Electrolytic conductivity of four imidazolium-based room-temperature ionic liquids and the effect of a water impurity

The electrolytic conductivities (κ) of four hydrophobic room-temperature ionic liquids (RTILs) were measured at temperatures from (288.15 to 323.15) K. The measurements were made with a commercial conductivity cell with platinum black electrodes. In order to exclude atmospheric moisture, the conductivity cell was modified so that it could be sealed during measurements. The RTILs studied were 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and 1-butyl-3-methylimidazolium hexafluorophosphate. The RTILs were dried under vacuum until the mass fraction of water (wH2O) was ⩽10−5. Coulometric Karl Fischer titration was used to determine wH2O in each RTIL before and after measurements of κ. Most measurements were made on dried RTILs, but some measurements were performed intentionally at higher wH2O in order to study the effect of a water impurity on κ. For {water + 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide}, κ was found to increase dramatically with increasing wH2O; for example, in going from wH2O=10-5to10-2, the fractional increase in κ was 0.36 {=(κwet − κdried)/κdried}. This work illustrates the importance of measuring the water content in RTILs both before and after measurements of κ.