Ionic Liquids, Stakes and Challenges, from Research to Industry

Fifteen years after a review that presented the main physico-chemical properties of ionic liquids (ILs), the new solvents of the day, it seemed necessary and important to revisit these compounds, which are as astonishing and multiform as ever, but no doubt less miraculous than researchers had imagined them to be. Starting with the definition of ionic liquids, an overview of their incredible structural diversity will then be given. After detailing new, cleaner and more efficient synthesis methods, new insights into their physico-chemical properties (density, viscosity, liquid phase domain, etc.) will be the focus of this review article, with some insights into the evolution of ideas. The article also describes the toxicity, volatility and flammability of ionic liquids, which were largely unknown in the past and have earned them the now disputed status of "green solvents". Faced with an ever-increasing number of ionic liquids, empirical laws have been proposed to describe their behavior. The limits of this approach will be discussed and compared with the efforts made by modeling techniques to try and pin down their structure in the liquid phase and their properties at interfaces. The solubility and partitioning properties of ionic liquids with traditional molecular solvents and other ionic liquids will then be summarized, followed by a description of some ionic liquid applications. Although they are struggling to make the transition from the laboratory to industry, their use is spreading in a variety of fields, from electrodeposition and liquid-liquid extraction of metals to analytical chemistry, biorefinery, tribology and the synthesis of inorganic nanomaterials.