(23d) Understanding Physical Properties of Mixtures of Ionic Liquids

Ionic
liquids (ILs), which are composed of only ions, are liquids with a melting
point less than 100 °C. In addition, most ILs have negligible vapor pressure. 
The anion and cation can both be adjusted, allowing for millions of different
ILs to exist.  Due to the enormous amount of potential ILs that can be made,
trends need to be discovered in order to tune the ILs properly to fit the needs
of various industrial applications.  One such application is CO₂-IL
co-fluid vapor compression refrigeration technology, which requires a certain
enthalpy of reaction between the IL and the CO₂ and a low viscosity
for the IL.  The vapor compression cycle involves CO₂ and IL to be
pump around a desorber, compressor, resorber, and expansion device.  The
benefit of CO₂ as
refrigerant instead of HFC-134a is that CO₂ has a Global Warming
Potential, GWP, of 1, instead of a GWP of 1410.  Of course, the GWP of ILs is
essentially 0 because of their low volatility. The temperature and pressure
range will vary in the cycle, from 10 °C to 50 °C, and from 10 bars to 30
bars.  Through proper tuning, ILs have been created to meet the desired
enthalpy.  However, the viscosity of these ILs are too high at lower
temperatures.  Such a high viscosity would cause a huge cost to pump the ILs
throughout the cycle.

Understanding
the properties of ILs and how the properties change once the ILs are adjusted
is important.  Also, examining how ILs properties change when another IL or a
diluent (e.g., tetraglyme) is added will offer another means to obtain the
desired properties.  Mixtures of ILs are important because it offers the option
to use desirable benefits from two different ILs.  In this work, the primarily
focus for the mixture properties is the viscosity. In addition, we present
several mixture densities and CO₂ solubilities.  The mixture glass
transition, T_g, the melting point, T_m, and the
decomposition, T_onset, are measured using Differential Scanning
Calorimetry, DSC, and Thermal Gravimetric Analysis, TGA,. 

            The
mixture densities always fall between the densities of the pure ILs or the IL
and the tetraglyme.  By contrast, the mixture viscosities exhibit a wide range
of behaviors.  Some mixtures have viscosities that smoothly transition between
the viscosities of the two pure compounds.  However, other mixtures exhibit
viscosities higher than either of the pure ILs.  In general, mixtures tend to
exhibit eutectic behavior, where the melting points of the mixtures are lower
than the melting point of either pure IL.  However, in many instances, the
addition of a second IL will eliminate a melting point, leading to a glass
transition temperature at much lower temperatures.  The trends in these
behaviors, in terms of alkyl chain length, fluorination, ether chains and ammonium
or phosphonium cation, will be discussed.