18: Review of Smart Ionic Liquids

Chapter 18 reviews smart ionic liquids. Ionic solids such as sodium chloride (table salt) have been known for centuries. To eliminate the role of a solvent, particularly in the realm of electrochemistry, molten salts became of interest. The very first examples were the groundbreaking endeavors of Sir Humphry Davy in the synthesis of alkali metals by electrolysis, which he discovered (isolated to be precise) via this approach. However, this needs a high temperature as the ionic bonds are strong. The electrolysis of sodium chloride should be conducted at a temperature higher than 801 °C. Since high temperatures are not technologically favorable, the melting point of such ionic solids can be reduced by weakening the ionic bonds in eutectic mixtures. One of the very first examples is the pioneering work of Charles Martin Hall in the synthesis of aluminum, which is still the dominant approach for the exploitation of metallic aluminum. The high melting point of these ionic liquids is due to the close arrangement of highly charged ions within the lattice. For instance, the sodium and chlorine are small atoms, which are closely located in a cubic lattice structure. Therefore, high energy is required to separate the anion and cation.