Phase transitions, molecular dynamics and structural properties of 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid

Abstract

Recently, ionic liquids are very promising substances mentioned in electrochemistry as electrolytes in supercapacitors. Due to the fact the operation conditions for supercapacitors require low temperatures, an important issue is an investigation of melting behavior of ionic liquid applied as an electrolyte. In the presented work, we report the experimental studies of the melting behavior of 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM TFSI); for this purpose the methods of dielectric spectroscopy (DS) and differential scanning calorimetry (DSC) in wide temperature ranges were applied. To investigate the structural properties of the system the temperature-controlled WAXS and Neutron Diffraction methods were adopted. The measurements of the complex permittivity of the studied EMIM TFSI allowed us to determine the relaxation of the system in wide temperature range from 140 K to 300 K. The results obtained from DSC and DS indicate that the ionic liquid on the heating process undergoes two phase transitions: solid-solid transition at temperature 230 K and the melting transition at 257 K, wherein the melting process is not homogeneous. Based on the structural studies results, performed in the temperatures range 10 K – room temperature, it was shown that in the temperature range 10 K–230 K EMIM TFSI forms a crystal monoclinic structure where the rotation motions of molecular groups are observed. At the higher temperature up to the melting temperature the crystal structure is undefined and the rotational motions of molecular groups are changed.