of the Poisson-Nernst-Planck formalism hint at ways in which turbulent drag could be controlled using electric fields.

Insight into the Electrical Double Layer of an Ionic Liquid on Graphene

Graphene is a promising next-generation conducting material with the potential to replace traditional electrode materials in supercapacitors. Since energy storage in supercapacitors relies on the electrolyte-electrode interface, here we elucidate the interfacial subnanometer structure of a single component liquid composed solely of cations and anions – an ionic liquid- on electrified graphene. We study the effect of applied potential on the interaction between graphene and a silicon tip in an ionic liquid and describe it within the framework of the Derjaguin-Landau-Verwey-Overbeck (DLVO) theory. The energy is stored in an electrical double layer composed of an extended Stern layer, which consists of multiple ion layers over ~2 nanometers, beyond which a diffuse layer forms to compensate the applied potential on graphene. The electrical double layer significantly responds to the applied potential, and it shows the transition from overscreening to crowding of counterions at the interface at the highest applied potentials. It is proposed that surface charging occurs through the adsorption of the imidazolium cation to unbiased graphene (likely due to π-π interactions) and that the surface potential is better compensated when counterion crowding happens. This study scrutinizes the electrified graphene-ionic liquid interface, with implications not only in the field of energy storage, but also in lubrication.

Nanoscale capillary freezing of ionic liquids confined between metallic interfaces and the role of electronic screening

Room-temperature ionic liquids (RTILs) are new materials with fundamental importance for energy storage and active lubrication. They are unusual liquids, which challenge the classical frameworks of electrolytes, whose behaviour at electrified interfaces remains elusive, with exotic responses relevant to their electrochemical activity. Using tuning-fork-based atomic force microscope nanorheological measurements, we explore here the properties of confined RTILs, unveiling a dramatic change of the RTIL towards a solid-like phase below a threshold thickness, pointing to capillary freezing in confinement. This threshold is related to the metallic nature of the confining materials, with more metallic surfaces facilitating freezing. This behaviour is interpreted in terms of the shift of the freezing transition, taking into account the influence of the electronic screening on RTIL wetting of the confining surfaces. Our findings provide fresh views on the properties of confined RTIL with implications for their properties inside nanoporous metallic structures, and suggests applications to tune nanoscale lubrication with phase-changing RTILs, by varying the nature and patterning of the substrate, and application of active polarization.

Electrostatic interactions between ions near Thomas-Fermi substrates and the surface energy of ionic crystals at imperfect metals

The electrostatic interaction between two charged particles is strongly modified in the vicinity of a metal. This situation is usually accounted for by the celebrated image charges approach, which was further extended to account for the electronic screening properties of the metal at the level of the Thomas-Fermi description. In this paper we build upon a previous approach [M. A. Vorotyntsev and A. A. Kornyshev, Zh. Eksp. Teor. Fiz., 1980, 78(3), 1008-1019] and successive works to calculate the 1-body and 2-body electrostatic energy of ions near a metal in terms of the Thomas-Fermi screening length. We propose workable approximations suitable for molecular simulations of ionic systems close to metallic walls. Furthermore, we use this framework to calculate analytically the electrostatic contribution to the surface energy of a one dimensional crystal at a metallic wall and its dependence on the Thomas-Fermi screening length. These calculations provide a simple interpretation for the surface energy in terms of image charges, which allows for an estimation of the interfacial properties in more complex situations of a disordered ionic liquid close to a metal surface. The counter-intuitive outcome is that electronic screening, as characterized by a molecular Thomas-Fermi length l_TF, profoundly affects the wetting of ionic systems close to a metal, in line with the recent experimental observation of capillary freezing of ionic liquids in metallic confinement.

