Download The Effect of Water and light Alcohols on the Viscosity of Ionic Liquids

The Effect of Water and light Alcohols on the
Viscosity of Ionic Liquids
Thesis Proposal
Bradley Buchheit
Ruth E. Baltus, PhD.
INTRODUCTION
Room temperature ionic liquids (RTIL’s) are salts that are in liquid form at room
temperature. Research involving these liquids has really only began to flourish since the
1990’s. This poses a slight problem: the scientific community has not had time to
effectively analyze these new compounds or explore their practical uses in chemical
processes. As more and more ionic liquids are synthesized, more opportunities arise to
research their various chemical properties.
Ionic liquids have potential uses that span from environmentally friendly solvents to
lubricants and electrochemical agents. As new combinations of cations and anions are
being manufactured, the availability of physical and chemical data about these unusual
liquids at different temperatures and under different conditions is often difficult to locate.
This data is needed before industrial applications can be developed. In most cases,
feasibility studies are not possible unless property data is available for the ionic liquid
being investigated.
It is proposed that the viscosity of different ionic liquids be obtained by use of a
gravitational viscometer at 10, 20, 25, 30, 40, and 50°C in equilibrium with ambient air,
50% relative humidity, and 100% relative humidity. The effect of methanol, ethanol and
propanol on the viscosity will also be considered. The ionic liquids to be used are: 1butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), 1-octyl-3-methylimidazolium
tetrafluoroborate ([omim][BF4]), 1-butyl-3-methylpyridinium
bis(trifluoromethylsulfonyl)imide ([BMPIm][Tf2N]), 1-hexyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]), and 1-butyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]). Experimental data will then be used to
develop a correlation for viscosity as a function of water/alcohol content.
Previous studies have shown that the introduction of various alcohols into ionic liquids
plays a role in their phase behavior (Crosthwaite, et al.2,3). Therefore, it is proposed that
ionic liquid/alcohol mixtures be studied to see if the introduction of an alcohol to an ionic
liquid alters the observed viscosity. Since water can be characterized as a very simple
alcohol (a hydroxide molecule attached to a hydrogen atom) and since it is very difficult
to keep water out of many ionic liquids, it too will be studied. This will give a better
understanding of how ionic liquid characteristics change when exposed to common
alcohols.
MATERIALS AND METHODS
The project consists of two different phases. In the first phase, the viscometer will be
calibrated using three different concentrations of aqueous sucrose solution (30%, 40%,
and 50% by weight) and the viscosities of the ionic liquids used in the study, in
equilibrium with ambient air, will be obtained. Phase two is comprised of the following:
analysis of the viscosity sensitivity to water/alcohol uptake, as well as measuring the
viscosity of the liquids after allowing them to equilibrate in 50% and 100% relative
humidity (RH) alcohol/water environments.
The kinematic viscosities will be calculated from the efflux times obtained using a
Cannon-Manning semi-micro, open capillary, gravitational viscometer. The viscometer
will be calibrated by recording the efflux times of 30%, 40%, and 50% by weight
solutions of sucrose at 10, 20, 25, 30, 40, and 50°C. The relationship between efflux
time and viscosity is:
η = ρ x Co(1-BΔT) x t
[1]
where η is the viscosity in cP, ρ is the density in g/cm3, ΔT=T-22 where T is the
temperature in °C, t is the efflux time in seconds, and Co and B are the calibrated
constants. Known viscosities and densities of the solutions at different temperatures are
given in literature(1), allowing for the calculation of the constants, Co and B. The data can
then be plotted in Excel with ΔT on the x-axis and η/(ρt) on the y-axis making the yintercept Co and the slope equal to – CoB.
η
 C o  C BT
o
ρt
In phase 2 of the experiment, water (and different alcohols) will be introduced into the
ionic liquids via a mixture of wet and dry nitrogen gas, using the apparatus shown in
figure 1.
[2]
Figure 1. Apparatus used to introduce water into the ionic liquids.
The N2 flow through each line is controlled by a mass flow meter (AALBORG, AFC2600).
The “wet” gas is bubbled through a gas dispersion tube into de-ionized water or alcohol.
The resulting gas will be mixed with the dry gas from the other line and introduced into
the ionic liquid using a gas sparger. The mixing of the 2 lines in different proportions
allows for different percent humidity. The ionic liquid will be allowed to equilibrate with
the product gas for 72 hours before viscosity analysis is performed. Water analysis will
be performed using the same procedure outlined in phase one. As for the alcohol
content of the liquids, experimental data for different alcohol solubilities, reported in
literature, will be used to calculate the percent by weight present in the mixture. Efflux
time measurements will be taken at the same temperature intervals as used in the first
phase. Before efflux times can be measured in the viscometer it will first need to be
cleaned with acetone, then methanol, and finally RBS 35® (Dichromate-Sulfuric mixture
alternative). The viscometer will then be placed in an evacuated oven at 60°C for 2
hours before charging. Three efflux time readings will be taken at each temperature for
accuracy. Viscometer and sample will be kept in an isothermal reservoir during the
entire experiment. The sample will be allowed to equilibrate with the water bath
temperature until temperature variations are negligible or approximately five minutes.
To ensure that the temperature is not affecting the ionic liquid structure, viscosity
measurements will be taken up to 50°C then back down to 10 °C for comparison. Water
content analysis will be performed on the liquids pre and post exposure to H2O using an
automatic volumetric Karl Fischer Titrator (Metrohm 795 Titrino). The sample will be
drawn into a syringe, massed, released into the titration vessel, and back weighing will
be performed. The mass of the sample can then be entered into the instrument, allowing
for the calculation of percent water by mass. Needles and syringes will be cleaned with
acetonitrile, and allowed to air dry for 12 hours in the hood. Alcohol concentrations will
be determined by using published solubility data.
TIMELINE
Summer 2006

