Nanostructure and Dynamics of the Locally Concentrated Ionic Liquid 2:1 (wt:wt) HMIM FAP:TFTFE and HMIM FAP on Graphite and Gold Electrodes as a Function of Potential

Video-rate atomic force microscopy (AFM) is used to record the near-surface nanostructure and dynamics of one pure ionic liquid (IL), 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (HMIM FAP), and a locally-concentrated IL comprising HMIM FAP with the low viscosity diluent 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (TFTFE), on highly oriented pyrolytic graphite (HOPG) and Au(111) electrodes as a function of potential. Over the potential range measured (open-circuit potential ± 1 V), different near-surface nanostructures are observed. For pure HMIM FAP, globular aggregates align in rows on HOPG, whereas elongated and worm-like nanostructures form on Au(111). For 2:1 (wt:wt) HMIM FAP:TFTFE, larger and less defined diluent swollen IL aggregates are present on both electrodes. Long-lived near-surface nanostructures for HMIM FAP and the 2:1 (wt:wt) HMIM FAP:TFTFE persist on both electrodes. 2:1 (wt:wt) HMIM FAP:TFTFE mixture diffuses more rapidly than pure HMIM FAP on both electrodes with obviously higher diffusion coefficients on HOPG than on Au(111) due to weaker electrostatic and solvophobic interactions between near-surface aggregates and Stern layer ions. These outcomes provide valuable insights for a wide range of IL applications in interface sciences, including electrolytes, catalysts, lubricants, and sensors.

Nanostructure and Dynamics of Aprotic Ionic Liquids at Graphite Electrodes as a Function of Potential

Atomic force microscope (AFM) videos reveal the near-surface nanostructure and dynamics of the ionic liquids (ILs) 1-butyl-3-methylimidazolium dicyanamide (BMIM DCA) and 1-hexyl-3-methylimidazolium dicyanamide (HMIM DCA) above highly oriented pyrolytic graphite (HOPG) electrodes as a function of surface potential. Molecular dynamics (MD) simulations reveal the molecular-level composition of the nanostructures. In combination, AFM and MD show that the near-surface aggregates form via solvophobic association of the cation alkyl chains at the electrode interface. The diffusion coefficients of interfacial nanostructures are ≈0.01 nm2 s-1 and vary with the cation alkyl chain length and the surface potential. For each IL, the nanostructure diffusion coefficients are similar at open-circuit potential (OCP) and OCP + 1V, but BMIM DCA moves about twice as fast as HMIM DCA. At negative potentials, the diffusion coefficient decreases for BMIM DCA and increases for HMIM DCA. When the surface potential is switched from negative to positive, a sudden change in the direction of the nanostructure motion is observed for both BMIM DCA and HMIM DCA. No transient dynamics are noted following other potential jumps. This study provides a new fundamental understanding regarding the dynamics of electrochemically stable ILs at electrodes vital for the rational development of IL-based electrochemical devices.

Disparate External Electric Field Effect on the Adsorption and Shear Behavior of Monovalent and Trivalent Ions in Electrolyte Solution

Reducing friction is of great interest, and an external potential applied to the friction pair can regulate lubricity. Electrochemical atomic force microscopy (EC-AFM) is used to study the tribological and adsorption behavior of monovalent and trivalent ionic solutions between charged surfaces. An opposite trend of coefficient of friction (COF) and normal force that varies with the applied electric potential is witnessed. Direct force measurements and theoretical models have disclosed that, for the NaCl solution, the negative electric field reduces the COF by increasing cation adsorption. As for LaCl3 solution, the positive electric field promotes the primary adsorption of anions on HOPG, resulting in the disappearance of the attractive ion-ion correlation between the trivalent ions, thereby reducing the COF. The shear behavior of adsorbed ions in electrolyte solution is sensitive to their valence, because of their different surface force contribution. The study further provides a framework to optimize the design of hydration lubrication.

Understanding the Electrode-Electrolyte Interfaces of Ionic Liquids and Deep Eutectic Solvents

Developing unconventional electrolytes such as ionic liquids (ILs) and deep eutectic solvents (DESs) has led to remarkable advances in electrochemical energy storage and conversion devices. However, the understanding of the electrode-electrolyte interfaces of these electrolytes, specifically the liquid structure and the charge/electron transfer mechanism and rates, is lacking due to the complexity of molecular interactions, the difficulty in studying the buried interfaces with nanometer-scale resolution, and the distribution of the time scales for the various interfacial events. This Feature Article outlines the standing questions in the field, summarizes some of the exciting approaches and results, and discusses our contributions to probing the electrified interfaces by electrochemical impedance spectroscopy (EIS), surface-enhanced Raman spectroscopy (SERS), and neutron reflectivity (NR). The related findings are analyzed within electrical double-layer models to provide a framework for studying ILs, DESs, and, more broadly, the concentrated hydrogen-bonded electrolytes.

