Department of Chemical Engineering, Imperial College London, South Kensington Campus, London, United Kingdom.
J Phys Chem B. 2011 Nov 10;115(44):12822-34. doi: 10.1021/jp203236q. Epub 2011 Oct 17.
Canonical ensemble molecular dynamics (MD) simulations are reported which compute both the vapor-liquid equilibrium properties (vapor pressure and liquid and vapor densities) and the interfacial properties (density profiles, interfacial tensions, entropy and enthalpy of surface formation) of four long-chained n-alkanes: n-decane (n-C(10)), n-eicosane (n-C(20)), n-hexacontane (n-C(60)), and n-decacontane (n-C(100)). Three of the most commonly employed united-atom (UA) force fields for alkanes (SKS: Smit, B.; Karaborni, S.; Siepmann, J. I. J. Chem. Phys. 1995,102, 2126-2140; J. Chem. Phys. 1998,109, 352; NERD: Nath, S. K.; Escobedo, F. A.; de Pablo, J. J. J. Chem. Phys. 1998, 108, 9905-9911; and TraPPE: Martin M. G.; Siepmann, J. I. J. Phys. Chem. B1998, 102, 2569-2577.) are critically appraised. The computed results have been compared to the available experimental data and those fitted using the square gradient theory (SGT). In the latter approach, the Lennard-Jones chain equation of state (EoS), appropriately parametrized for long hydrocarbons, is used to model the homogeneous bulk phase Helmholtz energy. The MD results for phase equilibria of n-decane and n-eicosane exhibit sensible agreement both to the experimental data and EoS correlation for all potentials tested, with the TraPPE potential showing the lowest deviations. However, as the molecular chain increases to n-hexacontane and n-decacontane, the reliability of the UA potentials decreases, showing notorious subpredictions of both saturated liquid density and vapor pressure. Based on the recommended data and EoS results for the heaviest hydrocarbons, it is possible to attest, that in this extreme, the TraPPE potential shows the lowest liquid density deviations. The low absolute values of the vapor pressure preclude the discrimination among the three UA potentials studied. On the other hand, interfacial properties are very sensitive to the type of UA potential thus allowing a differentiation of the potentials. Comparing the interfacial tension MD results to the available experimental data and SGT results, the TraPPE model exhibits the lowest deviations for all hydrocarbons.
报道了一种正则系综分子动力学（MD）模拟方法，用于计算四种长链正构烷烃（n-癸烷（n-C(10)）、n-二十烷（n-C(20)）、n-二十六烷（n-C(60)）和 n-三十烷（n-C(100)）的汽液平衡性质（蒸气压和液体与蒸汽密度）和界面性质（密度分布、界面张力、表面形成的熵和焓）。对于烷烃，通常使用三种最常用的统一原子（UA）力场（SKS：Smit，B.；Karaborni，S.；Siepmann，J. I. J. Chem. Phys. 1995,102, 2126-2140；J. Chem. Phys. 1998,109, 352；NERD：Nath，S. K.；Escobedo，F. A.；de Pablo，J. J. J. Chem. Phys. 1998, 108, 9905-9911；和 TraPPE：Martin M. G.；Siepmann，J. I. J. Phys. Chem. B1998, 102, 2569-2577.）进行了严格评估。计算结果与实验数据和使用平方梯度理论（SGT）拟合的数据进行了比较。在后一种方法中，适用于长链烃的 Lennard-Jones 链状态方程（EoS）用于模拟均匀本体的亥姆霍兹自由能。n-癸烷和 n-二十烷的相平衡的 MD 结果与所有测试的势的实验数据和 EoS 相关性都具有很好的一致性，TraPPE 势的偏差最小。然而，随着分子链增加到 n-二十六烷和 n-三十烷，UA 势的可靠性降低，饱和液体密度和蒸气压的预测值明显偏低。基于最重烃类的推荐数据和 EoS 结果，可以证明，在这种极端情况下，TraPPE 势的液体密度偏差最小。蒸气压的绝对值很低，无法区分所研究的三种 UA 势。另一方面，界面性质对 UA 势的类型非常敏感，因此可以区分势。将界面张力 MD 结果与可用的实验数据和 SGT 结果进行比较，TraPPE 模型对所有烃类的偏差最小。