Switching the Structural Force in Ionic Liquid-Solvent Mixtures by Varying Composition

The structure and interactions in electrolytes at high concentration have implications from energy storage to biomolecular interactions. However, many experimental observations are yet to be explained in these mixtures, which are far beyond the regime of validity of mean-field models. Here, we study the structural forces in a mixture of ionic liquid and solvent that is miscible in all proportions at room temperature. Using the surface force balance to measure the force between macroscopic smooth surfaces across the liquid mixtures, we uncover an abrupt increase in the wavelength above a threshold ion concentration. Below the threshold concentration, the wavelength is determined by the size of the solvent molecule, whereas above the threshold, it is the diameter of a cation-anion pair that determines the wavelength.

Physicochemical and tribophysical properties of trioctylalkylammonium bis(salicylato)borate (N888n-BScB) ionic liquids: effect of alkyl chain length

The alkyl chain length in trioctylalkylammonium bis(salicylato)borate ionic liquids plays an important role in controlling the viscosity, friction and wear characteristics.

Electrotunable lubricity with ionic liquids: the influence of nanoscale roughness

The properties of ionic liquids can be modified by applying an external electrostatic potential, providing a route to control their performance in nanolubrication applications. Most computational studies to date have focused on the investigation of smooth surfaces. Real surfaces are generally inhomogeneous and feature roughness of different length scales. We report here a study of the possible effects that surface roughness may have on electrotunable lubricity with ionic liquids, performed here by means of non-equilibrium molecular dynamics simulations. In order to advance our understanding of the interplay of friction and substrate structure we investigate coarse grained models of ionic liquids confined in model surfaces with nanometer roughness. The friction is shown to depend on the roughness of the substrate and the direction of shear. For the investigated systems, the friction coefficient is found to increase with roughness. These results are in contrast with previous studies, where roughness induced reduction of friction was reported, and they highlight the strong sensitivity of the friction process to the structure of the surfaces. The friction force features a maximum at a specific surface charge density. This behaviour is reminiscent of the one reported in ionic liquids confined by flat surfaces, showing the generality of this physical effect in confined ionic liquids. We find that an increase of the substrate–liquid dispersion interactions shifts the maximum to lower surface charges. This effect opens a route to control electrotunable friction phenomena by tuning both the electrostatic potential and the composition of the confining surfaces.

Dynamic shear force microscopy of confined liquids at a gold electrode

The confinement of liquids in nanometer-scale gaps can lead to changes in their viscous shear properties. For liquids of polar molecules, the charge state of the confining surfaces has a significant influence on the structure in the confined liquid. Here we report on the implementation of dynamic shear force microscopy in an electrochemical cell. Lateral oscillations of the tip of an atomic force microscope were magnetically activated at a frequency of about 50 kHz. The damping of the lateral tip oscillation was recorded as a function of the tip–sample distance and of the electrode potential at the surface of a Au(100) single crystal electrode. The influence of surface charges on the shear response of the nano-confined liquids was demonstrated for the ionic liquid [EMIM][NTf₂] and for aqueous Na₂SO₄ solution.

Is the boundary layer of an ionic liquid equally lubricating at higher temperature?

Atomic force microscopy has been used to study the effect of temperature on normal forces and friction for the room temperature ionic liquid (IL) ethylammonium nitrate (EAN), confined between mica and a silica colloid probe at 25 °C, 50 °C, and 80 °C. Force curves revealed a strong fluid dynamic influence at room temperature, which was greatly reduced at elevated temperatures due to the reduced liquid viscosity. A fluid dynamic analysis reveals that bulk viscosity is manifested at large separation but that EAN displays a nonzero slip, indicating a region of different viscosity near the surface. At high temperatures, the reduction in fluid dynamic force reveals step-like force curves, similar to those found at room temperature using much lower scan rates. The ionic liquid boundary layer remains adsorbed to the solid surface even at high temperature, which provides a mechanism for lubrication when fluid dynamic lubrication is strongly reduced. The friction data reveals a decrease in absolute friction force with increasing temperature, which is associated with increased thermal motion and reduced viscosity of the near surface layers but, consistent with the normal force data, boundary layer lubrication was unaffected. The implications for ILs as lubricants are discussed in terms of the behaviour of this well characterised system.