Another Piece of the Ionic Liquid's Puzzle: Adsorption of Cl- Ions

Classical electrochemical and microscopy methods were used to characterize the interfacial processes of the adsorption of chloride ions from ionic liquids at the Bi(111) single crystal electrode. The mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium chloride was electrochemically characterized by using cyclic voltammetry and electrochemical impedance spectroscopy. In situ scanning tunneling microscopy images showed the formation of superstructures at the electrode's surface over an extended period of time. The specific adsorption of chloride ions reaches an equilibrium state in a more viscous ionic liquid medium slower than in aqueous and organic solvents. Capacitance values increase considerably (also depending on alternative current frequency) at the potential region, where the specific adsorption of chloride ions with partial charge transfer occurs.

Nanostructure of Locally Concentrated Ionic Liquids in the Bulk and at Graphite and Gold Electrodes

The physical properties of ionic liquids (ILs) have led to intense research interest, but for many applications, high viscosity is problematic. Mixing the IL with a diluent that lowers viscosity offers a solution if the favorable IL physical properties are not compromised. Here we show that mixing an IL or IL electrolyte (ILE, an IL with dissolved metal ions) with a nonsolvating fluorous diluent produces a low viscosity mixture in which the local ion arrangements, and therefore key physical properties, are retained or enhanced. The locally concentrated ionic liquids (LCILs) examined are 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HMIM TFSI), 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (HMIM FAP), or 1-butyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate (BMIM FAP) mixed with 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (TFTFE) at 2:1, 1:1, and 1:2 (w/w) IL:TFTFE, as well as the locally concentrated ILEs (LCILEs) formed from 2:1 (w/w) HMIM TFSI-TFTFE with 0.25, 0.5, and 0.75 m lithium bis(trifluoromethylsulfonyl)imide (LiTFSI). Rheology and conductivity measurements reveal that the added TFTFE significantly reduces viscosity and increases ionic conductivity, and cyclic voltammetry (CV) reveals minimal reductions in electrochemical windows on gold and carbon electrodes. This is explained by the small- and wide-angle X-ray scattering (S/WAXS) and atomic force microscopy (AFM) data, which show that the local ion nanostructures are largely retained in LCILs and LCILEs in bulk and at gold and graphite electrodes for all potentials investigated.

Separating Convective from Diffusive Mass Transport Mechanisms in Ionic Liquids by Redox Pro-fluorescence Microscopy

The study of electrochemical reactivity requires analytical techniques capable of probing the diffusion of reactants and products to and from electrified interfaces. Information on diffusion coefficients is often obtained indirectly by modeling current transients and cyclic voltammetry data, but such measurements lack spatial resolution and are accurate only if mass transport by convection is negligible. Detecting and accounting for adventitious convection in viscous and wet solvents, such as ionic liquids, is technically challenging. We have developed a direct, spatiotemporally resolved optical tracking of diffusion fronts which can detect and resolve convective disturbances to linear diffusion. By tracking the movement of an electrode-generated fluorophore, we demonstrate that parasitic gas evolving reactions lead to 10-fold overestimates of macroscopic diffusion coefficients. A hypothesis is put forward linking large barriers to inner-sphere redox reactions, such as hydrogen gas evolution, to the formation of cation-rich overscreening and crowding double layer structures in imidazolium-based ionic liquids.

Molecular Resolution Nanostructure and Dynamics of the Deep Eutectic Solvent-Graphite Interface as a Function of Potential

Interest in deep eutectic solvents (DESs), particularly for electrochemical applications, has boomed in the past decade because they are more versatile than conventional electrolyte solutions and are low cost, renewable, and non-toxic. The molecular scale lateral nanostructures as a function of potential at the solid-liquid interface-critical design parameters for the use of DESs as electrochemical solvents-are yet to be revealed. In this work, in situ amplitude modulated atomic force microscopy complemented by molecular dynamics simulations is used to probe the Stern and near-surface layers of the archetypal and by far most studied DES, 1:2 choline chloride:urea (reline), at the highly orientated pyrolytic graphite surface as a function of potential, to reveal highly ordered lateral nanostructures with unprecedented molecular resolution. This detail allows identification of choline, chloride, and urea in the Stern layer on graphite, and in some cases their orientations. Images obtained after the potential is switched from negative to positive show the dynamics of the Stern layer response, revealing that several minutes are required to reach equilibrium. These results provide valuable insight into the nanostructure and dynamics of DESs at the solid-liquid interface, with implications for the rational design of DESs for interfacial applications.

