Temperature dependence of viscosity for room temperature ionic liquids

Abstract

We have measured the absolute viscosity of 23 room temperature ionic liquids (RTILs) (13 air/moisture-tolerant single salts and 10 air/moisture-sensitive binary salts) using a rolling-ball viscometer, which was constructed to enable the viscosity measurements to be performed outside a glove-box. The viscometer was non-destructive to the RTIL and the viscosity measurements were made under typical levels of O 2 and H 2O of less than 1 ppm. The viscometer was calibrated with Milli-Q water and sucrose solutions (20, 40 and 60% w/w) at 14 temperatures (10 °C ⩽ T⩽ 70 °C). Three balls of diameters 3.2, 4 and 5 mm were used for the calibration. We found that the ball with the smallest diameter (3.2 mm) was more accurate for measuring the viscosities. The activation energies for viscous flow ( E η ) for each RTIL were calculated, from the slopes of the Arrhenius plots. E η was correlated with changes in molecular structure of the RTIL; those containing high molar mass and less symmetric cations had higher E η values. The Arrhenius plot was not linear for the majority of the RTILs and some of these RTILs were better fitted with the Vogel–Tammann–Fulcher (VTF) equation. Those RTILs which obeyed the Arrhenius law typically contained asymmetric cations and the majority did not contain functional groups. Those RTILs which obeyed the VTF law contained small and symmetrical cations with low molar mass. Those RTILs which obeyed neither the Arrhenius nor the VTF law consisted of cations which were less symmetric, contained functional groups (e.g., OH and CO) and had higher molar mass. The glass transition temperatures T o (calculated from the VTF fittings) generally decreased with increasing size and molar mass of the cation and anion. The chloroaluminate RTILs containing oxygenated cations were more volatile than the corresponding de-oxygenated RTILs making them unsuitable for applications as solvents and/or electrolytes above ambient temperature.