Ionic Liquids: Technology Integration and Function

This special issue highlights current endeavors that employ ionic liquids in developing technologies. The interplay between properties and application underscores the historical development of ionic liquids over the past century and continues to be a driving force for onwards exploration. As new ionic liquids being synthesized, unique properties are discovered and then applied to new applications. The reverse is also true, where the stringent requirements of some applications force the discovery of new compounds. On the molecular level, the synthesis of ionic liquids containing conventional alkyl imidazolium, ammonium, and phosphonium cations have led to the development of taskspecific cations with tailored functionalization or completely new organic cations or anions with ever increasing applicability. The most salient features of ionic liquids are their low melting points, charged nature, and non-volatility. More broadly though, a liquid that is functionally non-volatile and displays excellent solubilizing properties is sufficiently unique in comparison with conventional solvents/liquids, which find niche applications. One early example was their use as molten electrolytes in the 1960s/1970s for use in batteries to replace conventional molten salts, which broadened the potential for these intriguing molecules beyond curiosities. A review in this special issue by Nakamura et al. presents an overview of the effect dielectric response of ionic liquids to distinguish them from common organic salts and the underlying reasons. Drawing upon a combination of theory and experimental data, they show that ionic liquids may be distinguished from conventional inorganic salts by their high dielectric responses and hydrogen bonding. Their use as tailored solvents became popularized over the past few decades and was broadened from small-molecule chemistry to applications in biopolymer processing. For example, an application-oriented review by Amarasekara et al. discusses the recent developments of ionic liquids in lignocellulosic biomass treatment. This includes depolymerization, biodiesel synthesis, and carbohydrate dehydration, among other chemical transformations. While ionic liquids do perform well, the practical challenges related to their high cost, toxicity, and recycling complexities leaves room for further exploration. Applications of ionic liquids beyond their use as solvents or battery electrolytes harness their other features such as tailor-made quality, non-volatility, and binding to smallmolecules of interest. Often the introduction of ionic liquid improves a technology by increasing the effectiveness of a given mechanism, thus leading to boosted performance. For example, Shaplov et al. provides some original work on the synthesis of silyl-functionalized ionic liquids and their incorporation within a polymer supported ionic liquid membrane (SILM) to determine their CO2/N2 permselectivities. The introduction of different atoms in the ionic liquid structure is an active research frontier with the potential to further broaden the property profile of ionic liquids for such applications. The SILM strategy seeks to compensate for the low melting point of ionic liquids, which makes their integration within functional devices problematic. The polymer itself does not fulfill any chemical function but instead acts as a support that can be impregnated with ionic liquid whose chemical properties dominate the function of the material. The functionalization of solid-supports is another approach to harness useful properties. Dedzo et al. presents a review on the improvement of kaolinite clay minerals by functionalization with ionic liquids to fabricate nanohybrid materials. Modification of clay with a variety of different organic cations has the potential to intercalate between clay sheets and improve dispersability or increase adsorptivity of small-molecules. One significant development in applying ionic liquids in technology has been the conversion of ionic liquids from a small-molecule in to a macromolecule. Such polymers combine useful properties of ionic liquids with the mechanical stability of macromolecule. This significantly broadened the applicability of ionic liquids into new domains and helped attracting attention among engineers and polymer chemists. In particular, polymerized ionic liquids became a route towards the fabrication of highly tunable cationic polyelectrolytes and have found use in porous materials with high surface areas. In such cases, the material takes advantage of the adsorption and exchange properties of the constituent cationic units. Hesemann et al. have demonstrated a facile approach to generate porous ionosilica from aminomethylsilanes for chromate adosprtion. The combination of high-surface area and anionexchange capabilities of the material led to impressively high amounts of chromate adsorption (up to 2.5 mmol/g). The