The solubility of carbon dioxide in ionic liquids of type 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cnmim][NTf2]) with varying chain length n=2, 4, 6, 8 is computed from molecular dynamics simulations. By applying both Bennett's overlapping distribution method and Widom's particle insertion technique, we determine solvation free energies that are in excellent agreement with available experimental solubility data over a large temperature range from 300 to 500 K. We find that the computed solvation free energy of carbon dioxide is remarkably insensitive to the alkane chain length, emphasizing the importance of solvent models with accurate volumetric properties. The simulations suggest that the "anomalous" temperature dependence of the CO2 solvation at infinite dilution is characterized by counter-compensating negative entropies and enthalpies of solvation. By systematically varying the interaction strength of CO2 with the solvent, we show that the negative solvation entropy of CO2 is not caused by solvation cavities, but enforced by Coulomb and van der Waals interactions. We observe that solvation free energies and enthalpies obtained for models with different solute-solvent interaction strengths are subject to a linear correlation, similar to an expression that has been suggested for gases in polymers. Despite the apparent chain length insensitivity of the solvation free energy, significant changes in the solvation shell of a CO2 molecule are observed. The chain length insensitivity is found to be a consequence of two counter-compensating effects: the increasing free energy of cavity formation is balanced by a favorable interaction of CO2 with the alkyl chain of the imidazolium cation.

Three imidazolium-based ionic liquids of the type [C n mim][NTf 2 ] with different alkyl chain lengths (n = 1, 2 and 8) at the first position of the imidazolium ring were studied applying infrared, linear Raman and multiplex coherent anti-Stokes Raman scattering spectroscopy. The focus has been on the CH-stretching region of the imidazolium ring, which is supposed to carry information about a possible hydrogen bonding network in the ionic liquid. The measurements are compared with calculations of the corresponding anharmonic vibrational spectra for a cluster of [C 2 mim][NTf 2 ] consisting of four ion pairs. The results support the hypothesis of weak hydrogen bonding involving the C(4)-H and C(5)-H groups and somewhat stronger hydrogen bonds of the C(2)-H groups.

We have determined the temperature dependence of the solvation behavior of a large collection of important light gases in imidazolium-based ionic liquids with the help of extensive molecular dynamics simulations. The motivation of our study is to unravel common features of the temperature dependent solvation under well controlled conditions, and to provide a guidance for cases, where experimental data from different sources disagree significantly. The solubility of molecular hydrogen, oxygen, nitrogen, methane, krypton, argon, neon and carbon dioxide in the imidazolium based ionic liquids of type 1-n-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cmim][NTf]) with varying alkyl side chain lengths n = 2, 4, 6, 8 is computed for a temperature range between 300 K and 500 K at 1 bar. By applying Widom's particle insertion technique and Bennet's overlapping distribution method, we are able to determine the temperature dependent solvation free energies of those selected light gases in simulated imidazolium based ionic liquids with high statistical accuracy. Our simulations demonstrate that the magnitude of the solvation free energy of a gas molecule at a chosen reference temperature and that of its temperature-derivatives are intimately related to one another. We conclude that this "universal" behavior is rooted in a solvation entropy-enthalpy compensation effect, which seems to be a defining feature of the solvation of small molecules in ionic liquids. The observations lead to simple analytical relations, determining the temperature dependence of the solubility data based on the absolute solubility at a certain reference temperature. By comparing our results with available experimental data from many sources, we can show that our approach is particularly helpful for providing reliable estimates for the solvation behavior of very light gases, such as hydrogen, where conflicting experimental data exist.

We explore the influence of small amounts of water dissolved in 1-alkyl-3-methyl-imidazolium based ionic liquids (ILs) on the solubility of CO2 as a function of water content and temperature. We observe a decreasing solubility of CO2 as function of increasing water content in almost quantitative agreement with recent experimental data by Husson et al. [Fluid Phase Eq. 294, 98 (2010)]. In addition, a purely repulsive variant of CO2 is employed as a test-case to elucidate the solvation behavior of small sized molecules in absence of favorable interactions. We find that the decreasing solubility is about one third due to the increasing solvent number density, and about two thirds due to the change in solvation free energy. In addition, we observe that the computed solvation free energy of a purely repulsive test-molecule increases even more strongly with increasing water content, indicating that small favorable “interaction sites” within the liquid, well suited for accommodating a CO2 molecule, become increasingly rare due to the competition with the water molecules.