Ionic liquids (ILs) have been widely investigated as novel solvents, electrolytes, and soft functional materials. Nevertheless, the widespread applications of ILs in most cases have been hampered by their liquid state. The confinement of ILs into nanoporous hosts is a simple but versatile strategy to overcome this problem. Nanoconfined ILs constitute a new class of composites with the intrinsic chemistries of ILs and the original functions of solid matrices. The interplay between these two components, particularly the confinement effect and the interactions between ILs and pore walls, further endows ILs with significantly distinct physicochemical properties in the restricted space compared to the corresponding bulk systems. The aim of this article is to provide a comprehensive review of nanoconfined ILs. After a brief introduction of bulk ILs, the synthetic strategies and investigation methods for nanoconfined ILs are documented. The local structure and physicochemical properties of ILs in diverse porous ...

Nanoconfined Ionic Liquids

Over recent years the Surface Force Apparatus (SFA) has been used to carry out model experiments revealing structural and dynamic properties of ionic liquids confined to thin films. Understanding characteristics such as confinement induced ion layering and lubrication is of primary importance to many applications of ionic liquids, from energy devices to nanoparticle dispersion. This Perspective surveys and compares SFA results from several laboratories as well as simulations and other model experiments. A coherent picture is beginning to emerge of ionic liquids as nano-structured in pores and thin films, and possessing complex dynamic properties. The article covers structure, dynamics, and colloidal forces in confined ionic liquids; ionic liquids are revealed as a class of liquids with unique and useful confinement properties and pertinent future directions of research are highlighted.

Ionic liquids in confined geometries

The microstructure of ionic liquids (ILs) has attracted much attention due to their relevance in physiochemical properties and behavior of ILs. The existence of clusters (or microheterogeneous structure) is one of the main features for many ILs, which provides fundamental information for understanding the performance of ILs in their applications. This perspective concentrates on the recent progresses in IL clusters research. Firstly, we give a brief introduction on the structure of clusters in neat ILs and IL solutions. Secondly, the possible formation mechanism of the IL clusters is described. Then, the effects of the clusters on the physicochemical properties, interfacial properties, confined geometry and assembly processes of ILs are discussed. Finally, we address the associated challenges and prospects on the future study of IL clusters.

Ionic liquid clusters: structure, formation mechanism, and effect on the behavior of ionic liquids

Ionic liquids (ILs) hold great promise in the fields of green chemistry, environmental science, and sustainable technology due to their unique properties, such as a tailorable structure, the various types available, and their environmentally friendly features. On the basis of multiscale simulations and experimental characterizations, two unique features of ILs are as follows: (1) strong coupling interactions between the electrostatic forces and hydrogen bonds, namely in the Z-bond, and (2) the unique semiordered structure and properties of ultrathin films, specifically regarding the quasi-liquid. In accordance with the aforementioned theoretical findings, many cutting-edge applications have been proposed: for example, CO2 capture and conversion, biomass conversion and utilization, and energy storage materials. Although substantial progress has been made recently in the field of ILs, considerable challenges remain in understanding the nature of and devising applications for ILs, especially in terms of e.g. in situ/real-time observation and highly precise multiscale simulations of the Z-bond and quasi-liquid. In this Perspective, we review recent developments and challenges for the IL research community and provide insights into the nature and function of ILs, which will facilitate future applications.

https://pubs.acs.org/doi/pdf/10.1021/jacsau.1c00538

Insights into Ionic Liquids: From Z-Bonds to Quasi-Liquids

Confining ionic liquids (ILs) with added lithium salt within silica host networks enhances their fragility and improves their conductivity. Overall, conductivity measurements, Raman spectroscopy of the TFSI anion and NMR spectroscopy of the lithium cation show segregative interaction of lithium ions with the SiO2 host matrix. This implies at IL/SiO2 interfaces a breakdown of aggregated regions that are found systematically in bulk ILs. Such destructuration due to the interface effect determines the fragility and thus results locally at the interface in short relaxation times, low viscosity, and good ionic conductivity. The “destructuration” of ion pairs or domains makes ILs within ionogels a competitive alternative to existing solid ionic conductors in all-solid devices, such as lithium batteries and supercapacitors.

Destructuring ionic liquids in ionogels: enhanced fragility for solid devices.

Solid-State NMR Concepts for the Investigation of Supported Transition Metal Catalysts and Nanoparticles

Ionic liquids confined in porous materials are important solvents which allow a simple heterogenization of homogenous liquids. The perdeuterated ionic liquid N-ethylpyridinium-bis(trifluoromethanesulfonyl)amide ([C2Py][BTA]-d10) was prepared and its bulk phase behavior was studied by differential scanning calorimetry (DSC) and temperature-resolved 2H and 19F solid-state NMR spectroscopy. Its bulk properties were compared to [C2Py][BTA]-d10 confined in a mesoporous silica support material as model material usable in SILP catalysts. The line shape analysis of the temperature-dependent NMR spectra of the bulk material reveals two phase transitions, one at 287–289 K (solid II/solid I) and one extending over a temperature range of 298–306 K (solid I/liquid). While the first phase transition is caused by the onset of an intramolecular rotation of the ethyl group of the cation, the second is due to the melting of the ionic liquid. In the bulk material, a hysteresis between the transition temperatures in heating and cooling scans occurs. In confinement, the dynamics of the ionic liquid changes considerably: no hysteresis is observed for [C2Py][BTA]-d10 confined in the mesopores. Instead, only a broad transition from solid II to the liquid state, which spans the temperature range of 215–245 K, is observed. This transition is identified as the result of a broad distribution of molecular environments of the confined ionic liquid, which thus forms an amorphous phase inside the pores. Hence, the behavior of the ionic liquid in confinement is similar to the behavior of non-ionic guest molecules in the mesoporous silica. Finally, it was found that the anion and cation of the ionic liquid exhibit the same dynamic behavior in confinement.

2H and 19F solid-state NMR studies of the ionic liquid [C2Py][BTA]-d10 confined in mesoporous silica materials

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Solid-State NMR Concepts for the Investigation of Supported Transition Metal Catalysts and Nanoparticles

2H and 19F solid-state NMR studies of the ionic liquid [C2Py][BTA]-d10 confined in mesoporous silica materials