Calculation of Free Energies and Chemical Potentials for Gas Hydrates Using Monte Carlo Simulations

• Published in: The journal of physical chemistry. B Vol. 111; no. 25; pp. 7274 - 7282
• Main Authors: Wierzchowski, S. J, Monson, P. A
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 28-06-2007
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp068325a

We describe a method for calculating free energies and chemical potentials for molecular models of gas hydrate systems using Monte Carlo simulations. The method has two components:  (i) thermodynamic integration to obtain the water and guest molecule chemical potentials as functions of the hydrate occupancy; (ii) calculation of the free energy of the zero-occupancy hydrate system using thermodynamic integration from an Einstein crystal reference state. The approach is applicable to any classical molecular model of a hydrate. We illustrate the methodology with an application to the structure-I methane hydrate using two molecular models. Results from the method are also used to assess approximations in the van der Waals−Platteeuw theory and some of its extensions. It is shown that the success of the van der Waals−Platteeuw theory is in part due to a cancellation of the error arising from the assumption of a fixed configuration of water molecules in the hydrate framework with that arising from the neglect of methane−methane interactions.