Nanoscale perturbations of room temperature ionic liquid structure at charged and uncharged interfaces

Molecular-Resolution Imaging of Ionic Liquid/Alkali Halide Interfaces with Varied Surface Charge Densities via Atomic Force Microscopy

When in contact with charged solid surfaces, ionic liquids (ILs) are known to form solvation structures consisting of alternating cation and anion layers. This phenomenon is considered to originate from the adsorption layer of counterions overcompensating the surface charge, so-called overscreening. However, the response of these layers to surfaces with near-zero or extremely high surface charge density (σ) remains inadequately understood. Here, we probe the solvation structure of ILs on alkali halide surfaces with varied surface orientations: nearly zero-charged RbI(100) and highly charged RbI(111), by employing frequency modulation atomic force microscopy with atomic resolution. Two commonly used ILs are examined in this study: 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C3mpyr][NTf2]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]). On RbI(100) surfaces with near zero σ, we observe alternating cation and anion layers, diverging from the previously proposed monolayer model for IL/alkali halide(100) interfaces. These results support the argument that overscreening occurs under low σ, even approaching zero, and reconcile conflicting experimental conclusions about low σ systems. On RbI(111) surfaces with high σ, we identify solvation structures consisting of two consecutive counterion layers. This structure aligns with the theoretically predicted crowding; a phenomenon rarely observed in commonly used ILs due to typically unreachable σ in electrochemical IL/electrode systems. Our findings indicate that alkali halide(111) surfaces are potentially valuable for exploring the crowding phenomenon in ILs, addressing the current scarcity of experimental observations.

Ordering of ionic liquids at a charged sapphire interface: Evolution with cationic chain length

HYPOTHESIS

Room Temperature Ionic Liquids (RTILs) bulk's molecular layering dominates their structure also at the RTIL/sapphire interface, increasing the layer spacing with the cationic alkyl chain length n. However, the negatively-charged sapphire surface compresses the layers, increases the layering range, and affects the intra-layer structure in yet unknown ways.

EXPERIMENTS

X-ray reflectivity (XR) off the RTIL/sapphire interface, for a broad homologous RTIL series 1-alkyl-3-methylimidazolium bis(trifluoromethansulfonyl)imide, hitherto unavailable for any RTIL.

FINDINGS

RTIL layers against the sapphire, exhibit two spacings: da and db. da is n-varying, follows the behavior of the bulk spacing but exhibits a downshift, thus showing significant layer compression, and over twofold polar slab thinning. The latter suggests exclusion of anions from the interfacial region due to the negative sapphire charging by x-ray-released electrons. The layering range is larger than the bulk's. db is short and near n-independent, suggesting polar moieties' layering, the coexistence mode of which with the da-spaced layering is unclear. Comparing the present layering with the bulk's and the RTIL/air interface's provides insight into the Coulomb and dispersion interaction balance dominating the RTIL's structure and the impact thereon of the presence of a charged solid interface.

Ion and water adsorption to graphene and graphene oxide surfaces

Graphene and graphene oxide (GO) are two particularly promising nanomaterials for a range of applications including energy storage, catalysis, and separations. Understanding the nanoscale interactions between ions and water near graphene and GO surfaces is critical for advancing our fundamental knowledge of these systems and downstream application success. This minireview highlights the necessity of using surface-specific experimental probes and computational techniques to fully characterize these interfaces, including the nanomaterial, surrounding water, and any adsorbed ions, if present. Key experimental and simulation studies considering water and ion structures near both graphene and GO are discussed. The major findings are: water forms 1-3 hydration layers near graphene; ions adsorb electrostatically to graphene under an applied potential; the chemical and physical properties of GO vary considerably depending on the synthesis route; and these variations influence water and ion adsorption to GO. Lastly, we offer outlooks and perspectives for these research areas.

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.

Molecular Dynamics Simulation of the Influence of External Electric Fields on the Glass Transition Temperature of the Ionic Liquid 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide

We present the results of molecular dynamics simulations of the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C₂C₁im][NTf₂] in the presence of external electric fields (EEFs) of varying strengths to understand the effects of EEFs on the glass transition temperature T_g. We compute T_g with an automated and objective method and observe a depression in T_g when cooling the IL within an EEF above a critical strength. The effect is reversible, and glasses prepared with EEFs recover their original zero-field T_g when heated. By examining the dynamics and structure of the liquid phase, we find that the EEF lowers the activation energy for diffusion, reducing the energetic barrier for movement and consequently T_g. We show that the effect can be leveraged to drive an electrified nonvapor compression refrigeration cycle.