Tribotronic control of an ionic boundary layer in operando extends the limits of lubrication

The effect of electric potential on the lubrication of a non-halogenated phosphonium orthoborate ionic liquid used as an additive in a biodegradable oil was studied. An in-house tribotronic system was built around an instrument designed to measure lubricant film thickness between a rolling steel ball and a rotating silica-coated glass disc. The application of an electric field between the steel ball and a set of customized counter-electrodes clearly induced changes in the thickness of the lubricant film: a marked decrease at negative potentials and an increase at positive potentials. Complementary neutron reflectivity studies demonstrated the intrinsic electroresponsivity of the adsorbate: this was performed on a gold-coated silicon block and made possible in the same lubricant system by deuterating the oil. The results indicate that the anions, acting as anchors for the adsorbed film on the steel surface, are instrumental in the formation of thick and robust lubricating ionic boundary films. The application of a high positive potential, outside the electrochemical window, resulted in an enormous boost to film thickness, implicating the formation of ionic multi-layers and demonstrating the plausibility of remote control of failing contacts in inaccessible machinery, such as offshore wind and wave power installations.

Structure of Deep Eutectic Solvents (DESs): What We Know, What We Want to Know, and Why We Need to Know It

Deep eutectic solvents (DESs) are a tunable class of solvents with many advantageous properties including good thermal stability, facile synthesis, low vapor pressure, and low-to-negligible toxicity. DESs are composed of hydrogen bond donors and acceptors that, when combined, significantly decrease the freezing point of the resulting solvent. DESs have distinct interfacial and bulk structural heterogeneity compared to traditional solvents, in part due to various intramolecular and intermolecular interactions. Many of the physiochemical properties observed for DESs are influenced by structure. However, our understanding of the interfacial and bulk structure of DESs is incomplete. To fully exploit these solvents in a range of applications including catalysis, separations, and electrochemistry, a better understanding of DES structure must be obtained. In this Perspective, we provide an overview of the current knowledge of the interfacial and bulk structure of DESs and suggest future research directions to improve our understanding of this important information.

Microstructural probing of phosphonium-based ionic liquids on a gold electrode using colloid probe AFM

Atomic force microscopy (AFM) with a gold colloid probe modeled as the electrode surface is employed to directly capture the contact resonance frequency of two phosphonium-based ionic liquids (ILs) containing a common anion [BScB]^- and differently lengthened cations ([P_6,6,6,14]⁺ and [P_4,4,4,8]⁺). The comparative interfacial studies are performed by creating IL films on the surface of gold, followed by measuring the wettability, thickness of the films, adhesion forces, surface morphology and AFM-probed contact resonance frequency. In addition, the cyclic voltammetry and impedance spectroscopy measurements of the neat ILs are measured on the surface of the gold electrode. The IL with longer cation alkyl chains exhibits a well-defined thin film on the electrode surface and enhanced the capacitance than the shorter chain IL. The AFM contact resonance frequency and force curves reveal that the longer IL prefers to form stiffer ion layers at the gold electrode surface, suggesting the "…anion-anion-cation-cation…" bilayer structure, in contrast, the shorter-chain IL forms the softer cation-anion alternating structure, i.e., "…anion-cation-anion-cation…".

Atomic force microscopy probing interactions and microstructures of ionic liquids at solid surfaces

Ionic liquids (ILs) are room temperature molten salts that possess preeminent physicochemical properties and have shown great potential in many applications. However, the use of ILs in surface-dependent processes, e.g. energy storage, is hindered by the lack of a systematic understanding of the IL interfacial microstructure. ILs on the solid surface display rich ordering, arising from coulombic, van der Waals, solvophobic interactions, etc., all giving near-surface ILs distinct microstructures. Therefore, it is highly important to clarify the interactions of ILs with solid surfaces at the nanoscale to understand the microstructure and mechanism, providing quantitative structure-property relationships. Atomic force microscopy (AFM) opens a surface-sensitive way to probe the interaction force of ILs with solid surfaces in the layers from sub-nanometers to micrometers. Herein, this review showcases the recent progress of AFM in probing interactions and microstructures of ILs at solid interfaces, and the influence of IL characteristics, surface properties and external stimuli is thereafter discussed. Finally, a summary and perspectives are established, in which, the necessities of the quantification of IL-solid interactions at the molecular level, the development of in situ techniques closely coupled with AFM for probing IL-solid interfaces, and the combination of experiments and simulations are argued.

Reservoir computing with dielectric relaxation at an electrode-ionic liquid interface

A physical reservoir device with tunable transient dynamics is strongly required to process time-series data with various timescales generated in the edge region. In this study, we proposed using the dielectric relaxation at an electrode–ionic liquid (IL) interface as the physical reservoir by making the most of designable physicochemical properties of ILs. The transient dynamics of a Au/IL/Au reservoir device were characterized as a function of the alkyl chain length of cations in the IL (1-alkyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide). By considering a weighted sum of exponentials expressing a superposition of Debye-type relaxations, the transient dynamics were well reconstructed. Although such complex dynamics governed by multiple relaxation processes were observed, each extracted relaxation time scales with a power law as a function of IL’s viscosity determined by the alkyl chain length of cations. This indicates that the relaxation processes are characterized by bulk properties of the ILs that obey the widely received Vogel-Fulcher-Tammann law. We demonstrated that the 4-bit time-series signals were transformed into the 16 classifiable data, and the data transformation, which enables to achieve higher accuracy in an image classification task, can be easily optimized according to the features of the input signals by controlling the IL’s viscosity.

