Thermophysical properties of room temperature ionic liquids with oligomeric formate and hydrogen sulfate

Brønsted acid room temperature ionic liquids (RTILs) comprised of oligomeric formate (or hydrogen sulfate) anions and pyrrolidinium-based cations are prepared. It is found that ion identity greatly affects the thermophysical properties. All RTILs studied here are liquid and have a high ionic conductivity (up to 55.4 mS/cm) and a low viscosity (down to 6.42 mPa s) at room temperature. The incorporation of oligomeric anions in ILs significantly increases the density and conductivity, and decreases the viscosity. The correlation between ionic conductivity and viscosity is based on the classical Walden rule; a relatively large deviation of the plots from the ideal Walden line is observed for formate-based ILs synthesized using equimolar amounts of acid and base, whereas the deviation decreases significantly when the formate-based ILs contain oligomeric anions. The Stokes–Einstein plot of DT−1 vs. η−1 for ferrocence in ILs shows that hydrodynamic radius of diffusing species is independent of temperature.