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.

Trivalent ion overcharging on electrified graphene

The structure of the electrical double layer (EDL) formed near graphene in aqueous environments strongly impacts its performance for a plethora of applications, including capacitive deionization. In particular, adsorption and organization of multivalent counterions near the graphene interface can promote nonclassical behaviors of EDL including overcharging followed by co-ion adsorption. In this paper, we characterize the EDL formed near an electrified graphene interface in dilute aqueous YCl₃solution usingin situhigh resolution x-ray reflectivity (also known as crystal truncation rod) and resonant anomalous x-ray reflectivity (RAXR). These interface-specific techniques reveal the electron density profiles with molecular-scale resolution. We find that yttrium ions (Y³⁺) readily adsorb to the negatively charged graphene surface to form an extended ion profile. This ion distribution resembles a classical diffuse layer but with a significantly high ion coverage, i.e., 1 Y³⁺per 11.4 ± 1.6 Ų, compared to the value calculated from the capacitance measured by cyclic voltammetry (1 Y³⁺per ∼240 Ų). Such overcharging can be explained by co-adsorption of chloride that effectively screens the excess positive charge. The adsorbed Y³⁺profile also shows a molecular-scale gap (⩾5 Å) from the top graphene surfaces, which is attributed to the presence of intervening water molecules between the adsorbents and adsorbates as well as the lack of inner-sphere surface complexation on chemically inert graphene. We also demonstrate controlled adsorption by varying the applied potential and reveal consistent Y³⁺ion position with respect to the surface and increasing cation coverage with increasing the magnitude of the negative potential. This is the first experimental description of a model graphene-aqueous system with controlled potential and provides important insights into the application of graphene-based systems for enhanced and selective ion separations.

Ion correlations drive charge overscreening and heterogeneous nucleation at solid-aqueous electrolyte interfaces

Ion distributions at charged solid–water interfaces, referred to as the electrical double layer (EDL), are poorly understood at high ion concentrations, in part due to the lack of molecular-scale descriptions of the interactions between adsorbed hydrated ions. Here, direct visualization of the salinity-dependent evolution of EDL structure reveals molecular origins of nonclassical transformation of the EDL, in which charge overscreening and heterogeneous nucleation are driven by ion–ion correlations at the interfaces. This manifestation of the atomistic basis of nonclassical behaviors provides a much-needed understanding of the impact of ion cooperativity at charged interfaces for the development of predictive models for element transport in natural environments and advanced technologies for material growth and synthesis in saline environments.

Classical electrical double layer (EDL) models are foundational to the representation of atomistic structure and reactivity at charged interfaces. An important limitation to these models is their dependence on a mean-field approximation that is strictly valid for dilute aqueous solutions. Theoretical efforts to overcome this limitation are severely impeded by the lack of visualization of the structure over a wide range of ion concentration. Here, we report the salinity-dependent evolution of EDL structure at negatively charged mica–water interfaces, revealing transition from the Langmuir-type charge compensation in dilute salt solutions to nonclassical charge overscreening in highly concentrated solutions. The EDL structure in this overcharging regime is characterized by the development of both lateral positional correlation between adsorbed ions and vertical layering of alternating cations and anions reminiscent of the structures of strongly correlated ionic liquids. These EDL ions can spontaneously grow into nanocrystalline nuclei of ionic compounds at threshold ion concentrations that are significantly lower than the bulk solubility limit. These results shed light on the impact of ion cooperativity that drives heterogeneous nonclassical behaviors of the EDL in high-salinity conditions.

Comment on "Bi-layering at ionic liquid surfaces: a sum - frequency generation vibrational spectroscopy - and molecular dynamics simulation-based study" by T. Iwahashi, T. Ishiyama, Y. Sakai, A. Morita, D. Kim and Y. Ouchi, Phys. Chem. Chem. Phys., 2020, 22, 12565

This Comment raises several questions concerning the surface structure concluded in the paper referenced in the title. Specifically, that paper ignores previous experiments and simulations which demonstrate for the same ionic liquids depth-decaying, multilayered surface-normal density profiles rather than the claimed molecular mono- or bi-layers. We demonstrate that the claimed structure does not reproduce the measured X-ray reflectivity, which probes directly the surface-normal density profile. The measured reflectivities are found, however, to be well-reproduced by a multilayered density model. These results, and previous experimental and simulation results, cast severe doubt on the validity of the surface structure claimed in the paper referenced in the title.

