Fragility of ionic liquids measured by Flash differential scanning calorimetry

Ionic liquids display outstanding properties and have a wide range of potential uses. Their unique ionic and organic nature also identify them as good glass-forming materials. In this work, a series of imidazolium-based ionic liquids with varying functionalities from aliphatic to aromatic groups and a fixed bis[(trifluoromethane)sulfonyl]amide ([NTf2]−) anion are characterized using a commercial rapid scanning nanocalorimetry instrument, the Mettler Toledo Flash DSC. The limiting fictive temperature Tf′, which is equivalent to the glass transition temperature Tg, is measured on heating as a function of cooling rate. The dynamic fragility is obtained for the series of ionic liquids, and using this data along with a compilation of data from the literature for a total of 50 ionic liquids reveals that aromatic structure increases fragility more than expected simply based on the change in Tg. Our results provide an approach to control fragility of ionic liquids through chemical modifications to the side group.