The thermodynamic stability of clathrate hydrate. III. Accommodation of nonspherical propane and ethane molecules

Abstract

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.