Functional Design of Ionic Liquids

Abstract Ionic liquids, molten salts at temperatures lower than 100 °C, preferably below room temperature, are unique liquids having very different characteristics from ordinary molecular liquids. Since these ionic liquids are prepared by coupling relatively large organic ions, there are many chances to create novel functionalities by changing structure of component ions. To develop such novel and functional ionic liquids, we first developed a facile preparation method, i.e., neutralization of tertiary amines with organic acids. By this convenient method, we prepared hundreds of salts having low melting points. We, then, tried to prepare ionic salts containing small ions such as lithium cation, proton, or chloride anion. Because small ions received stronger electrostatic force due to higher surface charge density, which results in the formation of salts with higher melting points, case must be taken. Ionic liquids containing halogen-free salts are another interesting subject to challenge from the viewpoint of environmentally friendly materials. Heteroaromatic rings such as azolates gave excellent ionic liquids having very low viscosity by coupling with suitable cations. Amino acids have also been used as anions to prepare salts with low melting points or amorphous glasses. They are helpful to analyze the relationship between ion structure and characteristics of the salts. Zwitterions, in which both cation and anion were tethered with a covalent bond, were prepared to function as ionic liquids. These ions should not migrate under a potential gradient and should be an excellent ion conductive matrix for added ions. Tethering of ions caused the Tm to be high, and the zwitterions with melting points around room temperature have not been synthesized yet. However, lots of solid zwitterions became liquids by adding suitable solid salts or acid such as lithium bis(trifluoromethanesulfonyl)imide. In this mixture, lithium cations had a transport number higher than 0.5, meaning that the lithium cations are the major mobile ions in the mixture. Some of the functionality is achieved by dimension control, such as polymerization of ions, and so on. Other functional modifications and design of ionic liquids are also mentioned.