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.