Investigation of Multilayered Structures of Ionic Liquids on Graphite and Platinum Using Atomic Force Microscopy and Molecular Simulations

The molecular-level orientation and structure of ionic liquids (ILs) at liquid-solid interfaces are significantly different than in the bulk. The interfacial ordering influences both IL properties, such as dielectric constants and viscosity, and their efficacy in devices, such as fuel cells and electrical capacitors. Here, we report the layered structures of four ILs on unbiased, highly ordered pyrolytic graphite (HOPG) and Pt(111) surfaces, as determined by atomic force microscopy. The ILs investigated are 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf₂N]), 1-ethyl-3-methylimidazolium perfluorobutylsulfonate ([emim][C₄F₉SO₃]), 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene bis(trifluoromethylsulfonyl)imide ([MTBD][Tf₂N]), and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene perfluorobutylsulfonate ([MTBD][C₄F₉SO₃]). Molecular dynamics simulations provide complementary information on the position and orientation of the ions. These ILs form a cation layer at the IL-solid interface, followed by a layer of anions. [Emim]⁺ and [MTBD]⁺ have similar orientations at the surface, but [MTBD]⁺ forms a thinner layer compared to [emim]⁺ on both HOPG and Pt(111). In addition, [Tf₂N]^- shows stronger interactions with Pt(111) surfaces than [C₄F₉SO₃]^-.

The Role of Energy Scales for the Structure of Ionic Liquids at Electrified Interfaces: A Theory-Based Approach

Ionic liquids offer unique bulk and interfacial characteristics as battery electrolytes. Our continuum approach naturally describes the electrolyte on a macroscale. An integral formulation for the molecular repulsion, which can be quantitatively determined by both experimental and theoretical methods, models the electrolyte on the nanoscale. In this article, we perform a systematic series expansion of this integral formulation, derive a description of chemical potentials in terms of higher-order concentration gradients, and rationalize the appearance of fourth-order derivative operators in modified Poisson equations, as recently proposed in this context. In this way, we formulate a rigorous multiscale methodology from atomistic quantum chemistry calculations to phenomenological continuum models. We apply our generalized framework to ionic liquids near electrified interfaces and perform analytical asymptotic analysis. Three energy scales describing electrostatic forces between ions, molecular repulsion, and thermal motion determine the shape and width of the long-ranging charged double layer. We classify the charge screening mechanisms dependent on the system parameters as dielectricity, ion size, interaction strength, and temperature. We find that the charge density of electrochemical double layers in ionic liquids either decays exponentially, for negligible molecular repulsion, or oscillates continuously. Charge ordering across several ion diameters occurs if the repulsion between molecules is comparable with thermal energy and Coulomb interactions. Eventually, phase separation of the bulk electrolyte into ionic layers emerges once the molecular repulsion becomes dominant. Our framework predicts the exact phase boundaries among these three phases as a function of temperature, dielectricity, and ion size.

Nanostructure, electrochemistry and potential-dependent lubricity of the catanionic surface-active ionic liquid [P6,6,6,14] [AOT]

HYPOTHESIS

A catanionic surface-active ionic liquid (SAIL) trihexyltetradecylphosphonium 1,4-bis(2-ethylhexoxy)-1,4-dioxobutane-2-sulfonate ([P_6,6,6,14] [AOT]) is nanostructured in the bulk and at the interface. The interfacial nanostructure and lubricity may be changed by applying a potential.

EXPERIMENTS

The bulk structure and viscosity have been investigated using small angle X-ray scattering (SAXS) and rheometry. The interfacial structure and lubricity as a function of potential have been investigated using atomic force microscopy (AFM). The electrochemistry has been investigated using cyclic voltammetry.

FINDINGS

[P_6,6,6,14] [AOT] shows sponge-like bulk nanostructure with distinct interdigitation of cation-anion alkyl chains. Shear-thinning occurs at 293 K and below, but becomes less obvious on heating up to 313 K. Voltammetric analysis reveals that the electrochemical window of [P_6,6,6,14] [AOT] on a gold micro disk electrode exceeds the potential range of the AFM experiments and that negligible redox activity occurs in this range. The interfacial layered structure of [P_6,6,6,14] [AOT] is weaker than conventional ILs and SAILs, whereas lubricity is better, confirming the inverse correlation between the near-surface structure and lubricity. The adhesive forces of [P_6,6,6,14] [AOT] are lower at -1.0 V than at open circuit potential and +1.0 V, likely due to reduced electrostatic interactions caused by shielding of charge centres via long alkyl chains.

Interfacial nanostructure and friction of a polymeric ionic liquid-ionic liquid mixture as a function of potential at Au(111) electrode interface

HYPOTHESIS

The polymeric cations of polymeric ionic liquids (PILs) can adsorb from the bulk of a conventional ionic liquid (IL) to the Au(111) electrode interface and form a boundary layer. The interfacial properties of the PIL boundary layer may be tuned by potential.

