Ionic liquids are pure salts that are also liquid at moderate temperatures. They have many interesting properties, including extraordinarily low vapor pressure and unique solvation ability. We have been using atomistic-level simulations to study ionic liquids for over fifteen 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.
We are using advanced molecular dynamics and Monte Carlo simulations to compute the thermophysical properties of these fascinating liquids, to discover new ionic liquids for a host of different applications and to learn how different molecular structures, compositions and mixtures govern the resulting physical properties. Some of our latest work has focused on the behavior of ionic liquid mixtures at interfaces and how water can cause liquid-liquid phase splits of ionic liquid mixtures having different degrees of hydrophobicity.
Ionic Liquid Mixtures
The figure below shows the computed density profile of acetate anions (OAc), bis(trifuoromethanesulfonyl)imide anions (TFSI) and 1-ethyl-2-methylimidazolium cations (C2mim) at a vacuum-ionic liquid interface. The ionic liquid is a 1:9 mixture of [(C2mim)(TFSI)] and [(C2mim)(OAc)]. Despite the fact that there are nine times the number of OAc anions in the mixture, TFSI clearly
segregates to the interface. These results were confirmed via X-ray photoemission spectroscopy carried out by experimental collaborators. For more information, see: Yong Zhang, Yehia Khalifa, Edward Maginn, and John Newberg, “Anion Enhancement at the Liquid-Vacuum Interface of an Ionic Liquid Mixture”, Journal of Physical Chemistry C, 2018, 122, 27392-27401. DOI: 10.1021/acs.jpcc.8b07995.
In another study, we used simulations to predict that the addition of water would cause a 50:50 mixture of [(C2mim)(TFSI)] and [(C2mim)(OAc)] to phase separate into a phase rich in TFSI and another phase rich in OAc. Water distributed mainly in the hydrophillic OAc phase. Similar behavior was predicted for a mixture of [(C2mim)(TFSI)] and [(C2mim)(Cl)] but a mixture of [(C2mim)(Cl)] and [(C2mim)(OAc)] would remain one phase upon adding water. Our experimental collaborators verified these computational predictions and measured the compositions of the two phases.
You can read more about this work here: Alejandra Rocha, Yong Zhang, Edward J. Maginn and Mark B. Shiflett, “Simulation and Measurement of Water Induced Liquid-Liquid Phase Separation of Imidazolium Ionic Liquid Mixtures”, Journal of Chemical Physics, 2018, 149, 164503. DOI:10.1063/1.5054786.
This work has been funded by the U.S. Department of Energy, Basic Energy Science, Joint Center for Energy Storage Research under contract no. DEAC0206CH11357.
Gas Solubility in Ionic Liquids
For many years, we have focused on gas solubility in ionic liquids and have also been particularly interested in CO2 solubility, with a focus on CO2 capture. We have shown that ionic liquids that chemically react with CO2 have the best chance of being used commercially. We recently showed that CO2 absorption isotherms in chemically reactive ionic liquids can be computed directly in two ways: using a thermodynamic cycle and using reactive ensemble Monte Carlo. The thermodynamic cycle is shown below:
The liquid phase reaction (step 1) is what we want to compute, but we can’t do this quantum mechanically because the solvation environment is important and there are too many atoms to do this in the bulk phase. We therefore compute the free energy of the reaction in the gas phase quantum mechanically (step 3) and compute the other “legs” of the cycle (vaporization, condensation, mixing) with a classical potential. The resulting isotherm agrees very well with experimental measurements. To learn more, see: Quintin Sheridan, Ryan Mullen, Tae Bum Lee, Edward Maginn, and William Schneider, “Hybrid Computational Strategy for Predicting CO2 Solubilities in Reactive Ionic Liquids”, Journal of Physical Chemistry C, 2018 DOI: 10.1021/acs.jpcc.8b02095.
We also showed that reactive CO2 isotherms can be computed directly with reaction ensemble Monte Carlo. In this approach, we not only obtain the total isotherm as a function of pressure and temperature but can compute the amount of CO2 that is chemically reacted and physically absorbed in the ionic liquid.
You can learn more about this work here: Ryan Gotchy Mullen, Steven Corcelli and Edward J. Maginn, “Reaction Ensemble Monte Carlo Simulations of CO2 Absorption in the Reactive Ionic Liquid Triethyl(octyl)phosphonium 2-Cyanopyrrolide”, Journal of Physical Chemistry Letters, 2018, 9, 5213-5218. DOI 10.1021/acs/jpclett.8b02304.
Currently, Bridgette Befort, Dr. Yong Zhang, Mary McKinley, Olivia Garcia-Velez, Leah Fast, Jacob Gerace and Yushan Zhang are all working on ionic liquid projects.