Lithium-ion battery electrolyte mobility at nano-confined graphene interfaces

Interfaces are essential in electrochemical processes, providing a critical nanoscopic design feature for composite electrodes used in Li-ion batteries. Understanding the structure, wetting and mobility at nano-confined interfaces is important for improving the efficiency and lifetime of electrochemical devices. Here we use a Surface Forces Apparatus to quantify the initial wetting of nanometre-confined graphene, gold and mica surfaces by Li-ion battery electrolytes. Our results indicate preferential wetting of confined graphene in comparison with gold or mica surfaces because of specific interactions of the electrolyte with the graphene surface. In addition, wetting of a confined pore proceeds via a profoundly different mechanism compared with wetting of a macroscopic surface. We further reveal the existence of molecularly layered structures of the confined electrolyte. Nanoscopic confinement of less than 4–5 nm and the presence of water decrease the mobility of the electrolyte. These results suggest a lower limit for the pore diameter in nanostructured electrodes.

Electrochemical processes strongly depend on the wetting and mobility phenomena at the involved nano-confined interfaces. Here, the authors use a surface forces apparatus to understand the wetting behavior of graphene, gold, and mica surfaces by Li-ion battery electrolytes.

Dynamic shear force microscopy of viscosity in nanometer-confined hexadecane layers

Hexadecane exhibits pronounced molecular layering upon confinement to gaps of a few nanometer width which is discussed for its role in boundary lubrication. We have probed the mechanical properties of the confined layers with the help of an atomic force microscope, by quasi-static normal force measurements and by analyzing the lateral tip motion of a magnetically actuated torsional cantilever oscillation. The molecular layering is modeled by a oscillatory force curve and the tip approach is simulated assuming thermal equilibrium correlations in the liquid. The shear response of the confined layers reveals gradually increasing stiffness and viscous dissipation for a decreasing number of confined layers.

Poly(ethylene oxide) Mushrooms Adsorbed at Silica-Ionic Liquid Interfaces Reduce Friction

The adsorbed layer conformation and lubricity of 35, 100, and 300 kDa PEO adsorbed to ionic liquid (IL)-silica interfaces from 0.01 wt % solutions have been investigated using colloid probe atomic force microscopy. The ILs used were propylammonium nitrate (PAN) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), which are protic and aprotic ILs, respectively. Normal force curves reveal steric interactions consistent with adsorbed polymer layers which are best fit using the mushroom model. Friction measurements show that the adsorbed polymer layer markedly reduces friction compared to surfaces sliding in the pure ILs and that lubricity increases with polymer length. When polymer is adsorbed to the sliding surfaces, friction is controlled by the creation and disruption of intermolecular interactions between entangled chains and the dragging of polymer chains through the interpenetration region. These experiments show that added polymer can reduce friction while maintaining the useful properties of ILs as lubricants.

Layering of ionic liquids on rough surfaces

Understanding the behavior of ionic liquids (ILs) either confined between rough surfaces or in rough nanoscale pores is of great relevance to extend studies performed on ideally flat surfaces to real applications. In this work we have performed an extensive investigation of the structural forces between two surfaces with well-defined roughness (<9 nm RMS) in 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide by atomic force microscopy. Statistical studies of the measured layer thicknesses, layering force, and layering frequency reveal the ordered structure of the rough IL-solid interface. Our work shows that the equilibrium structure of the interfacial IL strongly depends on the topography of the contact.

Influence of Environmental Humidity on the Wear and Friction of a Silica/Silicon Tribopair Lubricated with a Hydrophilic Ionic Liquid