EXPERIMENTS

Atomic force microscopy has been used to investigate the changes of surface morphology, normal and lateral forces of a 5 wt% PIL/IL mixture as a function of potential.

FINDINGS

Polymeric cations adsorb strongly to Au(111) and form a polymeric cation-enriched boundary layer at -1.0 V. This boundary layer binds less strongly to the surface at open circuit potential (OCP) and weakly at + 1.0 V. The polymeric cation chains are compressed at -1.0 V and OCP owing to electrical attractions with the electrode surface, but fully stretched at + 1.0 V due to electrical repulsions. The lateral forces of the 5 wt% PIL/IL mixture at -1.0 V and OCP are higher than at + 1.0 V as the polymeric cation-enriched boundary layer is rougher and has stronger interactions with the AFM probe; at + 1.0 V, the lateral force is low and comparable to pure conventional IL due to displacement of polymeric cations with conventional anions in the boundary layer.

Imidazolium Triflate Ionic Liquids' Capacitance-Potential Relationships and Transport Properties Affected by Cation Chain Lengths

In this paper we report the effects of five imidazolium cations with varying alkyl chain lengths to study the effects of cation size on capacitance versus voltage behavior. The cations include ethyl-, butyl-, hexyl-, octyl-, and decyl-3-methylimidazolium, all paired with a triflate anion. We analyze the capacitance with respect to the cation alkyl chain length qualitatively and quantitatively by analyzing changes in the capacitance-potential curvature shape and magnitude across several standard scanning protocols and electrochemical techniques. Further, three transport properties (viscosity, diffusion coefficient, and electrical conductivity) are experimentally determined and integrated into the outcomes. Ultimately, we find higher viscosities, lower diffusion coefficients, and lower electrical conductivities when the alkyl chain length is increased. Also, capacitance values increase with cation size, except 1-octyl-3-methylimidazolium, which does not follow an otherwise linear trend. This capacitive increase is most pronounced when sweeping the potential in the cathodic direction. These findings challenge the conventional hypothesis that increasing the length of the alkyl chain of imidazolium cations diminishes the capacitance and ionic liquid performance in charge storage.

Direct observation of the double-layering quantized growth of mica-confined ionic liquids

Since the interface between ionic liquids (ILs) and solids always plays a critical role in important applications such as coating, lubrication, energy storage and catalysis, it is essential to unravel the molecular structure and dynamics of ILs confined to solid surfaces. Here we report direct observation of a unique double-layering quantized growth of three IL (i.e. [Emim][FAP], [Bmim][FAP] and [Hmim][FAP]) nanofilms on mica. AFM results show that the IL nanofilms initially grow only by covering more surface areas at the constant film thickness of 2 monolayers (ML) until a quantized increase in the film thickness by another 2 ML occurs. Based on the AFM results, we propose a double-layering model describing the molecular structure of IL cations and anions on the mica surface. The interesting double-layering structure can be explained as the result of several competing interactions at the IL-mica interface. Meanwhile, the time-dependent AFM results indicate that the topography of IL nanofilms could change with time and mobility of the nanofilm is lower for ILs with longer alkyl chains, which can be attributed to the stronger solvophobic interaction. The findings here have important implications on the molecular structure and dynamics of ILs confined to solid surfaces.

Experimental Evidence of Long-Lived Electric Fields of Ionic Liquid Bilayers

Herein we demonstrate that ionic liquids can form long-lived double layers, generating electric fields detectable by straightforward open circuit potential (OCP) measurements. In imidazolium-based ionic liquids an external negative voltage pulse leads to an exceedingly stable near-surface dipolar layer, whose field manifests as long-lived (∼1-100 h) discrete plateaus in OCP versus time traces. These plateaus occur within an ionic liquid-specific and sharp potential window, defining a simple experimental method to probe the onset of interfacial ordering phenomena, such as overscreening and crowding. Molecular dynamics modeling reveals that the OCP arises from the alignment of the individual ion dipoles to the external electric field pulse, with the magnitude of the resulting OCP correlating with the product of the projected dipole moment of the cation and the ratio between the cation diffusion coefficient and its volume. Our findings also reveal that a stable overscreened structure is more likely to form if the interface is first forced through crowding, possibly accounting for the scattered literature data on relaxation kinetics of near-surface structures in ionic liquids.