Investigation of the Ionic Liquid Graphene Electric Double Layer in Supercapacitors Using Constant Potential Simulations

In this work, we investigate the effect of the cation structure on the structure and dynamics of the electrode–electrolyte interface using molecular dynamics simulations. A constant potential method is used to capture the behaviour of 1-ethyl-3-methylimidazolium bis (trifluoromethane)sulfonimide ([C2mim][NTf2]) and butyltrimethylammonium bis(trifluoromethane) sulfonimide ([N4,1,1,1][NTf2]) ionic liquids at varying potential differences applied across the supercapacitor. We find that the details of the structure in the electric double layer and the dynamics differ significantly, yet the charge profile and capacitance do not vary greatly. For the systems considered, charging results in the rearrangement and reorientation of ions within ∼1 nm of the electrode rather than the diffusion of ions to/from the bulk region. This occurs on timescales of O(10 ns) for the ionic liquids considered, and depends on the viscosity of the fluid.

Lateral Ordering in Nanoscale Ionic Liquid Films between Charged Surfaces Enhances Lubricity

Electric fields modify the structural and dynamical properties of room temperature ionic liquids (RTILs) providing a physical principle to develop tunable lubrication devices. Using nonequilibrium molecular dynamics atomistic simulations, we investigate the impact of the composition of imidazolium RTILs on the in-plane ordering of ionic layers in nanogaps. We consider imidazolium cations and widely used anions featuring different molecular structures, spherical ([BF₄]^-), elongated surfactant-like ([C₂SO₄]^-), and elongated with a more delocalized charge ([NTf₂]^-). The interplay of surface charge, surface polarity, and anion geometry enables the formation of crystal-like structures in [BF₄]^- and [NTf₂]^- nanofilms, while [C₂SO₄]^- nanofilms form disordered layers. We study how the ordering of the ionic liquid lubricant in the nanogap affects friction. Counterintuitively, we find that the friction force decreases with the ability of the RTILs to form crystal-like structures in the confined region. The crystallization can be activated or inhibited by changing the polarity of the surface, providing a mechanism to tune friction with electric fields.

Nanolubrication in deep eutectic solvents

We report surface force balance measurements of the normal surface force and friction between two mica surfaces separated by a nanofilm of the deep eutectic solvent ethaline. Ethaline, a 1 : 2 mixture of choline chloride and ethylene glycol, was studied under dry conditions, under ambient conditions and with added water, revealing surface structural layers and quantised frictional response highly sensitive to water content, including regions of super-lubric behaviour under dry conditions and with added water. We also report exceptionally long-ranged electrostatic repulsion far in excess of that predicted by Debye-Hückel theory for a system with such high electrolyte content, consistent with previously reported observations of "underscreening" in ionic liquid and concentrated aqueous electrolyte systems [Smith et al., J. Phys. Chem. Lett., 2016, 7(12), 2157].

How Viscous Is the Solidlike Structure at the Interface of Ionic Liquids? A Study Using Total Internal Reflection Fluorescence Spectroscopy with a Fluorescent Molecular Probe Sensitive to High Viscosity

Aiming at the evaluation of the viscosity of the interfacial solidlike structure of ionic liquids (ILs), we performed total internal reflection fluorescence (TIRF) spectroscopy for N,N-diethyl-N'-phenyl-rhodamine (Ph-DER), a fluorescent probe that is sensitive to viscosity in a high-viscosity range. TIRF spectra at the glass interface of trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide (TOMAC4C4N), a hydrophobic IL, showed that the fluorescence intensity of Ph-DER increases with the decrease of the evanescence penetration depth, suggesting that there exists a high-viscosity region at the interface. In contrast, glycerol, which is a molecular liquid with a bulk viscosity similar to that of TOMAC4C4N, did not show such a fluorescence increase, supporting that the formation of a highly viscous solidlike structure at the interface is intrinsic to ILs. A model analysis suggested that the high viscous region at the glass interface of TOMAC4C4N is at least twice thicker than the ionic multilayers at the air interface, implying that the solid substrate enhances the ordering of the interfacial structure of ILs. The viscosity at the glass interface of TOMAC4C4N was found to be at least 40 times higher than that of the liquid bulk.

