Ab initio approaches to understanding polarisability of ionic liquids

Due to the ability of ionic liquids (ILs) to polarize the environment, they have attracted much attention as electrolytes for use in high efficiency devices such as fuel cells, solar cells and batteries. One of the experimental techniques to determine polarisability-related properties of ILs is dielectric spectroscopy that provides an effective way of measuring dielectric constants of ionic liquids. Interestingly, from the dielectric spectroscopy data an unexpected trend is revealed: dielectric constants of routinely used ILs are normally in the range of 10 to 15, thus indicating remarkably low polarity. Electronic polarisabilities, dipole moments and molecular volumes have been calculated at different levels of ab initio theory for a wide set of ionic liquid ions as well as traditional solvents (such as water, THF and DMSO). Theoretical results show how electronic polarisability of a single ion/solvent molecule relates to molecular size and electronic dipole moment. The analysis of the theoretical data has shown that the sheer values of electronic polarisability cannot reliably predict in what range the corresponding dielectric constant should lie, as most of the IL ions studied turned out to have even larger numbers for this quantity than those of highly polar solvents like water and DMSO. It appears that the ratio of polarisability to molecular size together with the packing arrangement of ions in solution play an important role in defining the dielectric constant. As a result, these theoretical data can help us explain why dielectric constants of ionic liquids can, in fact, be rather low.