The Influence of Water Vapor on the Electrochemical Shift of an Ionic Liquid Measured by Ambient Pressure X-ray Photoelectron Spectroscopy

Ionic liquids (ILs) are considered to be one of the steppingstones to fabricate next generation electrochemical devices given their unique physical and chemical properties. The addition of water to ILs significantly impact electrochemical related properties including viscosity, density, conductivity, and electrochemical window. Herein we utilize ambient pressure X-ray photoelectron spectroscopy (APXPS) to examine the impact of water on values of the electrochemical shift (S), which is determined by measuring changes in binding energy shifts as a function of an external bias. APXPS spectra of C 1s, O 1s and N 1s regions are examined for the IL 1-butyl-3-methylimidazolium acetate, [C₄ mim][OAc], at the IL/gas interface as a function of both water vapor pressure and external bias. Results reveal that in the absence of water vapor there is an IL ohmic drop between the working electrode and quasi reference electrode, giving rise to chemical specific S values of less than one. Upon introducing water vapor, S values approach one as a function of increasing water vapor pressure, indicating a decrease in the IL ohmic drop as the IL/water mixture becomes more conductive and the potential drop is driven by the electric double layer at the electrode/IL interface.

Hysteresis in the MD Simulations of Differential Capacitance at the Ionic Liquid-Au Interface

In this Letter, we report the first observation of the capacitance-potential hysteresis at the ionic liquid | electrode interface in atomistic molecular dynamics simulations. While modeling the differential capacitance dependence on the potential scan direction, we detected two long-living types of interfacial structure for the BMImPF₆ ionic liquid at specific charge densities of the gold Au(111) surface. These structures differ in how counterions overscreen the surface charge. The high barrier for the transition from one structure to another slows down the interfacial restructuring process and leads to the marked capacitance-potential hysteresis.

Growth of Multilayers of Ionic Liquids on Au(111) Investigated by Atomic Force Microscopy in Ultrahigh Vacuum

Understanding the growth of ultrathin films of ionic liquids (ILs) on metal surfaces is of highest relevance for a variety of applications. We present a detailed study of the growth of the wetting layer and successive multilayers of 1,3-dimethylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C1C1Im][Tf2N]) on Au(111). By atomic force microscopy (AFM) in ultrahigh vacuum, we follow the temperature-dependent behavior between 110 and 300 K at defined coverages. We initially observe the formation of a wetting layer with a thickness of ∼0.37 nm with anions and cations arranged in a checkerboard structure. Stable AFM imaging up to 280 K allows us to follow the IL growing on top of the wetting layer in bilayers with an average thickness of ∼0.71 nm, that is, double the height of the wetting layer, in a bilayer-by-bilayer fashion. This growth behavior is independently confirmed from the surface morphology, as deduced from AFM and angle-resolved X-ray photoelectron spectroscopy. High-resolution AFM images at 110 K allow for identifying the molecular surface structure of the bilayers as a striped phase, which is one of the phases also seen for the wetting layer (Meusel, M.; Lexow, M.; Gezmis, A.; Schotz, S.; Wagner, M.; Bayer, A.; Maier, F.; Steinrück, H. P. Atomic Force and Scanning Tunneling Microscopy of Ordered Ionic Liquid Wetting Layers from 110 K up to Room Temperature. ACS Nano 2020, 14, 9000-9010).

Engineering high-energy-density sodium battery anodes for improved cycling with superconcentrated ionic-liquid electrolytes

Non-uniform metal deposition and dendrite formation in high-density energy storage devices reduces the efficiency, safety and life of batteries with metal anodes. Superconcentrated ionic-liquid electrolytes (for example 1:1 ionic liquid:alkali ion) coupled with anode preconditioning at more negative potentials can completely mitigate these issues, and therefore revolutionize high-density energy storage devices. However, the mechanisms by which very high salt concentration and preconditioning potential enable uniform metal deposition and prevent dendrite formation at the metal anode during cycling are poorly understood, and therefore not optimized. Here, we use atomic force microscopy and molecular dynamics simulations to unravel the influence of these factors on the interface chemistry in a sodium electrolyte, demonstrating how a molten-salt-like structure at the electrode surface results in dendrite-free metal cycling at higher rates. Such a structure will support the formation of a more favourable solid electrolyte interphase, accepted as being a critical factor in stable battery cycling. This new understanding will enable engineering of efficient anode electrodes by tuning the interfacial nanostructure via salt concentration and high-voltage preconditioning.

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.

Controlling the nanoscale friction by layered ionic liquid films

The nanofriction coefficient of ionic liquids (ILs), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) and 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), on the surfaces of mica and graphite was investigated using atomic force microscopy (AFM). A pronounced layered spatial distribution was found in the IL film formed on the solid substrates and can be divided into 3 well distinguishable regions exhibiting different physical properties with increasing distance from the substrate. We found that the friction coefficient (μ) increases monotonically as the layering thickness decreases, no matter what the thickness of the bulk IL is. This suggests that the layering assembled IL at solid surfaces is more important than the bulk phase in determining the magnitude of the nanoscale friction. The increase in the friction coefficient as the layering thickness decreases is most likely attributed to the assembled ordered IL layers closer to the substrate surfaces having a greater activation barrier for unlocking the surfaces to allow shear.

