Experimental and molecular modeling studies are conducted to investigate the underlying mechanisms for the high solubility of CO2 in imidazolium-based ionic liquids. CO2 absorption isotherms at 10, 25, and 50 degrees C are reported for six different ionic liquids formed by pairing three different anions with two cations that differ only in the nature of the "acidic" site at the 2-position on the imidazolium ring. Molecular dynamics simulations of these two cations paired with hexafluorophosphate in the pure state and mixed with CO2 are also described. Both the experimental and the simulation results indicate that the anion has the greatest impact on the solubility of CO2. Experimentally, it is found that the bis(trifluoromethylsulfonyl)imide anion has the greatest affinity for CO2, while there is little difference in CO2 solubility between ionic liquids having the tetrafluoroborate or hexafluorophosphate anion. The simulations show strong organization of CO2 about hexafluorophosphate anions, but only small differences in CO2 structure about the different cations. This is consistent with the experimental finding that, for a given anion, there are only small differences in CO2 solubility for the two cations. Computed and measured densities, partial molar volumes, and thermal expansion coefficients are also reported.

The discovery of materials that combine selectively, controllably, and reversibly with CO2 is a key challenge for realizing practical carbon capture from flue gas and other point sources. We report the design of ionic liquids (ILs) with properties tailored to this CO2 separation problem. Atomistic simulations predict that suitably substituted aprotic heterocyclic anions, or “AHAs,” bind CO2 with energies that can be controlled over a wide range suitable to gas separations. Further, unlike all previously known CO2-binding ILs, the AHA IL viscosity is predicted to be insensitive to CO2. Spectroscopic, temperature-dependent absorption, rheological, and calorimetric measurements on trihexyl(tetradecyl)-phosphonium 2-cyanopyrrolide ([P66614][2-CNpyr]) show CO2 uptakes close to prediction as well as insignificant changes in viscosity in the presence of CO2. A pyrazolide-based AHA IL behaves qualitatively similarly but with weaker binding energy. The results demonstrate the intrinsic design advantages of ILs as a platform for CO2 separations.

We report results from the first molecular simulation study of 1-n-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF 6 ], a widely studied ionic liquid. Monte Carlo simulations are carried out in the isothermal-isobaric (NPT) ensemble to calculate the molar volume, cohesive energy density, isothermal compressibility, cubic expansion coefficient and liquid structure as a function of temperature and pressure. A united atom forcefield is developed using a combination of ab initio calculations and literature parameter values. Calculated molar volumes are within 5% of experimental values, and a reasonable agreement is obtained between calculated and experimental values of the isothermal compressibility and cubic expansion coefficient. PF 6 anions are found to preferentially cluster in two favorable regions near the cation.

The Henry's constants of water, carbon dioxide, ethane, ethene, methane, oxygen, and nitrogen are computed in the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]) using test particle insertion and expanded ensemble Monte Carlo methods. The partial molar enthalpy and partial molar entropy of solvation are also computed for water, carbon dioxide, and oxygen. The results from the simulations are compared against experimental data from the literature. In addition, the accuracy and precision of the two methods in determining the Henry's constant are examined. Local organization of the ionic liquid around a solute molecule is analyzed, and the interactions responsible for the experimentally observed solubility trends are identified.

Current bottlenecks in the large-scale commercial use of many ionic liquids (ILs) include their high costs, low biodegradability, and often unknown toxicities. As a proactive effort to better understand the molecular mechanisms of ionic liquid toxicities, the work herein presents a comprehensive molecular simulation study on the interactions of 1-n-alkyl-3-methylimidazolium-based ILs with a phosphatidylcholine (PC) lipid bilayer. We explore the effects of increasing alkyl chain length (n = 4, 8, and 12) in the cation and anion hydrophobicity on the interactions with the lipid bilayer. Bulk atomistic molecular dynamics (MD) simulations performed at millimolar (mM) IL concentrations show spontaneous insertion of cations into the lipid bilayer regardless of the alkyl chain length and a favorable orientational preference once a cation is inserted. Cations also exhibit the ability to "flip" inside the lipid bilayer (as is common for amphiphiles) if partially inserted with an unfavorable orientation. Moreover, structural analysis of the lipid bilayer show that cationic insertion induces roughening of the bilayer surface, which may be a precursor to bilayer disruption. To overcome the limitation in the timescale of our simulations, free energies for a single IL cation and anion insertion have been determined based on potential of mean force calculations. These results show a decrease in free energy in response to both short and long alkyl chain IL cation insertion, and likewise for a single hydrophobic anion insertion, but an increase in free energy for the insertion of a hydrophilic chloride anion. Both bulk MD simulations and free energy calculations suggest that toxicity mechanisms toward biological systems are likely caused by ILs behaving as ionic surfactants. [Yoo et al., Soft Matter, 2014].