Structural Anomalies and Electronic Properties of an Ionic Liquid under Nanoscale Confinement

Ionic liquids (ILs) promise far greater electrochemical performance compared to aqueous systems, yet key physicochemical properties governing their assembly at interfaces within commonly used graphitic nanopores remain poorly understood. In this work, we combine synchrotron X-ray scattering with first-principles molecular dynamics simulations to unravel key structural characteristics of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([TFSI]^-) ionic liquids confined in carbon slit pores. X-ray scattering reveals selective pore filling due to size exclusion, while filled pores exhibit disruption in the IL intermolecular structure, the extent of which increases for narrower slit pores. First-principles simulations corroborate this finding and quantitatively describe how perturbations in the local IL structure, particularly the hydrogen-bond network, depend strongly on the degree of confinement. Despite significant deviations in structure under confinement, electrochemical stability remains intact, which is important for energy storage based on nanoporous carbon electrodes (e.g., supercapacitors).

Evolution and Reversible Polarity of Multilayering at the Ionic Liquid/Water Interface

Highly correlated positioning of ions underlies Coulomb interactions between ions and electrified interfaces within dense ionic fluids such as biological cells and ionic liquids. Recent work has shown that highly correlated ionic systems behave differently than dilute electrolyte solutions, and interest is focused upon characterizing the electrical and structural properties of the dense electrical double layers (EDLs) formed at internal interfaces. It has been a challenge for experiments to characterize the progressive development of the EDL on the nanoscale as the interfacial electric potential is varied over a range of positive and negative values. Here we address this challenge by measuring X-ray reflectivity from the interface between an ionic liquid (IL) and a dilute aqueous electrolyte solution over a range of interfacial potentials from -450 to 350 mV. The growth of alternately charged cation-rich and anion-rich layers was observed along with a polarity reversal of the layers as the potential changed sign. These data show that the structural development of an ionic multilayer-like EDL with increasing potential is similar to that suggested by phenomenological theories and MD simulations, although our data also reveal that the excess charge beyond the first ionic layer decays more rapidly than predicted.

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.

Molecular Dynamics Simulation of the Ionic Liquid 1-n-Butyl-3-Methylimidazolium Methylsulfate [Bmim][MeSO4 ]: Interfacial Properties at the Silica and Vacuum Interfaces

Molecular dynamics simulations are done to investigate the structure and dynamics of a thin [Bmim][MeO₄ ] film in contact with a hydroxylated silica surface on one side and with vacuum on the other. An examination of the microscopic structure of ionic liquid (IL) film shows that strong layered anionic/cationic structures are formed at both interfaces. At the silica interface, the imidazolium rings are closer to the silica surface (compared to anions) and are coplanar with it. At the vacuum interface, the charged imidazolium ring more concentrates in the interior of the film, but the butyl side chain stretches out toward the vacuum interface. While there exists an excess concentration of the cations at the silica interface, at the vacuum interface an excess concentration of anions (dissolved in the butyl chain) is found. The influence of the interface on the dynamical properties is shown to depend on their time scales. A short-time dynamical property, such as hydrogen bond formation is not noticeably perturbed at the interface. In contrary, long-time properties such as ion-pair formation/rupture and translation of ions across the film are largely decelerated at the silica interface but are accelerate at the vacuum interface. Our findings indicate that the structural relaxation time of ion-pairs, is comparable to diffusion time scale in the IL film. Therefore, ion-pairs are not stable species; the IL is composed of short-lived ion-pairs and freely diffusing ions. However, the structural relaxation times of ion-pairs is still long enough (comparable to the time scale of diffusion) to conclude that correlated motions of counterions influence the macroscopic properties of IL, such as diffusion and ionic conductivity. In this respect, we have shown that correcting the Nernst-Einstein equation for the joint translation of ion-pairs considerably improves the accuracy of calculated ionic conductivities.

Electric double layer formation and storing energy processes on graphene-based supercapacitors from electrical and thermodynamic perspectives

Atomistic molecular dynamics simulations were used to investigate the processes of electrical double layer formation and electrolyte confinement in graphene-based supercapacitors. For both processes, free energy calculations were used to analyze the thermodynamics involved in the electrolyte confinement and its re-arrangement in a double layer on the electrode surface. The value of the free energy of the formation of the double electric layer was related to the energy required to charge the supercapacitor, i.e., the energy density stored, and compared with values obtained using Poisson's electrostatic formalism, which is the conventionally employed approach. Both analyzes were consistent with each other, presenting compatible values for the stored energy.

