Thermodynamic Stobility and Melting Mechanism of Clathrate Hydrate

通过考察笼子占据自由能的各种分量，研究了笼状水合物包裹非球形分子的热力学稳定性。在本研究中，我们发展了一种方法来处理非球形客体分子的笼状水合物，方法是扩展了广义的van der Waals和Platteeuw理论[Tanaka和Kiyohara，J.Chem.Phys。98,4086和8110(1993)]。考察了非球形丙烷和乙烷分子旋转运动的势能面。与客体分子的非球形性质相关的自由能被评估为来自包围球形客体的参考系的微扰。客体分子和主体分子的振动自由能都分为简谐和非简谐贡献。对受阻转动自由能中必不可少的后一项进行了估算，热力学性质与实测值符合得很好。通过计算重定向关联函数和速度关联函数，研究了其动力学行为。用超正则蒙特卡罗模拟计算了占有率，并与直接计算的结果进行了比较，符合较好。

The thermodynamic stability of clathrate hydrates encaging nonspherical molecules has been investigated by examining the various components of the free energy of cage occupancy. In the present study, we develop a method to treat clathrate hydrates of nonspherical guest molecules by extending a generalized van der Waals and Platteeuw theory [Tanaka and Kiyohara, J.Chem.Phys. 98,4086 and 8110 (1993) ] . The potential energy surfaces for the rotational motions of nonspherical propane and ethane molecules are examined. The free energy associated with the nonspherical nature of the guest molecules is evaluated as a perturbation from the reference system encaging spherical guests. The vibrational free energy of both guest and host molecules is divided into harmonic and anharmonic contributions. The latter term that is essential in the free energy of the hindered rotation for the guests is evaluated and thermodynamic properties are found to be in good agreement with the measured values. The dynamical behaviors are investigated by calculating the reorientational and velocity correlation functions. The occupation rates are calculated by grandcanonical Monte Carlo simulations and are in reasonable agreement with those from the direct calculation.