The Association of Water in Ionic Liquids: A Reliable Measure of Polarity

摘要

Ionic liquids (ILs) are liquids formed solely of ions. Unlike inorganic molten salts such as NaCl, they are fluid at room temperature, or at least within the typical temperature range of most organic chemical reactions.[1], [2] A number of properties make ionic liquids attractive in different areas of chemistry. They exhibit an almost vanishing vapor pressure, the characteristic electrical conductivity of an ionic conductor, and a considerable gap between the melting point and the temperature of decomposition.[1], [3] The possibility of directing the course of chemical reactions along with the low vapor pressure make ILs an attractive alternative to traditional organic solvents for both laboratory and industrial purposes. Today ILs are regarded as green solvents and designer media for chemical reactions.[4]-[6] To expand the utility of ionic liquids, IL-based mixed solvents have come into focus.[7] In particular the addition of water can strongly affect the physical and chemical properties of ionic liquids such as viscosity,[8], [9] electrical conductivity,[10] and reactivity,[11] as well as solvation and solubility properties.[12]-[16] Water is also an impurity commonly present in ILs since all known ILs are hygroscopic; traces of water are thought to be ubiquitous in these materials.