Electrical charging effects on the sliding friction of a model nano-confined ionic liquid

Thin-Film Rheology and Tribology of Imidazolium Ionic Liquids

Ionic liquids (ILs) are organic molten salts with low-temperature melting points that hold promise as next-generation environmentally friendly boundary lubricants. This work examines the relationship between tribological and rheological behavior of thin films of five imidazolium ILs using a surface force apparatus to elucidate lubrication mechanisms. When confined to films of a few nanometers, the rheological properties change drastically as a function of the number of confined ion layers; not only the viscosity increases by several orders of magnitude but ILs can also undergo a transition from Newtonian to viscoelastic fluid and to an elastic solid. This behavior can be justified by the confinement-induced formation of supramolecular clusters with long relaxation times. The quantized friction coefficient is explained from the perspective of the strain relaxation via diffusion of these supramolecular clusters, where higher friction correlates with longer relaxation times. A deviation from this behavior is observed only for 1-ethyl-3-methylimidazolium ethylsulfate ([C2C1Im][EtSO4]), characterized by strong hydrogen bonding; this is hypothesized to restrict the reorganization of the confined IL into clusters and hinder (visco)elastic behavior, which is consistent with the smallest friction coefficient measured for this IL. We also investigate the contrasting influence of traces of water on the thin-film rheology and tribology of a hydrophobic IL, 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate, [C2C1Im][FAP], and a hydrophilic IL, [C2C1Im][EtSO4]. [C2C1Im][EtSO4] remains Newtonian under both dry and humid conditions and provides the best lubrication, while [C2C1Im][FAP], characterized by a prominent solid-like behavior under both conditions, is a poor lubricant. The results of this study may inspire molecular designs to enable efficient IL lubrication.

Theory and Simulations of Ionic Liquids in Nanoconfinement

Room-temperature ionic liquids (RTILs) have exciting properties such as nonvolatility, large electrochemical windows, and remarkable variety, drawing much interest in energy storage, gating, electrocatalysis, tunable lubrication, and other applications. Confined RTILs appear in various situations, for instance, in pores of nanostructured electrodes of supercapacitors and batteries, as such electrodes increase the contact area with RTILs and enhance the total capacitance and stored energy, between crossed cylinders in surface force balance experiments, between a tip and a sample in atomic force microscopy, and between sliding surfaces in tribology experiments, where RTILs act as lubricants. The properties and functioning of RTILs in confinement, especially nanoconfinement, result in fascinating structural and dynamic phenomena, including layering, overscreening and crowding, nanoscale capillary freezing, quantized and electrotunable friction, and superionic state. This review offers a comprehensive analysis of the fundamental physical phenomena controlling the properties of such systems and the current state-of-the-art theoretical and simulation approaches developed for their description. We discuss these approaches sequentially by increasing atomistic complexity, paying particular attention to new physical phenomena emerging in nanoscale confinement. This review covers theoretical models, most of which are based on mapping the problems on pertinent statistical mechanics models with exact analytical solutions, allowing systematic analysis and new physical insights to develop more easily. We also describe a classical density functional theory, which offers a reliable and computationally inexpensive tool to account for some microscopic details and correlations that simplified models often fail to consider. Molecular simulations play a vital role in studying confined ionic liquids, enabling deep microscopic insights otherwise unavailable to researchers. We describe the basics of various simulation approaches and discuss their challenges and applicability to specific problems, focusing on RTIL structure in cylindrical and slit confinement and how it relates to friction and capacitive and dynamic properties of confined ions.

Electrotunable friction with ionic liquid lubricants

Room-temperature ionic liquids and their mixtures with organic solvents as lubricants open a route to control lubricity at the nanoscale via electrical polarization of the sliding surfaces. Electronanotribology is an emerging field that has a potential to realize in situ control of friction-that is, turning the friction on and off on demand. However, fulfilling its promise needs more research. Here we provide an overview of this emerging research area, from its birth to the current state, reviewing the main achievements in non-equilibrium molecular dynamics simulations and experiments using atomic force microscopes and surface force apparatus. We also present a discussion of the challenges that need to be solved for future applications of electrotunable friction.

