Water-Controlled Structural Transition and Charge Transfer of Interfacial Ionic Liquids

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.

Molecular Understanding of Charging Dynamics in Supercapacitors with Porous Electrodes and Ionic Liquids

Porous electrodes and ionic liquids could significantly enhance the energy storage of supercapacitors. However, they may reduce the charging dynamics and power density due to the nanoconfinement of porous electrodes and the high viscosity of ionic liquids. A comprehensive understanding of the charging mechanism in porous supercapacitors with ionic liquids provides a crucial theoretical foundation for their design optimization. Here, we review the progress of molecular simulations of the charging dynamics in supercapacitors consisting of porous electrodes and ionic liquids. We highlight and delve into the breakthroughs in the ion transport and charging mechanism for electrodes with subnanometer pores and realistic porous structures. We also discuss future directions for the charging dynamics of supercapacitors.

Wettability of Ionic Liquids in High Magnetic Fields

Here, we demonstrate that high magnetic fields alter the wettability of water and ionic solutions on the single-crystal α-Al2O3. We investigated the relationship between the substrate crystal orientation, material magnetism, liquid conductivity, and the surface contact angle. Applying high magnetic fields decreased the water contact angles on all of the surface orientations studied, and the reduction was larger for more magnetic substrates. For ionic solutions, high magnetic fields increased the contact angle on the (0001) α-Al2O3 surface but decreased the contact angles on the (112̅0), (101̅0), and (011̅2) surfaces. We attribute these orientation-dependent ionic solution responses to competition between the field-induced sample magnetization energy and the Lorentz force acting on the ionic solution. Overall, this work provides new magnetic-field-based strategies for changing the wettability and provides guidelines for fabricating novel microfluidic systems or biointerfaces with in situ magnetic control.

Free-Standing Two-Dimensional Crystals Formed from Self-Assembled Ionic Liquids

The fabrication of two-dimensional crystals (2DCs) has attracted very large interest because it creates materials with various surface structural features and special surface properties. Normally, this is limited to sheets networked together with strong covalent or coordination bonds. Against this understanding, we discovered macroscopic scale free-standing 2DCs in the aqueous dispersions of [C_nmim]X (X = Br, NO₃; n = 14, 16, 18) using simultaneous synchrotron small- and wide-angle X-ray scattering techniques. On the other hand, the 2DCs are also a kind of novel hydrogel holding water content up to 98 wt %. This unusual phenomenon is attributed to the weak interactions between imidazole headgroups and counterions. The observation reported in this work is expected to contribute to theorists in their pursuit of the general principles governing the stability of 2D materials. It may also enlighten experimentalists in designing new free-standing 2DCs for various applications.