Thermodynamic Stobility and Melting Mechanism of Clathrate Hydrate

本文研究了笼形水合物包覆非球形分子的热力学稳定性。在本研究中，我们发展了一种通过扩展广义van der Waals和Platteeuw理论来处理非球形客体分子的笼形水合物的方法[Tanaka和Kiyohara， j . chemistry . phys]。[98,4086和8110(1993)]。研究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有关的自由能被评价为来自包裹球形客体的参考系统的扰动。客分子和主分子的振动自由能分为谐波和非谐波贡献。后一项在来宾的阻碍旋转的自由能中是必不可少的，被评估，并且发现热力学性质与测量值很好地一致。通过计算重定向和速度相关函数来研究其动力学行为。利用蒙特卡罗模拟计算了占用率，与直接计算结果基本吻合。

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.