Effect of Hydrogen Bonding between Ions of Like Charge on the Boundary Layer Friction of Hydroxy-Functionalized Ionic Liquids

Atomic force microscopy has been used to measure the lubricity of a series of ionic liquids (ILs) at mica surfaces in the boundary friction regime. A previously unreported cation bilayer structure is detected at the IL-mica interface due to the formation of H-bonds between the hydroxy-functionalized cations [(c-c) H-bonds], which enhances the ordering of the ions in the boundary layer and improves the lubrication. The strength of the cation bilayer structure is controlled by altering the strength of (c-c) H-bonding via changes in the hydroxyalkyl chain length, the cation charge polarizability, and the coordination strength of the anions. This reveals a new means of controlling IL boundary nanostructure via H-bonding between ions of the same charge, which can impact diverse applications, including surface catalysis, particle stability, electrochemistry, etc.

Lubricating Properties of Cyano-Based Ionic Liquids against Tetrahedral Amorphous Carbon Film

Ionic liquids have unique characteristics, which render them ideal candidates as new base oils or additives. In particular, there are great expectations from the combination of diamond-like carbon and cyano-based ionic liquids. Lubricating properties of cyano-based ionic liquids have been studied on specific tetrahedral amorphous carbon (ta-C) films. After lubrication, ta-C film/ta-C film contact interface exhibits exceedingly low friction. Therefore, it is necessary to understand this low friction phenomenon. The current study evaluated the lubricating mechanism of cyano-based ionic liquids against ta-C films. 1-Butyl-3-methylimidazolium dicyanamide ((BMIM)(DCN)) and 1-butyl-3-methylimidazolium tricyanomethane ((BMIM)(TCC)) were used as lubricants, with the latter exhibiting low friction coefficient of 0.03. Steel cylinders and disks with ta-C films were used as test specimens. Raman spectroscopy, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and thermogravimetric analysis (TGA) helped us understand the mechanism of low friction induced by (BMIM)(TCC). Graphitization of the ta-C film at high temperatures might have caused the reduction in friction between the films. Similarly, anion adsorption on the worn surface at high temperatures also led to reduced friction. However, the TGA result showed a different trend than that of the sliding test. Our results indicate that the cyano-based ionic liquids underwent tribo-decomposition at low temperatures. Further, a minimum temperature was required for the adsorption of anions onto the sliding surface.

Charge oscillations in ionic liquids: A microscopic cluster model

In spite of their enormous applications as alternative energy storage devices and lubricants, room-temperature ionic liquids (ILs) still pose many challenges from a pure scientific viewpoint. We develop an IL microscopic theory in terms of ionic clusters, which describes the IL behavior close to charged interfaces. The full structure factor of finite-size clusters is considered and allows us to retain fine and essential details of the system as a whole. Beside the reduction in the screening, it is shown that ionic clusters cause the charge density to oscillate near charged boundaries, with alternating ion-size thick layers, in agreement with experiments. We distinguish between short-range oscillations that persist for a few ionic layers close to the boundary, as opposed to long-range damped oscillations that hold throughout the bulk. The former can be captured by finite-size ion pairs, while the latter is associated with larger clusters with a pronounced quadrupole (or higher) moment. The long-wavelength limit of our theory recovers the well-known Bazant-Storey-Kornyshev (BSK) equation in the linear regime, and elucidates the microscopic origin of the BSK phenomenological parameters.

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.

Ionic liquid-induced nanoporous structures of polymer films

Nanoporous polymer thin films can be fabricated using strong polyelectrolyte pairs in ionic liquid aqueous solutions.

Potential Screening at Electrode/Ionic Liquid Interfaces from In Situ X-ray Photoelectron Spectroscopy

A new approach to investigate potential screening at the interface of ionic liquids (ILs) and charged electrodes in a two-electrode electrochemical cell by in situ X-ray photoelectron spectroscopy has been introduced. Using identical electrodes, we deduce the potential screening at the working and the counter electrodes as a function of applied voltage from the potential change of the bulk IL, as derived from corresponding core level binding energy shifts for different IL/electrode combinations. For imidazolium-based ILs and Pt electrodes, we find a significantly larger potential screening at the anode than at the cathode, which we attribute to strong attractive interactions between the imidazolium cation and Pt. In the absence of specific ion/electrode interactions, asymmetric potential screening only occurs for ILs with different cation and anion sizes as demonstrated for an imidazolium chloride IL and Au electrodes, which we assign to the different thicknesses of the electrical double layers. Our results imply that potential screening in ILs is mainly established by a single layer of counterions at the electrode.

