Physicochemical features of short-chain 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide ionic liquids containing equilibrium water absorbed from air

Abstract

In the present research we studied ionic liquids of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [CnMIm][TFSI], where n = 2 (ethyl, C2), 3 (propyl, C3) and 4 (butyl, C4) with small water impurity absorbed though the contact with air. IR spectra, thermodynamic, electrochemical and the bulk properties, such as density and viscosity, were determined by means of FT-IR spectroscopy, conductometry, densimetry, viscosimetry, TG and DSC methods. The influence of temperature, absorbed water and elongation of the cation alkyl chain on the physicochemical properties was studied. It was found that the absorbed water (approximately 0.24–0.25% by weight) is frozen at a temperature from −40.5 to −46.3 °C, has little effect on thermal stability and significantly affects the phase behavior of ionic liquids. The linearly dependence of the density on the temperature was observed. The conductivity increases and the viscosity decreases with increasing temperature according with the VFT equation. Physicochemical properties depend on alkyl chain length, but not linearly, so that C3 is close to the transition between C2 and C4. Such tendency was explained in terms of Van der Waals forces and hydrogen bonds.