In this study, the tribological behavior of silica/silicon surfaces lubricated with the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM] EtSO4) was investigated. Tests were carried out in the presence of either humid air (45-55% relative humidity) or in a nitrogen atmosphere, and the results were compared with those obtained using pure water as a lubricant. The cross-sectional analysis of the contact area performed by focused-ion-beam scanning electron microscopy indicated the presence of cracks in the subsurface region, showing that brittle fracture contributed to wear. Sliding promoted the formation of a third body, the presence of which was indicated by optical and secondary electron microscopy. X-ray photoelectron spectroscopy showed that the third body was mostly composed of silicon oxides. The accumulation of the debris was controlled by the presence of water: in the presence of a nitrogen atmosphere, particles were trapped between the sliding surfaces, whereas in the case of humid air, the debris was progressively removed from the contact. Notably, the presence of trapped particles was associated with higher values of wear coefficients of both disks and pins. In addition, a lower roughness was observed along the direction of sliding in the case of water-containing ionic liquid. The observed trends in wear and the combined results of the various techniques, as well as the comparison with tests carried out in the presence of pure water, all point to the characteristic tribochemical reactions of water with silicon-based materials, namely, the formation of a sacrificial layer of hydrated oxide and the dissociative adsorption of water at crack tips of SiO2. In the absence of water, the lack of a tribochemical mechanism forming a sacrificial layer leads to a microfracture-dominated wear mechanism over the entire duration of the test, thus leading to more severe wear. The possible occurrence of stress-induced phase transformation of silicon during sliding is also discussed.

Metal ion adsorption at the ionic liquid-mica interface

Mica has been employed in many studies of ionic liquid (IL) interfaces on account of its atomic smoothness and well defined surface properties. However, until now it has been unclear whether ions dissolved in ILs can compete with the IL cation and adsorb to mica charge sites. In this work amplitude modulated atomic force microscopy (AM-AFM) has been used to probe metal ion adsorption at the interface of mica with propylammonium nitrate (PAN), a room temperature IL. Lithium, sodium, potassium, magnesium and calcium nitrate salts were added to PAN at a concentration of ∼60 mM. Aluminum nitrate was also investigated, but only at 5 mM because its solubility in PAN is much lower. The AM-AFM images obtained when the metal ions were present are strikingly different to that of pure PAN, indicating that the ions compete effectively with the propylammonium cation and adsorb to negatively charged sites on the mica surface despite their much lower concentration. This is a consequence of electrostatic attractions between the mica charge sites and the metal ions being significantly stronger than for the propylammonium cation; compared to the metal ions the propylammonium charged group is relatively constrained sterically. A distinct honeycomb pattern is noted for the PAN + Al(3+) system, less obviously for the divalent ions and not at all for monovalent ions. This difference is attributed to the strength of electrostatic interactions between metal ions and mica charge sites increasing with the ion charge, which means that divalent and (particularly) trivalent ions are located more precisely above the charged sites of the mica lattice. The images obtained allow important distinctions between metal ion adsorption at mica-water and mica-PAN interfaces to be made.

Effect of the environmental humidity on the bulk, interfacial and nanoconfined properties of an ionic liquid

Structural and dynamical properties of ILs are altered by the weakening of ion–ion correlations in the presence of water.

Electrotunable Friction with Ionic Liquid Lubricants: How Important Is the Molecular Structure of the Ions?

Using nonequilibrium molecular dynamics simulations and a coarse-grained model of ionic liquids, we have investigated the impact that the shape and the intramolecular charge distribution of the ions have on the electrotunable friction with ionic liquid nanoscale films. We show that the electric field induces significant structural changes in the film, leading to dramatic modifications of the friction force. Comparison of the present work with previous studies using different models of ionic liquids indicate that the phenomenology presented here applies to a wide range of ionic liquids. In particular, the electric-field-induced shift of the slippage plane from the solid-liquid interface to the interior of the film and the nonmonotonic variation of the friction force are common features of ionic lubricants under strong confinement. We also demonstrate that the molecular structure of the ions plays an important role in determining the electrostriction and electroswelling of the confined film, hence showing the importance of ion-specific effects in electrotunable friction.