Uncovering the Underlying Mechanisms Governing the Solidlike Layering of Ionic Liquids (ILs) on Mica

Significant progress has been made in understanding the IL-solid interface in the past three decades, and a key finding is that ILs can form solidlike layers at the interface. It has been recognized that the electrostatic forces at the solid-IL interface and self-assembly of ILs are key enablers of the IL layering. However, regarding the layering structure of ILs, research from different laboratories is not consistent; i.e., the number of solidlike layers could range from 0 to ∼60, indicating the complexity of the underlying mechanisms and/or the existence of overlooked key parameters. In the current review, we will discuss the underlying mechanisms and key parameters governing the layering of ILs on mica, the most studied model solid. First, we will present the experimental findings from various laboratories, both consistent and contradictory ones, and summarize the current understanding of the governing mechanisms. Then, we will discuss the possible key parameters, including the structure of ILs, surface modification and contamination of mica, and cosolvent impacting the solidlike layering of ILs. Finally, we will discuss future research directions in uncovering the underlying mechanisms.

An electric double layer structure and differential capacitance at the electrode interface of tributylmethylammonium bis(trifluoromethanesulfonyl)amide studied using a molecular dynamics simulation

A molecular dynamics simulation at the electrode interface of a quaternary ammonium ionic liquid, tributylmethylammonium bis(trifluoromethanesulfonyl)amide ([N₁₄₄₄⁺][TFSA^-]), has been performed. Unlike the commonly used cations, such as 1-alkyl-3-methylimidazolium and 1,1-alkylmethylpyrrolidinium cations, N₁₄₄₄⁺ has multiple long-alkyl groups (three butyl groups). The behavior of ions at the electrode interface, especially these butyl groups, has been investigated. N₁₄₄₄⁺ at the first layer mainly has two types of orientations, lying and standing. The lying orientation is dominant at moderately negative potentials. However, the standing one becomes dominant at the more negative potentials. Due to this orientational change, the number of N₁₄₄₄⁺ increases at the first layer as the potential becomes negative even at the potentials where the anions are completely depleted there. The change in orientation results in the upward deviation of the differential capacitance from the theoretical prediction at the negative potentials. The results suggest that the orientational preference caused by the steric constraint between alkyl groups plays an important role in the behavior of the electric double layer of the ionic liquids.

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.

Interfacial structuring of non-halogenated imidazolium ionic liquids at charged surfaces: effect of alkyl chain length

Control of the interfacial structures of ionic liquids (ILs) at charged interfaces is important to many of their applications, including in energy storage solutions, sensors and advanced lubrication technologies utilising electric fields.

Ionic liquid lubricants: when chemistry meets tribology

Ionic liquids (ILs) have emerged as potential lubricants in 2001. Subsequently, there has been tremendous research interest in ILs from the tribology society since their discovery as novel synthetic lubricating materials. This also expands the research area of ILs. Consistent with the requirement of searching for alternative and eco-friendly lubricants, IL lubrication will experience further development in the coming years. Herein, we review the research progress of IL lubricants. Generally, the tribological properties of IL lubricants as lubricating oils, additives and thin films are reviewed in detail and their lubrication mechanisms discussed. Considering their actual applications, the flexible design of ILs allows the synthesis of task-specific and tribologically interesting ILs to overcome the drawbacks of the application of ILs, such as high cost, poor compatibility with traditional oils, thermal oxidization and corrosion. Nowadays, increasing research is focused on halogen-free ILs, green ILs, synthesis-free ILs and functional ILs. In addition to their macroscopic properties, the nanoscopic performance of ILs on a small scale and in small gaps is also important in revealing their tribological mechanisms. It has been shown that when sliding surfaces are compressed, in comparison with a less polar molecular lubricant, ion pairs resist "squeeze out" due to the strong interaction between the ions of ILs and oppositely charged surfaces, resulting in a film that remains in place at higher shear forces. Thus, the lubricity of ILs can be externally controlled in situ by applying electric potentials. In summary, ILs demonstrate sufficient design versatility as a type of model lubricant for meeting the requirements of mechanical engineering. Accordingly, their perspectives and future development are discussed in this review.

Interfacial Forces across Ionic Liquid Solutions: Effects of Ion Concentration and Water Domains †

Using the surface force apparatus (SFA), the interaction forces between mica surfaces across ionic liquid (IL) solutions are studied. The IL solution, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide in propylene carbonate solvent, is used at different concentrations to elucidate the ions' conformation at the interface from the analysis of short-range structural forces. A direct correlation between the ion layer thickness at the interface and the IL molar fraction in the solution is observed, suggesting conformational changes relative to the ion packing density. In addition, effects of large microscopic and macroscopic water domains at the interface are investigated. The microscopic water domains induced significant adhesion at contact because of the long-range capillary forces, which are found to depend on solvent concentration. The macroscopic water domains entirely cover the interaction area, ensuring that the long-range interfacial interactions occur entirely across the aqueous electrolyte solution with dissolved IL ions as the electrolyte. These results help elucidate the interfacial interactions in IL-charged solid interfaces with practical importance in green energy storage, catalysis, and lubrication.

