Controlling the nanoscale friction by layered ionic liquid films

Quantifying and Decoupling Molecular Interactions of Ionic Liquids with Gold Electrodes

This work combined gold colloid probe atomic force microscopy (AFM) with a quartz crystal microbalance (QCM) to accurately quantify the molecular interactions of fluorine-free phosphonium-based ionic liquids (ILs) with gold electrode surfaces. First, the interactions of ILs with the gold electrode per unit area (F A ' , N/m2) were obtained via the force-distance curves measured by gold probe AFM. Second, a QCM was employed to detect the IL amount to acquire the equilibrium number of IL molecules adsorbed onto the gold electrode per unit area (NIL, Num/m2). Finally, the quantified molecular interactions of ILs with the gold electrode (F0, nN/Num) were estimated. F0 is closely related to the IL composition, in which the IL with the same anion but a longer phosphonium cation exhibits a stronger molecular interaction. The changes in the quantified interactions of gold with different ILs are consistent with the interactions predicted by the extended Derjaguin-Landau-Verwey-Overbeek theory, and the van der Waals interaction was identified as the major contribution of the overall interaction. The quantified molecular interaction is expected to enable the direct experimental-derived interaction parameters for molecular simulations and provide the virtual design of novel ILs for energy storage applications.

Integrative studies of ionic liquid interface layers: bridging experiments, theoretical models and simulations

Ionic liquids (ILs) are a class of salts existing in the liquid state below 100 °C, possessing low volatility, high thermal stability as well as many highly attractive solvent and electrochemical capabilities, etc., making them highly tunable for a great variety of applications, such as lubricants, electrolytes, and soft functional materials. In many applications, ILs are first either physi- or chemisorbed on a solid surface to successively create more functional materials. The functions of ILs at solid surfaces can differ considerably from those of bulk ILs, mainly due to distinct interfacial layers with tunable structures resulting in new ionic liquid interface layer properties and enhanced performance. Due to an almost infinite number of possible combinations among the cations and anions to form ILs, the diversity of various solid surfaces, as well as different external conditions and stimuli, a detailed molecular-level understanding of their structure-property relationship is of utmost significance for a judicious design of IL-solid interfaces with appropriate properties for task-specific applications. Many experimental techniques, such as atomic force microscopy, surface force apparatus, and so on, have been used for studying the ion structuring of the IL interface layer. Molecular Dynamics simulations have been widely used to investigate the microscopic behavior of the IL interface layer. To interpret and clarify the IL structure and dynamics as well as to predict their properties, it is always beneficial to combine both experiments and simulations as close as possible. In another theoretical model development to bridge the structure and properties of the IL interface layer with performance, thermodynamic prediction & property modeling has been demonstrated as an effective tool to add the properties and function of the studied nanomaterials. Herein, we present recent findings from applying the multiscale triangle "experiment-simulation-thermodynamic modeling" in the studies of ion structuring of ILs in the vicinity of solid surfaces, as well as how it qualitatively and quantitatively correlates to the overall ILs properties, performance, and function. We introduce the most common techniques behind "experiment-simulation-thermodynamic modeling" and how they are applied for studying the IL interface layer structuring, and we highlight the possibilities of the IL interface layer structuring in applications such as lubrication and energy storage.

Effect of Electrode Surface Chemistry on Ion Structuring of Imidazolium Ionic Liquids

Surface chemistry plays a critical role in the ion structuring of ionic liquids (ILs) at the interfaces of electrodes and controls the overall energy storage performance of the system. Herein, we functionalized the gold (Au) colloid probe of an atomic force microscope with -COOH and -NH₂ groups to explore the effect of different surface chemical properties on the ion structuring of an IL. Aided by colloid-probe atomic force microscopy (AFM), the ion structuring of an imidazolium IL, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆], abbreviated as BP hereafter), on the Au electrode surface and the ion response to the change in the surface chemistry are investigated. AFM morphologies, contact angles, and approaching force-distance curves of the BP IL on the functionalized Au surfaces exhibited that the IL forms a more obvious layering structure on the -COOH-terminated Au surface (Au-COOH), while it forms heterogeneous and aggregating droplets on the -NH₂ surface (Au-NH₂). The formed uniform and aggregation-free ion layers in the vicinity of the Au-COOH surface are due to the π-π⁺ stacking interaction between the delocalized π⁺ electrons from the imidazolium ring in the IL [BMIM]⁺ cation and the localized π electrons from the sp² carbon on the -COOH group. The in situ observation of nano-friction and torsional resonance frequency at the IL-electrode interfaces further demonstrated the ion structuring of the IL at Au-COOH, which results in a more sensitive electrochemical response associated with a faster capacitive process.