Potential Dependence of Surfactant Adsorption at the Graphite Electrode/Deep Eutectic Solvent Interface

Atomic force microscopy and cyclic voltammetry are used to probe how ionic surfactant adsorbed layer structure affects redox processes at deep eutectic solvent (DES)/graphite interfaces. Unlike its behavior in water, sodium dodecyl sulfate (SDS) in DESs only adsorbs as a complete layer of hemicylindrical hemimicelles far above its critical micelle concentration (CMC). Near the CMC it forms a tail-to-tail monolayer at open-circuit potential (OCP) and positive potentials, and it desorbs at negative potentials. In contrast, cetyltrimethylammonium bromide (CTAB) adsorbs as hemimicelles at low concentrations and remains adsorbed at both positive and negative potentials. The SDS horizontal monolayer has little overall effect on redox processes at the graphite interface, but hemimicelles form an effective and stable barrier. The stronger solvophobic interactions between the C₁₆ versus C₁₂ alkyl chains in the DES allow CTAB to self-assemble into a robust coating at low concentrations and illustrate how the structure of the DES/electrode interface and electrochemical response can be engineered by controlling surfactant structure.

Atomic Force Spectroscopy on Ionic Liquids

Ionic liquids have become of significant relevance in chemistry, as they can serve as environmentally-friendly solvents, electrolytes, and lubricants with bespoke properties. In particular for electrochemical applications, an understanding of the interface structure between the ionic liquid and an electrified interface is needed to model and optimize the reactions taking place on the solid surface. As with ionic liquids, the interplay between electrostatic forces and steric effects leads to an intrinsic heterogeneity, as the structure of the ionic liquid above an electrified interface cannot be described by the classical electrical double layer model. Instead, a layered solvation layer is present with a structure that depends on the material combination of the ionic liquid and substrate. In order to experimentally monitor this structure, atomic force spectroscopy (AFS) has become the method of choice. By measuring the force acting on a sharp microfabricated tip while approaching the surface in an ionic liquid, it has become possible to map the solvation layers with sub-nanometer resolution. In this review, we provide an overview of the AFS studies on ionic liquids published in recent years that illustrate how the interface is formed and how it can be modified by applying electrical potential or by adding impurities and solvents.

MicroMegascope based dynamic surface force apparatus

Surface force apparatus (SFA) allows accurate resolving of the interfacial properties of fluids confined between extended surfaces. The accuracy of the SFA makes it an ubiquitous tool for the nanoscale mechanical characterization of soft matter systems. The SFA traditionally measures force-distance profiles through interferometry with subnanometric distance precision. However, these techniques often require a dedicated and technically demanding experimental setup, and there remains a need for versatile and simple force-distance measurement tools. Here we present a MicroMegascope based dynamic SFA capable of accurate measurement of the dynamic force profile of a liquid confined between a millimetric sphere and a planar substrate. Normal and shear mechanical impedance is measured within the classical frequency modulation framework. We measure rheological and frictional properties from micrometric to molecular confinement. We also highlight the resolution of small interfacial features such as ionic liquid layering. This apparatus shows promise as a versatile force-distance measurement device for exotic surfaces or extreme environments.

Fine probing the effect of replacing [PF6]- with [PF3(C2F5)3]- on the local structure and nanoscale organization of [bmim]+-based ionic liquids using MD simulation

Comparative all-atom molecular dynamics simulations are used to study the microscopic local structure and interionic interactions of two ionic liquids (ILs) composed of the 1-butyl-3-methylimidazolium cation, [bmim]+, coupled with the hexafluorophosphate, [PF6]-, or tris(pentafluoroethyl)trifluorophosphate, [FAP]-, anions. Respective distribution functions clearly reveal that the structural correlations between the cation and anion decrease when (i) replacing [PF6]- with [FAP]-, (ii) scaling the partial atomic charges, and (iii) considering the anion's structural flexibility versus rigidity. Replacement of [PF6]- with [FAP]- expands the nonpolar domains totally and causes the decreasing of the three-dimensional polar networks as well as the diminishing of the nano-aggregation of cation side chains. Current simulations show that with increasing the anion size and its charge delocalization, the probability of the in-plane cation-anion conformation, its related hydrogen bond acceptor ability, and the cation-cation π-π interaction decreases in accordance with the fluidity enhancements of the corresponding imidazolium-based IL. Hence, structural findings can explain and justify rationally the origins of the observed trends in the simulated dynamical properties of these ILs in our previous report. A complete understanding of the microscopic structure of ILs is necessary to control the outstanding properties of ILs as designer solvents that will support experimentalists for the best engineering design and a breakthrough efficiency of IL-related processes.

Spreading of Nanodroplets of Ionic Liquids on the Mica Surface

Previously, the molecular structure of ionic liquids (ILs) next to a solid surface has been extensively studied because the IL-solid interfaces are critical to many important applications. What is largely unknown is the mobility of ILs confined to a solid surface. The solid-like stable structure of ILs on the solid surfaces has been reported in previous research, which indicates that the mobility of ILs next to a solid surface is quite low and the structure of ILs will not change with time. However, here, we show that the nanodroplets of ILs flow on the mica surface, which suggest that, at a small scale, the solid-like stable structure of ILs on the solid surfaces indeed changes with time. Interestingly, although the IL nanodroplets show a layered structure, no terrace-like flow of the droplet has been observed, which has been attributed to the weak mica-IL interaction at low humidity.