Comparative study of atmospheric ionic liquids based on bis(trifluoromethylsulfonyl)imide anion and alkyl substituted cations of ammonium, pyrrolidinium and imidazolium

Abstract

Ionic liquids (ILs) with single bis(trifluoromethylsulfonyl)imide anion (TFSI−) and different cation, such as methyltrioctylammonium (MTOAm+), butylmethylpyrrolidinium (BMPyrr+) and 1-butyl-3-methylimidazolium (BMIm+), were studied using FT-IR, TG, DSC, densimetry, viscosimetry and conductometry methods. All ionic liquids contained equilibrium water absorbed from air in an amount of about 0.13–0.24 wt%. It was found that, the density, conductivity and crystallization ability decrease in the following order MTOAm+ ≪ BMPyrr+ < BMIm+, the viscosity and ionisity increase in the following order MTOAm+ ≫ BMPyrr+ > BMIm+. A small amount of water (<0.25 wt%) does not significantly affect the density, thermostability and glass transition, but reduces viscosity, increases conductivity and can promotes the formation of metastable mesophase, which includes both ionic liquid and water. The BMPyrr+ and BMIm+ ionic liquids form at least three crystalline mesophases with close melting points from −7 to 10 °C. Using IR spectroscopy, the expected H–bonds, such as CH··O(F), CH··OH and OH··O(F), and strong interactions of the octyl chains in MTOAm+ cation were confirmed. The effect of the cation and water impurity on the properties ionic liquids was discussed in the terms of the cation size and expected interactions in the ILs–H2O system.