Impact of Dissolved Species on Ionic Liquid Viscosity
Daniel Wang1 and Dr. Ruth Baltus2
Department of Chemical and Biomolecular Engineering
Introduction
Ionic liquids are a new class of solvents, typically composed of bulky, organic cations and
inorganic anions. They remain liquid at room temperature and are characterized by strong conductivity,
thermal chemical stability, and no detectable vapor pressure. These properties give ionic liquids potential
for numerous applications in catalytic biological reactions, separation processes, and electrochemistry.
Often they are more efficient solvents than their traditional organic counterparts. Furthermore, because
ionic liquids are nonvolatile, they are considered to be environmentally benign alternatives.
Before industrial applications can be properly engineered, more must be understood about ionic
liquid properties. Viscosity, in particular, is known to be highly variable among ionic liquids containing
dissolved species. This project investigates the effect of three different species on viscosity: lithium,
silver, and water.
Motivations for Species Selection
Lithium-ion batteries are a common power source in many consumer electronics. They currently
contain volatile, organic solvents that serve as electrolytes. These batteries deteriorate rapidly at high
temperatures. Under certain mistreatment, they can even explode. Ionic liquids have a very wide
temperature range, up to 250 degrees Celsius, and also a wide electrochemical window, or voltage range.
Table 1 - Ionic liquids planned for adding lithium to.
Ionic Liquids for Lithium
1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
1-ethyl-3-methylimidazolium tetrafluoroborate
1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
1-butyl-3-methylimidazolium tetrafluoroborate
1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
1-hexyl-3-methylimidazolium tetrafluoroborate
1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide
1-butyl-2,3-dimethylimidazolium tetrafluoroborate
1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
1-ethyl-3-methylimidazolium tetrafluoroborate
Abbreviation
emim Tf2N
emim BF4
bmim Tf2N
bmim BF4
hmim Tf2N
hmim BF4
bmmim Tf2N
bmmim BF4
empy Tf2N
empy BF4
Silver is commonly used as a reference electrode for aqueous systems. For continued ionic liquid
research in electrochemistry, it would be useful to create an ionic liquid reference electrode. Due to
chemical supply constraints, the following list is more limited.
Table 2 - Ionic liquids planned for adding silver to.
Daniel Wang, Chemical Engineering, University of Rochester Class of 2010,
Clarkson University Environmental Science and Engineering REU 2008
2 Dr. Ruth Baltus, Professor and Chair, Department of Chemical and Biomolecular Engineering, Clarkson University
1
Ionic Liquids for Silver
1-ethyl-3-methylimidazolium tetrafluoroborate
1-butyl-3-methylimidazolium tetrafluoroborate
1-hexyl-3-methylimidazolium tetrafluoroborate
1-octyl-3-methylimidazolium tetrafluoroborate
Abbreviation
emim BF4
bmim BF4
hmim BF4
omim BF4
Many potential industrial processes utilizing ionic liquids may involve exposure to water, through
condensation, humid air, or other means. Previous research has shown that very small water
concentrations can have drastic effects on ionic liquid viscosity, a serious concern when designing
chemical processes involving ionic liquids.
Table 3 - Ionic liquids planned for adding water to.
Ionic Liquids for Water
1-butyl-3-methylimidazolium tetrafluoroborate
1-hexyl-3-methylimidazolium tetrafluoroborate
1-octyl-3-methylimidazolium tetrafluoroborate
Abbreviation
bmim BF4
hmim BF4
omim BF4
Procedures
Samples are prepared of 1M lithium ion solution in four ionic liquids, simply by measuring and
adding the appropriate mass fraction of ions. This is the same concentration of lithium currently used in
batteries. Silver ion solutions are similarly prepared, but with different ionic liquids because of limited
supply.
Water is introduced by bubbling humidified nitrogen through a gas sparger into ionic liquid
samples for 72 hours. This should simulate equilibrium with humid air. A mass flow regulator controls
the nitrogen humidity by combining two gas streams in different proportions, one stream of pure nitrogen
and another of water saturated nitrogen. The two liquids used are 1-butyl-3-methylimidazolium
tetrafluoroborate (bmim BF4) and 1-octyl-3-methylimidazolium tetrafluoroborate (omim BF4), for which
previous data was already collected at 50% and 100% humidity. This study adds 25% and 75% to that
data. Water content was measured through Karl Fisher titration (Metrohm 795 Titrino). Viscosities of all
the samples are measured by a cone and plate viscometer at various shear rates, and at various
temperatures controlled by an oil bath: 0, 10, 25, 40, 50 C.
Discussion and Results
Partial results for viscosity of clean ionic liquids and ionic liquids with added lithium ions are
show in Figure 1. These results indicate that the addition of lithium to clean ionic liquids causes a very
large increase in viscosity at low temperatures, but a small increase at temperatures above 25 C. Because
higher viscosity is correlated to lower ionic conductivity, this suggests that batteries containing ionic
liquids would work most efficiently at higher temperatures. Further results are pending.
The data gathered at 0 C has been omitted. At this low temperature, the ionic liquid viscosity appeared to
change with different shear rates, characteristic of non-Newtonian fluids. More specifically, ionic liquid
behavior at 0 C closely fit a Bingham plastic non-Newtonian model. However, the viscosity
Daniel Wang, Chemical Engineering, University of Rochester Class of 2010,
Clarkson University Environmental Science and Engineering REU 2008
2 Dr. Ruth Baltus, Professor and Chair, Department of Chemical and Biomolecular Engineering, Clarkson University
1
measurements are unusually widespread and unreliable, so this theory is not conclusive. The widespread
data may have been due to a machine failure, or perhaps is a direct effect of unusual non-Newtonian
behavior.
Figure 1 - Viscosity vs Temp for clean ionic liquid and for ionic liquid with 1M Li
Future Work
Time constraints limited the investigation of water content. This study would be made more complete by
adding data on Hmim BF4. Adding this third sample may make a theoretical analysis of the molecular
effect of water content on ionic liquids possible. The study of silver’s effects on viscosity may be made
more complete by investigating silver ions in Tf2N containing ionic liquids, as was done for lithium ions.
It may also be important to further investigate the possible non-Newtonian behavior of ionic liquids at
low temperatures.
Daniel Wang, Chemical Engineering, University of Rochester Class of 2010,
Clarkson University Environmental Science and Engineering REU 2008
2 Dr. Ruth Baltus, Professor and Chair, Department of Chemical and Biomolecular Engineering, Clarkson University
1
References
Buchheit, Bradley. Ruth E. Baltus. The Effect of Water and Light Alcohols on the Viscosity of
Tetrafluoroborate Containing Ionic Liquids. Department of Chemical and Biomolecular Engineering,
Clarkson University.
Moganty, Sekhar. Personal interview. July 2008.
Morrison, Faith A. Understanding Rheology. Oxford University Press, Inc. Copyright 2001. New York
New York.
Daniel Wang, Chemical Engineering, University of Rochester Class of 2010,
Clarkson University Environmental Science and Engineering REU 2008
2 Dr. Ruth Baltus, Professor and Chair, Department of Chemical and Biomolecular Engineering, Clarkson University
1