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

Abstract

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 ( w H 2 O ) was ⩽10 −5. Coulometric Karl Fischer titration was used to determine w H 2 O in each RTIL before and after measurements of κ. Most measurements were made on dried RTILs, but some measurements were performed intentionally at higher w H 2 O 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 w H 2 O ; for example, in going from w H 2 O = 10 - 5 to 10 - 2 , the fractional increase in κ was 0.36 {=( κ wet − κ dried)/ κ dried}. This work illustrates the importance of measuring the water content in RTILs both before and after measurements of κ.