Microstructural and Dynamical Heterogeneities in Ionic Liquids

Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of ILs lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

Electrotunable Lubrication with Ionic Liquids: the Effects of Cation Chain Length and Substrate Polarity

Electrotunable lubrication with ionic liquids (ILs) provides dynamic control of friction with the prospect to achieve superlubrication. We investigate the dependence of the frictional and structural forces with 1-n,2-methyl-imidazolium tetrafluoroborate [C_nMIM]⁺[BF₄]^- (n = 2, 4, 6) ILs as a lubricant on the molecular structure of the liquid, normal load, and polarity of the electrodes. Using non-equilibrium molecular dynamics simulations and coarse-grained force-fields, we show that the friction force depends significantly on the chain length of the cation. ILs containing cations with shorter aliphatic chains show lower friction forces, ∼40% for n = 2 as compared to the n = 6 case, and more resistance to squeeze-out by external loads. The normal load defines the dynamic regime of friction, and it determines maxima in the friction force at specific surface charges. At relatively low normal loads, ∼10 MPa, the velocity profile in the confined region resembles a Couette type flow, whereas at high loads, >200 MPa, the motion of the ions is highly correlated and the velocity profile resembles a "plug" flow. Different dynamic regimes result in distinctive slippage planes, located either at the IL-electrode interface or in the interior of the film, which ultimately lead, at high loads, to the observation of maxima in the friction force at specific surface charge densities. Instead, at low loads the maxima are not observed, and the friction is found to monotonously increase with the surface charge. Friction with [C_nMIM]⁺[BF₄]^- as a lubricant is reduced when the liquid is confined between positively charged electrodes. This is due to better lubricating properties and enhanced resistance to squeeze out when the anion [BF₄]^- is in direct contact with the electrode.

Structural Forces in Mixtures of Ionic Liquids with Organic Solvents

Using molecular dynamics simulations, we study the impact of electrode charging and addition of solvent (acetonitrile, ACN) on structural forces of the BMIM PF₆ ionic liquid (IL) confined by surfaces at nanometer separations. We establish relationships between the structural forces and the microscopic structure of the confined liquid. Depending on the structural arrangements of cations and anions across the nanofilm, the load-induced squeeze-out of liquid layers occurs via one-layer or bilayer steps. The cations confined between charged plates orient with their aliphatic chain perpendicular to the surface planes and link two adjacent IL layers. These structures facilitate the squeeze-out of single layers. For both pure IL and IL-ACN mixtures, we observe a strong dependence of nanofilm structure on the surface charge density, which affects the simulated pressure-displacement curves. Addition of solvent to the IL modifies the layering in the confined film. At high electrode charges and high dilution of IL (below 10% molar fraction), the layered structure of the nanofilm is less well defined. We predict a change in the squeeze-out mechanism under pressure, from a discontinuous squeeze-out (for high IL concentrations) to an almost continuous one (for low IL concentrations). Importantly, our simulations show that charged electrodes are coated with ions even at low IL concentrations. These ion-rich layers adjacent to the charged plate surfaces are not squeezed out even under very high normal pressures of ∼5 GPa. Hence, we demonstrate the high performance of IL-solvent mixtures to protect surfaces from wear and to provide lubrication at high loads.

Recent highlights in nanoscale and mesoscale friction

Friction is the oldest branch of non-equilibrium condensed matter physics and, at the same time, the least established at the fundamental level. A full understanding and control of friction is increasingly recognized to involve all relevant size and time scales. We review here some recent advances on the research focusing of nano- and mesoscale tribology phenomena. These advances are currently pursued in a multifaceted approach starting from the fundamental atomic-scale friction and mechanical control of specific single-asperity combinations, e.g., nanoclusters on layered materials, then scaling up to the meso/microscale of extended, occasionally lubricated, interfaces and driven trapped optical systems, and eventually up to the macroscale. Currently, this "hot" research field is leading to new technological advances in the area of engineering and materials science.

Are Ionic Liquids Good Boundary Lubricants? A Molecular Perspective

The application of ionic liquids as lubricants has attracted substantial interest over the past decade and this has produced a rich literature. The aim of this review is to summarize the main findings about frictional behavior of ionic liquids in the boundary lubrication regime. We first recall why the unusual properties of ionic liquids make them very promising lubricants, and the molecular mechanisms at the origin of their lubricating behavior. We then point out the main challenges to be overcome in order to optimise ionic liquid lubricant performance for common applications. We finally discuss their use in the context of electroactive lubrication.

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.

