Thermal Properties and Ionic Conductivity of Imidazolium Bis(trifluoromethanesulfonyl)imide Dicationic Ionic Liquids

We report on the thermal and transport properties of new dicationic ionic liquids. The new ionic liquids are based on the bis(trifluoromethanesulfonyl)imide [NTf(2)](-) anion and a cation that contains two imidazolium rings, connected by either a pentane or a decane hydrocarbon chain and different side groups. We have investigated the conductivity and the thermal properties by dielectric spectroscopy and differential scanning calorimetry, respectively. Our results show that the length of the alkyl chain on the cation has no, or weak, influence on the glass transition temperature, T(g), whereas the presence of rigid aromatic side groups has a strong influence increase T(g). The highest ionic conductivity is 5.9 x 10(-4) S cm(-1) at 298 K for an ionic liquid with a decane chain and one methyl group on each imidazolium ring. The conductivity results correlate well with the glass transition temperatures. This shows that the flexibility of the geminal cations is very important for the conductivity. However, the presence of nonflexible aromatic side groups on the imidazolium ring decreases the flexibility and hence the mobility.