Kerlé Daniela, Ludwig Ralf, Geiger Alfons, Paschek Dietmar
Institut for Chemie, Abteilung Physikalische Chemie, Universität Rostock, Dr.-Lorenz-Weg 1, D-18051 Rostock, Germany.
J Phys Chem B. 2009 Sep 24;113(38):12727-35. doi: 10.1021/jp9055285.
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.
通过分子动力学模拟计算了二氧化碳在链长n = 2、4、6、8的1-烷基-3-甲基咪唑双（三氟甲基磺酰）亚胺（[Cnmim][NTf2]）型离子液体中的溶解度。通过应用贝内特重叠分布方法和维登粒子插入技术，我们确定了溶剂化自由能，该自由能在300至500K的大温度范围内与现有的实验溶解度数据高度吻合。我们发现，计算得到的二氧化碳溶剂化自由能对烷烃链长非常不敏感，这突出了具有精确体积性质的溶剂模型的重要性。模拟结果表明，无限稀释下二氧化碳溶剂化的“反常”温度依赖性表现为相互抵消的负溶剂化熵和焓。通过系统地改变二氧化碳与溶剂的相互作用强度，我们表明二氧化碳的负溶剂化熵不是由溶剂化空穴引起的，而是由库仑相互作用和范德华相互作用导致的。我们观察到，对于具有不同溶质 - 溶剂相互作用强度的模型所获得的溶剂化自由能和焓呈现线性相关性，类似于针对聚合物中气体所提出的表达式。尽管溶剂化自由能对链长明显不敏感，但观察到二氧化碳分子的溶剂化壳层有显著变化。发现链长不敏感性是两种相互抵消效应的结果：空穴形成自由能的增加被二氧化碳与咪唑阳离子烷基链的有利相互作用所平衡。
