Temperature Dependence of the Solubility of Carbon Dioxide in Imidazolium-Based Ionic Liquids

Institut for Chemie, Abteilung Physikalische Chemie, Universität Rostock, Dr.-Lorenz-Weg 1, D-18051 Rostock, Germany.
The Journal of Physical Chemistry B (Impact Factor: 3.3). 10/2009; 113(38):12727-35. DOI: 10.1021/jp9055285
Source: PubMed


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.

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    • "The equation of state (EOS) (Andreu and Vega, 2007, 2008; Karakatsani et al., 2008; Kroon et al., 2006; Kumelan et al., 2008) and_ENREF_8 activity coefficient models (Kato and Gmehling, 2005; Nebig et al., 2007; Wang et al., 2008) are capable of accurately describing the solubility of CO 2 in ILs with analytical expressions, provided that the model parameters are fitted to some (or all) the available experimental data. Molecular simulations (Bhargava and Balasubramanian, 2007; Kerle et al., 2009; Shah and Maginn, 2005; Shi and Maginn, 2008) determine the solubility based on fundamental molecular interactions; however, they are often quite computational demanding. Quantitative Structure Property Relationship (QSPR) models (Eike et al., 2003, 2004; Katritzky et al., 2001) identifies the dominating molecular properties (referred to as descriptors ) in order to construct semi-theoretical relationships between solubility and molecular properties. "
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