Interfacial Structure and Boundary Lubrication of a Dicationic Ionic Liquid

We report measurements of the normal surface forces and friction forces between two mica surfaces separated by a nanofilm of dicationic ionic liquid using a Surface Force Balance. The dicationic ionic liquid 1,10-bis(3-methylimidazolium)decane di[bis(trifluoromethylsulfonyl)imide] forms a layered structure in nanoconfinement, revealed by oscillatory structural forces. Friction measurements performed at different film thicknesses display quantized friction, i.e., discontinuities in friction as layers are squeezed out and friction coefficients dependent on the number of liquid layers confined between the surfaces. The details of the friction traces indicate a liquidlike film, and, surprisingly, decreasing friction with increasing water content; we discuss possible mechanisms underlying these observations. This latter trait may be helpful in applications where ionic liquid lubricants cannot be insulated against humid environments.

Self-assembled nanostructures in ionic liquids facilitate charge storage at electrified interfaces

Driven by the potential applications of ionic liquids (ILs) in many emerging electrochemical technologies, recent research efforts have been directed at understanding the complex ion ordering in these systems, to uncover novel energy storage mechanisms at IL-electrode interfaces. Here, we discover that surface-active ILs (SAILs), which contain amphiphilic structures inducing self-assembly, exhibit enhanced charge storage performance at electrified surfaces. Unlike conventional non-amphiphilic ILs, for which ion distribution is dominated by Coulombic interactions, SAILs exhibit significant and competing van der Waals interactions owing to the non-polar surfactant tails, leading to unusual interfacial ion distributions. We reveal that, at an intermediate degree of electrode polarization, SAILs display optimum performance, because the low-charge-density alkyl tails are effectively excluded from the electrode surfaces, whereas the formation of non-polar domains along the surface suppresses undesired overscreening effects. This work represents a crucial step towards understanding the unique interfacial behaviour and electrochemical properties of amphiphilic liquid systems showing long-range ordering, and offers insights into the design principles for high-energy-density electrolytes based on spontaneous self-assembly behaviour.

Studies on Possible Ion-Confinement in Nanopore for Enhanced Supercapacitor Performance in 4V EMIBF4 Ionic Liquids

Supercapacitors have the rapid charge/discharge kinetics and long stability in comparison with various batteries yet undergo low energy density. Theoretically, square dependence of energy density upon voltage reveals a fruitful but challenging engineering tenet to address this long-standing problem by keeping a large voltage window in the compositionally/structurally fine-tuned electrode/electrolyte systems. Inspired by this, a facile salt-templating enables hierarchically porous biochars for supercapacitors filled by the high-voltage ionic liquids (ILs). Resultant nanostructures possess a coherent/interpenetrated framework of curved atom-thick sidewalls of 0.8-/1.5-nanometer pores to reconcile the pore-size-dependent adlayer structures of ILs in nanopores. Surprisingly, this narrow dual-model pore matches ionic radii of selected ILs to accommodate ions by unique coupled nano-/bi-layer nanoconfinements, augmenting the degree of confinement (DoC). The high DoC efficiently undermines the coulombic ordering networks and induces the local conformational oscillations, thus triggering an anomalous but robust charge separation. This novel bi-/mono-layer nanoconfinement combination mediates harmful overscreening/overcrowding effects to reinforce ion-partitioning, mitigating long-lasting conflicts of power/energy densities. This interesting result differs from a long-held viewpoint regarding the sieving effect that ion-in-pore capacitance peaks only if pore size critically approaches the ion dimension. Optimal biocarbon finally presents a very high/stable operational voltage up to 4 V and specific energy/power rating (88.3 Whkg^-1 at 1 kWkg^-1, 47.7 Whkg^-1 albeit at a high battery-accessible specific power density of 20 kWkg^-1), overwhelmingly outperforming most hitherto-reported supercapacitors and some batteries. Such attractive charge storage level can preliminarily elucidate an alternative form of a super-ionic-state high-energy storage linked with both the coordination number and coulombic periodism of the few ion-sized mesopores inside carbon electrodes, escalating supercapacitors into a novel criterion of charge delivery.