Immobilizing Ionic Liquids onto Functionalized Surfaces for Sensing Volatile Organic Compounds

Herein, a highly sensitive volatile organic compound (VOC) gas sensor is demonstrated using immobilized ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate, onto surfaces functionalized by the quaternary ammonium group -N⁺R, -COOH, and -NH₂, i.e., N⁺-IL, COOH-IL, and NH₂-IL, respectively. These functional groups ensure highly tunable interactions between the IL and surfaces, efficiently modulating the electrical resistance of the immobilized IL upon exposure to acetone and toluene. The immobilized IL to both acetone and toluene displays significant electronic resistance changes at a concentration of 150 ppm, falling in the order NH₂-IL > N⁺-IL > COOH-IL for acetone while COOH-IL > NH₂-IL > N⁺-IL for toluene. A better gaseous sensing ability is achieved in COOH-IL for toluene than acetone, while this does not hold in the case of NH₂-IL and N⁺-IL surfaces because of the completely different ion structuring of the IL at these functionalized surfaces. The accelerated ion mobility in the IL that is immobilized onto functionalized surfaces is also responsible for the strong gaseous sensing response, which is demonstrated further by the atomic force microscopy-measured smaller friction coefficient. This is highly encouraging and suggests that ILs can be immobilized by a network formed by surface functionalization to easily and cheaply detect VOCs at ppm concentrations.

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.

Probing the hydration friction of ionic interfaces at the atomic scale

Despite the extensive studies conducted in exploring friction in the aqueous environment, the mechanism of hydration friction remains not well understood. Herein, we directly probed hydration friction on mica-electrolyte interfaces with different hydrated alkali cations through a combination of three-dimensional atomic force microscopy and friction force microscopy. The atomic scale imaging of the hydration layers at the mica surface in different electrolyte solutions clearly revealed a correlation between the alkali cations and the structure of the hydration layers. Our detailed analysis showed that the hydration force was much higher at high ionic concentrations than that at low concentrations. The hydration friction coefficient was found to follow the trend K⁺< Na⁺< Li⁺< Cs⁺, which contrasts with the Hofmeister series, indicating that the hydration friction depends not only on the hydration strength of the alkali cations but also on the arrangement of the alkali cations at the interface. The results of this study provide deep insights into the origins of hydration friction, with potential implications for the development of new boundary lubrication in aqueous media.

Effects of shear flow on the structure and dynamics of ionic liquids in a metallic nanoconfinement

It has been shown that a weak shear can induce crystallisation in a disordered, glassy state. In this study, we use molecular dynamics simulations in order to investigate the out-of-equilibrium properties of [BMIM][BF₄] confined between metal slabs. In particular, we want to understand the extent to which the shear flow modifies the interfacial properties. In particular, the questions we address here are (i) is the shear able to promote the crystalline phase in [BMIM][BF₄]? (ii) Can, as a consequence of shear flow, a solid-like layer develop at the interface with a metallic surface? (iii) What are the tribological properties of nanoconfined [BMIM][BF₄]? We find that the system behaves quite differently from the ideal linear Couette flow. Indeed, the portion of fluid closer to the shearing slabs behaves as a disordered, solid-like layer, which, under the investigated conditions extends to a few nanometres. The linear velocity regime is only recovered in the central region of the ionic liquid slab. The formation of such a solid-like glassy rather than crystalline layer is in agreement with recent mechanical impedance measurements performed on nano-confined ionic liquids.

On the physicochemical origin of nanoscale friction: the polarizability and electronegativity relationship tailoring nanotribology

Friction is a ubiquitous manifestation of nature, and when it is studied at the nanoscale, complex and interesting effects arise from fundamental physical and chemical surface properties. Surprisingly, and probably due to the complexity of nanofriction studies, this aspect has not been completely discussed in prior studies. To fully consider the physicochemical influence in nanoscale friction, amorphous carbon films with different amounts of hydrogen and fluorine were prepared, chemically characterized, and evaluated via lateral force microscopy. Hydrogen and fluorine were selected because although they exhibit different physicochemical properties, both contribute to frictional force reduction. Indeed, to explain the experimental behavior, it is necessary to propose a new damping constant unifying both polarizability (physical) and electronegativity (chemical) properties. The satisfactory agreement between theory and experiments may encourage and enhance deeper discussion and new experiments that take into account the chemical peculiarities of frictional behavior relating to nanoscale elastic regimes.