(776d) Vapor-Liquid Coexistence and Critical Behavior of Ionic Liquids

Since the famous publication of Earle et al. in 2006 [1] in which it was shown that
ionic liquids could be distilled, interest in the nature of the vapor phase of ionic
liquids has grown. There have been many efforts to measure enthalpies of
vaporisation and vapor pressures. Studies also have been carried out to examine
the nature of the ions in the gas phase, and attempts have been made to estimate the
critical point of ionic liquids using either empirical correlations or extrapolation of
experimental properties such as surface tensions to high temperature. These latter
efforts have been made because, even though it is expected that most ionic liquids
will decompose before reaching their critical temperature, equation of state models
and other correlations rely extensively on the use of critical points.

In this talk, we present Gibbs ensemble Monte Carlo results in which vapor‐liquid coexistence curves of ionic
liquids are computed using an atomistic force field. The simulations provide information on the enthalpy
of vaporisation and vapor pressure as a function of temperature, and an estimate of a critical point of an
ionic liquid. The gas phase ion cluster
distribution is computed, and it is shown that the predominate species at low
temperature is the single ion pair. At higher temperatures and gas densities,
multiple ions aggregate into larger clusters. We examine the dependence of all these
properties on the structure and composition of the cations and anions.

[1] Earle, M. J.; Esperanc¸ a, J. M. S. S.; Gilea, M. A.; Canongia Lopes, J. N.; Rebelo, L.
P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. Nature 2006, 439, 831