Irreversible structural change of a dry ionic liquid under nanoconfinement

Studies of 1-hexyl-3-methyl-imidazolium ethylsulfate ([HMIM] EtSO4) using an extended surface forces apparatus show, for the first time, an ordered structure within the nanoconfined ionic liquid (IL) between mica surfaces that extends up to ∼60 nm from the surface. Our measurements show the growth of this ordered IL-film upon successive nanoconfinements-the structural changes being irreversible upon removal of the confinement-and the response of the structure to shear. The compressibility of this system is lower than that typically measured for ILs, while creep takes place during shear, both findings supporting a long-range liquid-to-solid transition. AFM (sharp-tip) studies of [HMIM] EtSO4 on mica only reveal ∼2 surface IL-layers, with order extending only ∼3 nm from the surface, indicating that confinement is required for the long-range IL-solidification to occur. WAXS studies of the bulk IL show a more pronounced ordered structure than is the case for [HMIM] with bis(trifluoromethylsulfonyl)imide as anion, but no long-range order is detected, consistent with the results obtained with the sharp AFM tip. These are the first force measurements of nanoconfinement-induced long-range solidification of an IL.

Squeezout phenomena and boundary layer formation of a model ionic liquid under confinement and charging

Electrical charging of parallel plates confining a model ionic liquid down to nanoscale distances yields a variety of charge-induced changes in the structural features of the confined film. That includes even-odd switching of the structural layering and charging-induced solidification and melting, with important changes of local ordering between and within layers, and of squeezout behavior. By means of molecular dynamics simulations, we explore this variety of phenomena in the simplest charged Lennard-Jones coarse-grained model including or excluding the effect a neutral tail giving an anisotropic shape to one of the model ions. Using these models and open conditions permitting the flow of ions in and out of the interplate gap, we simulate the liquid squeezout to obtain the distance dependent structure and forces between the plates during their adiabatic approach under load. Simulations at fixed applied force illustrate an effective electrical pumping of the ionic liquid, from a thick nearly solid film that withstands the interplate pressure for high plate charge to complete squeezout following melting near zero charge. Effective enthalpy curves obtained by integration of interplate forces versus distance show the local minima that correspond to layering and predict the switching between one minimum and another under squeezing and charging.

Electrotunable lubricity with ionic liquid nanoscale films

One of the main challenges in tribology is finding the way for an in situ control of friction without changing the lubricant. One of the ways for such control is via the application of electric fields. In this respect a promising new class of lubricants is ionic liquids, which are solvent-free electrolytes, and their properties should be most strongly affected by applied voltage. Based on a minimal physical model, our study elucidates the connection between the voltage effect on the structure of the ionic liquid layers and their lubricating properties. It reveals two mechanisms of variation of the friction force with the surface charge density, consistent with recent AFM measurements, namely via the (i) charge effect on normal and in-plane ordering in the film and (ii) swapping between anion and cation layers at the surfaces. We formulate conditions that would warrant low friction coefficients and prevent wear by resisting "squeezing-out" of the liquid under compression. These results give a background for controllable variation of friction.

Near surface properties of mixtures of propylammonium nitrate with n-alkanols 2. Nanotribology and fluid dynamics

Colloid probe friction force microscopy (FFM) has been used to study the lubricity of propylammonium nitrate (PAN) mixed with n-alkanols confined between sliding silica and mica surfaces.

Nanolubrication by ionic liquids: molecular dynamics simulations reveal an anomalous effective rheology

All-atom molecular dynamics simulations of an ionic liquid confined between iron oxide surfaces reveal an anomalous effective rheology induced by hydrodynamic and thermal slip at the walls.

Molecular Friction Mechanisms Across Nanofilms of a Bilayer-Forming Ionic Liquid

The prevailing paradigm in boundary lubrication asserts, in essence, that surfaces coated in amphiphiles slide past each other by way of the "slippery" exposed alkyl chains while the polar head group remains anchored at the surface. Here we show, for ionic liquid boundary lubricants, that the molecular mechanism of shearing is more subtle; while a monolayer on each surface gives rise to alkyl plane shearing, a bilayer on each surface shears at the ionic (nonalkyl) interface. The incorporation of water from the environment dramatically alters the shear at ionic interfaces but leaves alkyl plane shearing unaffected. Our experiments involve shearing two identical and atomically smooth surfaces past one another with films of an ionic liquid between, with subnanometer control of the film thickness and ultrasensitive shear stress resolution. With this, we uncover molecular mechanistic details relevant to boundary lubrication in general and the development of ionic liquid lubricants in particular.

