Ionic liquids (IL) 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 IL consists of cations based on alkylimidazolium or alkylpyridinium ions and anions such as BF4Â¯, PF6Â¯, N(CF3SO2)2Â¯, CF3SO3Â¯. Chloroaluminate anions are also important provided moisture can be excluded. A large number of ionic combinations are possible for designing compounds with specific properties.
IL 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 IL 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:
a) thermodynamic and transport properties of the pure IL
b) solubility of gases in IL
c) miscibility gaps of IL with organic liquids and water (liquid-liquid equilibria)
d) densities, activity coefficients and excess properties of IL +- organic substances (and water)
e) viscosities, diffusion coefficients, and electrical conductivities of IL +- organic substance (and water)
f) 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 ionic liquids have been held but they have been primarily directed towards the use of IL as solvent systems for chemical reactions with emphasis on kinetics and homogeneous organometallic catalysis. There has been a minimal emphasis on thermophysical property information. Of the few duplicate thermophysical property data published agreement was generally poor.
The Task Group plans to hold three workshops within the 3 year life of the Project. Each workshop will include a number of invited speakers, short oral and poster contributions, and a panel discussion. The aim of the workshop series would be to stimulate discussion and cooperation concerning items a) to f) above between chemical engineers, thermodynamicists, theoreticians, and organic chemists active in research on IL and those who are dealing with IL in technical and industrial applications. Standardized IL and IL +- liquid mixtures will be studied and the results will be used to establish reference data for testing measurement techniques.
A session and workshop on ionic liquids was held at the International Conference on Chemical Thermodynamics (ICCT) in Rostock, Germany, 27 July to 2 August, 2002 (see annex 1 – pdf 16KB). The present project objectives were finalized at that workshop.
The selection of the reference ionic liquid was not straightforward. Ionic liquids containing boron tetrafluoride and phosphorus hexafluoride were found to decompose to produce HF, which was likely one major reason for the discordant results reported in the literature. After preliminary measurements and discussion, it was agreed at a meeting of some of the Task Group members at the PPEPPD Conference in Snowbird, Utah, May 2004, that the reference material would be 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [hmim][Tf2N]. (See annex 2 – pdf 6KB). It was decided not to recommend the butyl compound [bmim][Tf2N] because of patent concerns in the US. The Physical and Chemical Properties Division of NIST at Boulder agreed to purchase the starting materials and Mark Muldoon of Joan Brennecke’s group at Notre Dame agreed to prepare and characterize up to 1 L of high purity [hmim][Tf2N].
A meeting of the Task Group was held August 17, 2004 at the ICCT, Beijing (see annex 3 – pdf 10KB). The selection of the reference material was confirmed and the coordinators of the various property categories as well as the participants in the measurement program were agreed upon.
GAS SOLUBILITY (Joan Brennecke, Coordinator)
VLE, GAMMA INFINITY, AND EXCESS ENTHALPY (Andreas Heintz, Coordinator)
Andreas Heintz (VLE, gamma infinity, excess enthalpy)
Trevor Letchor (gamma infinity)
DENSITY, VISCOSITY, HEAT CAPACITY (Joseph Magee, Coordinator)
Ken Marsh (density and viscosity)
Jason Widegren (viscosity)
Don Archer (heat capacity)
Tooru Atake (heat capacity)
Luis Paulo Rebelo (density)
Gennady Kabo (heat capacity)
Carlos Nieto de Castro (viscosity)
THERMAL CONDUCTIVITY, RELATIVE PERMITIVITY, ELECTROLYTIC CONDUCTANCE, SURFACE TENSION, SPEED OF SOUND (Ken Marsh, Coordinator)
Ken Marsh (thermal conductivity, relative permittivity, electrolytic conductance)
Andreas Heintz (surface tension)
Luis Paulo Rebelo (speed of sound)
Richard Perkins (thermal conductivity)
Jason Widegren (electrolytic conductance)
Carlos Nieto de Castro (thermal conductivity, relative permittivity)
LIQUID-LIQUID EQUILIBRIA, EXCESS VOLUMES (Luis Paulo Rebelo, Coordinator)
Luis Paulo Rebelo (LLE and VE)
Andreas Heintz (LLE)
Joan Brennecke (LLE)
DECOMPOSITION TEMPERATURE, THERMAL ANALYSIS (Ken Seddon, Coordinator)
Ken Seddon (decomposition temperature, Tg, Tm)
Joan Brennecke (decomposition temperature, Tg, Tm, Tf)
Between September and November 2004 samples with known water content, determined by coulometric Karl Fischer titration, were distributed from NIST to the participants in vacuum sealed Schlenk tubes. It was agreed that measurements would be completed by July 31, 2005 and that the results would be collected and analyzed by the coordinators and a detailed report made to a Task Group meeting with the results reported at a Workshop on Ionic Liquids scheduled at the next ICCT conference (Boulder, July 30 to August 4, 2006). It was agreed that water content would be measured by each participant before and after their measurements. The ASTM standards for measurement of water content by both volumetric and coulometric Karl Fischer titration were distributed.
A report on the progress of the measurement program was given at the Thermo2005 conference in Sesimbra, Portugal, April 8 2005 (annex 4 – pdf 53KB). The status of the experimental program at the end of May 2005 is detailed in annex 5 (pdf 14KB).
Software for submission of data to the Ionic liquids Database is in the testing stage and should be available from the NIST web site at the end on June 2005.
> July 2007 update – An informal meeting of a number of members of the Task Group was held at the 1st International Congress on Ionic Liquids (COIL) June 21, 2005, Salzburg, Austria. See minutes as annex 6 (pdf 7KB).
During the last two years most of the planned measurements have been completed. Many of the results have been published in the open literature and some of the results have been submitted to NIST with the intention that they be published in the final report. The only measurements than have not been completed are thermal conductivity, enthalpy of mixing, and surface tension. The hot wire technique is not suitable for conducting liquids without major modification and researchers using the parallel plate method did not join the project. The enthalpy of mixing measurements requires an excessive amount of the ionic liquid to be able to adequately cover the whole mole fraction range.
A meeting attended by Dr Marsh, Dr Magee, Dr Frenkel and Dr Chirico was held at NIST on 28 June 2007 with the express aim of finalizing the project, and with final draft papers preapred for submission in December 2007.
> List of papers published or submitted and reports resulting from this project [pdf file – 12KB; update 2 July 2007]