The Effect of Water Content on the Viscosity of Ionic Liquids

Ionic liquids are organic salts that are liquid at room temperature. Our understanding of the physical and chemical properties of ionic liquids is limited. In this study, the effect of water on the viscosity of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), 1-octyl-3-methylimidazolium tetrafluoroborate ([omim][BF4]), and 1-butyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide ([BMPIm][Tf2N]) was investigated. The viscosity of ionic liquids was measured for samples equilibrated with ambient air, with a 50% relative humidity environment, and with a 100% relative humidity environment at 10, 20, 25, 30, 40 and 50 degrees Celsius. Water content was determined using volumetric Karl Fischer Titration. The viscosities of the ionic liquids exhibited Arrhenius behavior over the given temperature range. Unlike most liquids, a significant change in viscosity was observed when even a small amount of water was introduced. It was found that the viscosity of the liquids containing a tetrafluoroborate anion were more sensitive to water than the ionic liquid containing the Tf2N- anion. Findings also suggest that the viscosity of [omim][BF4] was more sensitive to water than the viscosity of [bmim][BF4], leading to the conclusion that the cation plays a role in the sensitivity of the viscosities of these liquids to water.