27 No. 5
Thermodynamics of Ionic Liquids, Ionic Liquid Mixtures, and the Development of Standardized Systems
Ionic liquids (ILs) represent a new class of liquid solvents having some characteristics of molten salts. Recently synthesized compounds are moisture, air, and temperature stable. Their melting points are distinctly below room temperature. Most of these ionic liquids consist of cations based on alkylimidazolium or alkylpyridinium ions and anions such as BF4¯, PF6¯, N(CF3SO2)2¯, CF3SO3¯. Chloroaluminate anions are also important, provided moisture can be excluded. A large number of ionic combinations are possible for designing compounds with specific properties.
ILs have attracted considerable interest during the last few years. They have no detectable vapor pressure and therefore exhibit favorable solvent properties for new homogeneous catalytic reactions and other chemical production processes with respect to “green chemistry.” An increasing number of successful applications are described in the literature. The utilization of ionic liquids in industrial chemistry requires a systematic study of their thermodynamic and thermophysical properties, which are required for chemical process design. The most important properties are as follows:
- thermodynamic and transport properties of the pure ionic liquids
- solubility of gases in ionic liquids
- miscibility gaps of ionic liquids with organic liquids and water (liquid-liquid equilibria)
- densities, activity coefficients and excess properties of ionic liquids + organic substances (and water)
- viscosities, diffusion coefficients, and electrical conductivities of ionic liquids + organic substance (and water)
- data required by theoreticians for predicting properties using computer simulation and/or ab-initio calculation methods
Prior to 2003, a number of workshops and special sessions on ILs were held, but they were primarily directed towards the use of ionic liquids as solvent systems for chemical reactions, with an emphasis on kinetics and homogeneous organometallic catalysis. There has been minimal emphasis on thermophysical property information. Of the little duplicate thermophysical property data published, agreement was generally poor.
The objectives of this project are as follows:
- encourage systematic studies of thermodynamic and thermophysical properties of ionic liquids based on the needs of industrial chemical processes
- recommend a reference ionic liquid and make reference quality measurements on selected thermophysical properties of both the pure ionic liquid and its mixtures
- establish recommended equations for the properties measured and provide recommendations on measurement methods
- suggest directions of future research and encourage cooperation to avoid unnecessary duplication of measurements
- organize workshops and special sessions devoted to the thermophysical properties of ionic liquids
- provide software for submission of data to the ionic liquids database
project, which was initiated in March 2003, is expected to
be completed by the end of 2005. Software for submission of
data to the ILs Database (see project # 2003-020-2-100)
is in the testing stage and should be made available from
the NIST Web site.
For more information and comments, contact Task Group Chairman Kenneth N. Marsh <[email protected]>.
last modified 30 August 2005.
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