Interfacial Structures of Trihexyltetradecylphosphonium-bis(mandelato)borate Ionic Liquid Confined between Gold Electrodes

Atomistic molecular dynamics simulations have been performed to study microscopic the interfacial ionic structures, molecular arrangements, and orientational preferences of trihexyltetradecylphosphonium-bis(mandelato)borate ([P_6,6,6,14][BMB]) ionic liquid confined between neutral and charged gold electrodes. It was found that both [P_6,6,6,14] cations and [BMB] anions are coabsorbed onto neutral electrodes at different temperatures. The hexyl and tetradecyl chains in [P_6,6,6,14] cations lie preferentially flat on neutral electrodes. The oxalato and phenyl rings in [BMB] anions are characterized by alternative parallel-perpendicular orientations in the mixed innermost ionic layer adjacent to neutral electrodes. An increase in temperature has a marginal effect on the interfacial ionic structures and molecular orientations of [P_6,6,6,14][BMB] ionic species in a confined environment. Electrifying gold electrodes leads to peculiar changes in the interfacial ionic structures and molecular orientational arrangements of [P_6,6,6,14] cations and [BMB] anions in negatively and positively charged gold electrodes, respectively. As surface charge density increases (but lower than 20 μC/cm²), the layer thickness of the mixed innermost interfacial layer gradually increases due to a consecutive accumulation of [P_6,6,6,14] cations and [BMB] anions at negatively and positively charged electrodes, respectively, before the formation of distinct cationic and anionic innermost layers. Meanwhile, the molecular orientations of two oxalato rings in the same [BMB] anions change gradually from a parallel-perpendicular feature to being partially characterized by a tilted arrangement at an angle of 45° from the electrodes and finally to a dominant parallel coordination pattern along positively charged electrodes. Distinctive interfacial distribution patterns are also observed accordingly for phenyl rings that are directly connected to neighboring oxalato rings in [BMB] anions.

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.

Molecular Dynamics Simulation of Bismuth Telluride Exfoliation Mechanisms in Different Ionic Liquid Solvents

Bismuth telluride (Bi₂Te₃) is a well-known thermoelectric material with potential applications in several different emerging technologies. The bulk structure is composed of stacks of quintuple sheets (with weak interactions between neighboring sheets), and the performance of the material can be significantly enhanced if exfoliated into two-dimensional nanosheets. In this study, eight different imidazolium-based ionic liquids are evaluated as solvents for the exfoliation and dispersion of Bi₂Te₃ at temperatures ranging from 350 to 550 K. Three distinct exfoliation mechanisms are evaluated (pulling, shearing, and peeling) using steered molecular dynamics simulations, and we predict that the peeling mechanism is thermodynamically the most favorable route. Furthermore, the [Tf₂N^-]-based ionic liquids are particularly effective at enhancing the exfoliation, and this performance can be correlated to the unique molecular-level solvation structures developed at the Bi₂Te₃ surfaces. This information helps provide insight into the molecular origins of exfoliation and solvation involving Bi₂Te₃ (and possibly other layered chalcogenide materials) and ionic liquid solvents.

Influence of electric potential on the apparent viscosity of an ionic liquid: facts and artifacts

According to recent findings, the steady shear viscosity of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim][Tf2N]) decreases significantly under the influence of electric potential. This implies a causal connection between nanoscale ordering at the electrified interface and a macroscopic change of transport properties. To study this phenomenon in more detail, we reproduced the above-mentioned measurements; however, we find no evidence that the viscosity of [Emim][Tf2N] is a function of electric potential. Additionally, our results show that steady shear measurements can lead to artifacts that, at first glance, may appear to be potential-induced changes in viscosity. We demonstrate that the artifacts result from a sliding electrical contact at the working electrode of the electrochemical cell and we suggest to consider our findings for future viscosity measurements of ionic liquids.

Friction and nonlinear dynamics

The nonlinear dynamics associated with sliding friction forms a broad interdisciplinary research field that involves complex dynamical processes and patterns covering a broad range of time and length scales. Progress in experimental techniques and computational resources has stimulated the development of more refined and accurate mathematical and numerical models, capable of capturing many of the essentially nonlinear phenomena involved in friction.

Tribotronic control of friction in oil-based lubricants with ionic liquid additives

Atomic force microscopy (AFM) reveals that tribotronic control of friction using an external potential applied to a gold surface is possible for ionic liquid (IL) concentrations as low as 5 mol% in hexadecane.