Charge Storage Mechanisms of Single-Layer Graphene in Ionic Liquid

Graphene-based carbon materials are promising candidates for electrical double-layer (EDL) capacitors, and there is considerable interest in understanding the structure and properties of the graphene/electrolyte interface. Here, electrochemical impedance spectroscopy (EIS) and electrochemical quartz crystal microbalance (EQCM) are used to characterize the ion fluxes and adsorption on single-layer graphene in neat ionic liquid (EMI-TFSI) electrolyte. It is found that a positively charged ion-species desorption and ion reorganization dominate the double-layer charging during positive and negative polarizations, respectively, leading to the increase in EDL capacitance with applied potential.

Toward Electrochemical Studies on the Nanometer and Atomic Scales: Progress, Challenges, and Opportunities

Electrochemical reactions and ionic transport underpin the operation of a broad range of devices and applications, from energy storage and conversion to information technologies, as well as biochemical processes, artificial muscles, and soft actuators. Understanding the mechanisms governing function of these applications requires probing local electrochemical phenomena on the relevant time and length scales. Here, we discuss the challenges and opportunities for extending electrochemical characterization probes to the nanometer and ultimately atomic scales, including challenges in down-scaling classical methods, the emergence of novel probes enabled by nanotechnology and based on emergent physics and chemistry of nanoscale systems, and the integration of local data into macroscopic models. Scanning probe microscopy (SPM) methods based on strain detection, potential detection, and hysteretic current measurements are discussed. We further compare SPM to electron beam probes and discuss the applicability of electron beam methods to probe local electrochemical behavior on the mesoscopic and atomic levels. Similar to a SPM tip, the electron beam can be used both for observing behavior and as an active electrode to induce reactions. We briefly discuss new challenges and opportunities for conducting fundamental scientific studies, matter patterning, and atomic manipulation arising in this context.

Molecular Insights into the Regulatable Interfacial Property and Flow Behavior of Confined Ionic Liquids in Graphene Nanochannels

The understanding of confined structure and flow property of ionic liquid (IL) in a nanochannel are essential for the efficient application of ILs in the green chemical processes. In this work, the ionic structure and various flow behaviors of ILs inside graphene nanochannels via molecular dynamics simulations are shown. The effect of the nanochannel structure on confined flow is explored, showing that the width mainly heightens the viscosity while the oxidation degree primarily enhances the interfacial friction coefficient. Tuning the width and oxidation degree of nanochannel, three different flow behaviors including Poiseuille, partial plunger and full plunger flow can be achieved, where the second one does not occur in water or other organic solvents. To describe the special flow behavior, an effective influence extent of the nanochannel (w _EIE ) is defined, whose value can distinguish the above flows effectively. Based on w _EIE , the phase diagrams of flow behavior for the nanochannel structure and pressure gradient are obtained, showing that the critical pressure gradient decreases with width and increases with the oxidation degree. Based on the quantitative relations between confined structures, viscosity, friction coefficient, flow behavior, and nanochannel structure, the intrinsic mechanism of regulating the flow behavior and rational design of nanochannel are finally discussed.

Utilizing Infrared Spectroscopy to Analyze the Interfacial Structures of Ionic Liquids/Al₂O₃ and Ionic Liquids/Mica Mixtures under High Pressures

The interfacial interactions between ionic liquids (1,3-dimethylimidazolium methyl sulfate and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate) and solid surfaces (mesoporous aluminum oxide and mica) have been studied by infrared spectroscopy at high pressures (up to 2.5 GPa). Under ambient pressure, the spectroscopic features of pure ionic liquids and mixtures of ionic liquids/solid particles (Al₂O₃ and mica) are similar. As the pressure is increased, the cooperative effect in the local structure of pure 1,3-dimethylimidazolium methyl sulfate becomes significantly enhanced as the imidazolium C–H absorptions of the ionic liquid are red-shifted. However, this pressure-enhanced effect is reduced by adding the solid particles (Al₂O₃ and mica) to 1,3-dimethylimidazolium methyl sulfate. Although high-pressure IR can detect the interactions between 1,3-dimethylimidazolium methyl sulfate and particle surfaces, the difference in the interfacial interactions in the mixtures of Al₂O₃ and mica is not clear. By changing the type of ionic liquid to 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, the interfacial interactions become more sensitive to the type of solid surfaces. The mica particles in the mixture perturb the local structure of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate under high pressures, forcing 1-ethyl-3-methylimidazolium trifluoromethanesulfonate to form into an isolated structure. For Al₂O₃, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate tends to form an associated structure under high pressures.

