We have been using atomistic-level simulations to study ionic liquids for over ten years. Since then we have helped advance our understanding of these fascinating fluids by making predictions of thermodynamic and transport properties and providing insight into the link between the structure and properties of these liquids.
In very early work, we showed that volumetric properties such as density and isobaric expansivity could be computed with a high level of accuracy using simple intermolecular potential functions of the kind used for biological simulations (see Morrow and Maginn, and Shah et al.). In later work, we computed the solubility of various gases in ionic liquids, and explained the origin of the high solubility of CO2 in these liquids, a feature that has subsequently been exploited for use in gas separations and CO2 capture.
Using advanced Monte Carlo methods, we have computed full isotherms of Co2 in an ionic liquid (see an example on the left). Agreement with experimental measurements (open symbols) is remarkably good. We have applied these and related techniques to many other gases and ionic liquids.
In developing new ionic liquids for CO2 capture, it was proposed to add reactive amine groups to the cations to enable Co2 to be removed from flue gas at low partial pressures. While this does remove Co2, the resulting reacted fluids become extremely viscous. By carrying out molecular dynamics simulations of the reacted and unreacted species, we discovered that the large increase in viscosity was caused by the formation of salt bridges (like the "B-C" salt bridge shown on the right) between the reacted carbamate and ammonium groups on the cation. Details of this study by Gutowski and Maginn can be found here.
We have also carried out simulations to study the properties of a new class of ionic liquids based on "aprotic heterocyclic anions" (AHAs). These anions react with CO2 but do not form salt bridges, thereby elimminating the problem of viscosity increase. A recent paper on this work was published by Wu et al.
In some of our early work in 2002, we computed solubility parameters and the internal energy of vaporization. It was long thought that ionic liquids were "non-volatile" liquids, but these early simulations showed that while the enthalpy of vaporization is high, it is not infinite. Working with Steve Leone's group at Berkeley, we showed that the most likely volatile species from an ionic liquid at low temperature and pressure is a single ion pair (see image below).
We also computed the enthalpy of vaporization for a series of ionic liquids, and showed that the trends agree with experimental data, although there is a lot of variation.
If ionic liquids are volatile, what is their vapor pressure? Boiling point? Critical point? Do these latter quantities even exist? We used advanced Gibbs ensemble Monte Carlo simulations with Cassandra to address these questions (the graphic below depicts this type of simulation). We find that the boiling points and critical points are "theoretical" in that the estimates from our simulations are above the decomposition temperature. Nevertheless, the simulations give insight into the critical behavior and phase equilibria of these liquids, and it is different from conventional liquids like alkanes. We find that as the size of the cation increases, the boiling point and critical temperature decrease (due to a competition between van der Waals and Coulombic forces). At the highest temperaratures and pressures, significant clustering is observed, while at very low pressures, single ion pairs dominate. This work was described in two publications: a Faraday Discussions article (for TF2N ionic liquids) and a J. Phys. Chem. Lett. paper (for BF4 ionic liquids).