The impact of anion electronic structure: similarities and differences in imidazolium based ionic liquids

In this paper the structural and energetic landscapes of ion-pair dimer conformers of 1,3-dimethylimidazolium based ionic liquids have been explored ([C1C1im][A])2, A = Cl(-), [NO3](-), [MeSO4](-), [OTf](-) and [BF4](-)). A common low-energy conformer has been selected for full electronic structure analysis. We have compared and contrasted each cluster based on the relative hydrogen bonding ability (β-value) of the anion, which varies experimentally as Cl(-) > [NO3](-) ≈ [MeSO4](-) > [OTf](-) ≈ [BF4](-). Correlations between experimental β-values, computed binding energies, charge transfer and various hydrogen bonding data have been made and outliers have been explained in terms of environmental effects present in the liquid phase. This is most evident in the structurally similar [MeSO4](-) and [OTf](-) anions that have very similar hydrogen bonding motifs, but significantly different β-values. Moreover, detailed analysis of the cluster molecular orbitals, for each anion, reveals a subtle interplay between two modes of interaction, an in-plane traditional H-bonding and inter-planar anion-π interaction. Inter-planar anion-π interactions are particularly prominent for the [NO3](-) cluster. We have rationalized how the full range of interactions could impact on the structuring of ILs at surfaces and the effect these may have on viscosity.

Force microscopy of layering and friction in an ionic liquid

The mechanical properties of the ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([Py1,4][FAP]) in confinement between a SiOx and a Au(1 1 1) surface are investigated by means of atomic force microscopy (AFM) under electrochemical control. Up to 12 layers of ion pairs can be detected through force measurements while approaching the tip of the AFM to the surface. The particular shape of the force versus distance curve is explained by a model for the interaction between tip, gold surface and ionic liquid, which assumes an exponentially decaying oscillatory force originating from bulk liquid density correlations. Jumps in the tip-sample distance upon approach correspond to jumps of the compliant force sensor between branches of the oscillatory force curve. Frictional force between the laterally moving tip and the surface is detected only after partial penetration of the last double layer between tip and surface.

Interface controls spontaneous crystallization in thin films of the ionic liquid [C₂C₁Im][OTf] on atomically clean Pd(111)

A total of 5-30 monolayer thick films of the ionic liquid (IL) [C2C1Im][OTf] were vaporized in vacuo onto an atomically clean Pd(111) single crystal surface at 220 K. Time- and temperature-resolved infrared reflection-absorption spectroscopy reveals growth, interactions with the metallic support, and the macroscopic phase behavior of the layer. At 220 K, the IL layer first grows in the form of a glassy phase. Crystallization of the IL was witnessed above a critical thickness of about 10 monolayers. On the basis of the known bulk crystal structure of the IL, we suggest the formation of well-oriented checkerboard-like crystalline film structures on the surface. The preferential orientation manifested by the crystal phase with regard to the macroscopic metallic surface is attributed to strong interactions between anionic headgroups and the metal.

Microslips to "Avalanches" in Confined, Molecular Layers of Ionic Liquids

We have measured forces between mica surfaces across two hydrophobic ionic liquids with a surface forces apparatus. Both surface-adsorbed water and alkyl-chain length on the imidazolium cation influence the structure of the nanoconfined film and the dynamics of film-thickness transitions. Friction shows accumulative microslips as precursors to collective "avalanches" that abruptly reduce friction momentarily. This behavior is interpreted as a consequence of interlayer ion correlations within the 1 to 2 nm thick film; we identify this to be analogous to the friction response of crackling noise systems over a broad range of sizes.

Shear dynamics of nanoconfined ionic liquids

The solid–liquid interfacial structure of ILs, determined by their molecular shape, results in different flow dynamics and friction under shear stress.