The Effect of Water and Light Alcohols on the Viscosity of Tetrafluoroborate Containing Ionic Liquids

Ionic liquids (IL's) are organic salts that are liquid at room temperature. Our understanding of the physical and chemical properties of IL's is limited. In this study, the effect of water on the viscosity of 1-butyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide ([BMPIm][Tf2N]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), and 1-octyl-3-methylimidazolium tetrafluoroborate ([omim][BF4]) was investigated. The effect of methanol and butanol on the viscosity of [bmim][BF4] and [omim][BF4] was also studied. Viscosity measurements were recorded for all three IL's exposed to water at 50% and 100% relative humidities, and for [bmim][BF4] and [omim][BF4] samples that were exposed to methanol and butanol at equivalent partial pressures. Viscosity was measured at temperatures ranging from 10 to 50 degree Celsius. Water content was measured using volumetric Karl Fischer Titration. The viscosities of the IL's exhibited Arrhenius behavior over the given temperature range. Unlike most liquids, a significant change in viscosity was observed in the IL's containing the tetrafluoroborate anion when only a small amount of water or alcohol was introduced. Our findings show that the viscosity of [omim][BF4] was more sensitive to water than the viscosity of [bmim][BF4]. Results also support the conclusion that addition of butanol results in a greater drop in viscosity than does methanol.