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

Small Molecules, Non-Covalent Interactions, and Confinement

This review gives an overview of current trends in the investigation of small guest molecules, confined in neat and functionalized mesoporous silica materials by a combination of solid-state NMR and relaxometry with other physico-chemical techniques. The reported guest molecules are water, small alcohols, and carbonic acids, small aromatic and heteroaromatic molecules, ionic liquids, and surfactants. They are taken as characteristic role-models, which are representatives for the typical classes of organic molecules. It is shown that this combination delivers unique insights into the structure, arrangement, dynamics, guest-host interactions, and the binding sites in these confined systems, and is probably the most powerful analytical technique to probe these systems.

Selective Control of Ion Transport by Nanoconfinement: Ionic Liquid in Mesoporous Resorcinol-Formaldehyde Monolith

Thermal and dynamic properties of ionic liquid (IL)-based electrolytic solution (Li⁺TFSI^- in Pyr13⁺TFSI^-; 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide = Pyr13⁺TFSI^-) confined in nanoporous polymer hosts were investigated with respect to the pore size/porosity and the surface chemistry of the polymer host. As host material, mesoporous resorcinol-formaldehyde (RF) polymer monoliths with three-dimensionally connected pore structure were prepared, with precise control of the pore size ranging from ca. 7 to 60 nm. Thermal analysis of RF polymer-ionic liquid composites showed stability up to almost 400 °C and a melting point depression proportional to the inverse of the pore diameter. Good ionic conductivity comparable to that of a commercial separator is obtained, which is dependent on the porosity (i.e., pore volume) of the confining host material (i.e., the number of charge carriers available in the system). Further pulsed field gradient (PFG) NMR experiments revealed that the diffusion coefficient of Pyr13⁺ cation becomes smaller than that of TFSI^- anion inside RF pores, which is contradictory to the bulk IL system. This change in the ionic motion is due to electrostatic attraction between the pore walls and Pyr13⁺ cations, resulting in a layer structure composed of a Pyr13⁺ cation-rich layer adsorbed at the pore wall surface and a TFSI^- anion-enriched bulklike layer at the pore center. Our study suggests that transport characteristics of the ions of interest can be controlled by optimizing the surface chemistry of the host framework and their motion can be separately monitored by PFG NMR spectroscopy.

Distribution of water in the pores of periodic mesoporous organosilicates – a proton solid state MAS NMR study

Proposed model of water layers and pore filling in ethane substituted periodic mesoporous organosilicates (PMO_E) based on analysis of solid state magic angle spinning (MAS) proton NMR spectra.

Encapsulation of an ionic liquid into the nanopores of a 3D covalent organic framework

An ionic liquid [Emim][Tf₂N] confined into the nanopores of three-dimensional COF-320 demonstrated an increased melting point.

Solid-ionic liquid interfaces: pore filling revisited

The properties of ionic liquids on ordered and non-ordered mesoporous silicas (silica gel, MCM-41, SBA-15) were studied by nitrogen sorption, mercury intrusion and thermogravimetric analyses, as well as (129)Xe-NMR spectroscopy. The ionic liquids investigated are based on the 1-hexyl-3-methylimidazolium cation, which was combined with anions of low (bis(trifluoromethanesulfonyl)imide; [NTf2](-)), medium (trifluoromethylsulfonate; [CF3SO3](-)) to high (acetate; [OAc](-)) basicity. The surface coverage depends on both the type of ionic liquid and support used. This results not only in layer or droplet formation, but also in different physico-chemical properties of the ionic liquid when compared to the bulk, depending mainly on the strength of interaction at the interface. Furthermore, the mercury intrusion analysis of mesopores is shown not to be suitable for supported ionic liquids.

Destructuring ionic liquids in ionogels: enhanced fragility for solid devices

The ionogel approach harnesses ionic liquid’s properties and strikingly enhances them. Confined ionic liquids show high fragility and good lithium transport, in relation to the percolating silica interface.

Structure and dynamics of 1-butyl-3-methylimidazolium hexafluorophosphate phases on silica and laponite clay: from liquid to solid behavior

Solid-state NMR experiments show that the behavior of supported 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid phases depends on the type of support and the phase thickness. A mobile nearly liquid phase is obtained on silica, on the basis of the line widths of the bands in (1)H, (31)P, and (13)C spectra. However, the mobility is somehow restricted, as shown by the possibility of using the cross-polarization technique, although with slow dynamics. On laponite clay, a layered material with a negatively charged surface, a truly solid phase is obtained at low coverage, whereas the increase in ionic liquid loading leads to a second liquid phase, as shown by the presence of two contributions with very different line widths. These two phases seem to coexist without exchange in the NMR time frame. Heteronuclear correlation experiments evidence different relative dispositions of the imidazolium-surface-PF(6) system, with only aromatic protons involved in all the interactions on silica but participation of the benzylic groups (N-CH(3) and/or N-CH(2)) in the case of laponite clay.

Ionic liquids in confined geometries

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.

Loading-Controlled Stiffening in Nanoconfined Ionic Liquids

An important strategy for using ionic liquids is to immobilize them by impregnation of supports or incorporation into porous solids to obtain materials called "ionogels". Of considerable importance for applications (electrolyte membranes, supported catalysts, etc.), such confinement results in dramatic changes in the physicochemical properties of the ionic liquid. Here, we report molecular simulations of a silica nanopore that is gradually filled with a typical imidazolium salt ionic liquid to obtain a realistic model of these ionogels. Despite the significant layering and stiffening of the ionic liquid in the vicinity of the silica surface, the pair correlation functions and magnitude of its dynamics clearly evidence liquid-like behavior. An increase in the self-diffusivity and ionic conductivity, associated with a decrease in the characteristic residence times of ions at the silica surface, is observed upon increasing the loading as the ionic liquid fills the nanopore center and tends to recover its bulk properties.