Develop apparatus to introduce alcohols and water.

Measure sensitivity of water content on viscosity.
Spring 2007

Finish measurements using water.

Start viscosity measurements of ionic liquid/alcohol systems.
Fall 2007

Finish measurements of viscosities of ionic liquid/alcohol systems.

Write thesis.
RESULTS TO DATE
The constant values for equation one, listed in table one, were obtained through the
experimental procedure outlined above. These values will be used in all viscosity
calculations throughout the project.
Table 1. Defined constants for equation 1.
Constant
Value
Co
.222744
B
413.72 x 10-6 /°C
Figure 2 contains initial data on the effect of water concentrations on the viscosity of
BmimBF4. Here, viscosity is plotted on a log scale versus the inverse temperature. As
the water content increases the viscosity of the ionic liquid decreases.
Viscosity (cP)
1000
Amb ie n t Air (.1 2 1 2 % Wa te r)
5 0 % RH (3 .3 7 6 4 % Wa te r)
1 0 0 % RH (9 .1 7 7 8 % Wa te r)
100
10
1
3 .0 8
3 .1 8
3 .2 8
3 .3 8
-1
1000/T (K )
3 .4 8
Figure 2. BmimBF4 Viscosity in Equilibrium with Ambient Air and 50% and 100% RH
It can therefore be hypothesized that this same behavior will be observed when other
ionic liquids are exposed to water. Since water is a simple alcohol it is expected that a
decrease in viscosity of ionic liquids exposed to different alcohols will also be observed.
It has also been found that different ionic liquids are affected differently by water
concentration than others. After testing multiple ionic liquids containing the
tetrafluoroborate anion, it has shown a considerably higher affinity towards water than
BmimTf2N. This in hand led to a greater affect on viscosity.
REFERENCES
1. Barber, E.J. “Calculation of Density and Viscosity of Sucrose Solutions as a
Function of Concentration and Temperature.” Natl. Cancer Inst. Monogr. 21
(1966): 219-239.
2. Crosthwaite, et al. "Liquid Phase Behavior of Ionic Liquids with Alcohols:
Experimental Studies and Modeling." Journal of Physical Chemistry B 110
(2006): 9354-9361.
3. Crosthwaite, et al. "Liquid Phase Behavior of Imadazolium-Based Ionic Liquids
with Alcohols." Journal of Physical Chemistry B 108 (2004): 5113-5119.
4. Jacquemin, J., P. Husson, A. A. H. Padua, and V. Majer. "Density and Viscosity
of Several Pure and Water-Saturated Ionic Liquids." Green Chemistry 8 (2006):
172-180.
5. Harris, K.R. “Temperature and Pressure Dependence of the Viscosity of the Ionic
Liquids 1-Methyl-3-octylimidazolium Hexafluorophosphate and 1-Methyl-3octylimidazolium Tetrafluoroborate.” J. Chem. Eng. Data 51 (2006): 1161-1167.