Molecular Mechanisms Underlying Lubrication by Ionic Liquids: Activated Slip and Flow

The present study provides molecular insight into the mechanisms underlying energy dissipation and lubrication of a smooth contact lubricated by an ionic liquid. We have performed normal and lateral force measurements with a surface forces apparatus and by colloidal probe atomic force microscopy on the following model systems: 1-ethyl-3-methyl imidazolium bis-(trifluoro-methylsulfonyl) imide, in dry state and in equilibrium with ambient (humid) air; the surface was either bare mica or functionalized with a polymer brush. The velocity-dependence of the friction force reveals two different regimes of lubrication, boundary-film lubrication, with distinct characteristics for each model system, and fluid-film lubrication above a transition velocity V∗. The underlying mechanisms of energy dissipation are evaluated with molecular models for stress-activated slip and flow, respectively. The stress-activated slip assumes that two boundary layers (composed of ions/water strongly adsorbed to the surface) slide past each other; the dynamics of interionic interactions at the slip plane and the strength of the interaction dictate the change in friction -decreasing, increasing or remaining constant- with velocity in the boundary-film lubrication regime. Above a transition velocity V∗, friction monotonically increases with velocity in the three model systems. Here, multiple layers of ions slide past each other (“flow”) under a shear stress and friction depends on a shear-activation volume that is significantly affected by confinement. The proposed friction model provides a molecular perspective of the lubrication of smooth contacts by ionic liquids and allows identifying the physical parameters that control friction.

Extended hierarchical solvent perturbations from curved surfaces of mesoporous silica particles in a deep eutectic solvent

HYPOTHESIS

Many applications of deep eutectic solvents (DES) rely on exploitation of their unique yet complex liquid structures. Due to the ionic nature of the DES components, their diffuse structures are perturbed in the presence of a charged surface. We hypothesize that it is possible to perturb the bulk DES structure far (>100 nm) from a curved, charged surface with mesoscopic dimensions.

EXPERIMENTS

We performed in situ, synchrotron-based ultra-small angle X-ray scattering (USAXS) experiments to study the solvent distribution near the surface of charged mesoporous silica particles (MPS) (≈0.5 µm in diameter) suspended in both water and a common type of DES (1:2 choline Cl-:ethylene glycol).

FINDINGS

A careful USAXS analysis reveals that the perturbation of electron density distribution within the DES extends ≈1 μm beyond the particle surface, and that this perturbation can be manipulated by the addition of salt ions (AgCl). The concentration of the pore-filling fluid is greatly reduced in the DES. Notably, we extracted the real-space structures of these fluctuations from the USAXS data using a simulated annealing approach that does not require a priori knowledge about the scattering form factor, and can be generalized to a wide range of complex small-angle scattering problems.

Structural and dynamic properties of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide/mica and graphite interfaces revealed by molecular dynamics simulation

It has been observed that the properties of room temperature ionic liquids near solid substrates are different from those of bulk liquids, and these properties play an important role in the development of catalysts, lubricants, and electrochemical devices. In this paper, we report microscopic studies of ionic liquid/solid interfaces performed using molecular dynamics simulations. The structural and dynamic properties of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMIM-TFSI) on mica and graphite interfaces were thoroughly investigated to elucidate the microscopic origins of the formation of layered structures at the interfaces. Our investigation included the observation of structural and orientational changes of ions as a function of distance from the surfaces, and contour mappings of ions parallel and perpendicular to the surfaces. By virtue of such detailed analyses, we found that, during the 5 ns simulation, the closest layer of BMIM-TFSI behaves as a two-dimensional ionic crystal on mica and as a liquid or liquid crystal on graphite.

Direct measurements of ionic liquid layering at a single mica-liquid interface and in nano-films between two mica-liquid interfaces

The layering of ionic liquids close to flat, charged interfaces has been identified previously through theoretical and some experimental measurements. Here we present evidence for oscillations in ion density ('layering') in a long chain ionic liquid (1-decyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide) near the interface with mica using two complementary approaches. Neutron reflection at the ionic liquid-mica interface is used to detect structure at a single interface, and surface force balance (SFB) measurements carried out with the same ionic liquid reveal oscillatory density in the liquid confined between two mica sheets. Our findings imply the interfacial structure is not induced by confinement alone. Structural forces between two mica surfaces extend to approximately twice the distance of the density oscillations measured at a single interface and have similar period in both cases.

Interfacial structure and structural forces in mixtures of ionic liquid with a polar solvent

Oscillatory and monotonic decay in mixtures of salt and solvent at interfaces with varying surface charge.

Molecular scale structure and dynamics at an ionic liquid/electrode interface

The structural arrangement and dynamics of ions near the IL/electrode interface during charging and discharging was studied by a combination of time resolved X-ray reflectivity and impedance spectroscopy.