A hybrid type of theory for calculating hydrate equilibrium was recently proposed. The theory has been extended to cover the region of equilibrium between ice and hydrate as well as liquid water and hydrate. In this theory the cell partition-function is evaluated by different procedures depending on the size of the molecules relative to the size of the free volume in the cavity. Movements for molecules that are large compared to the free volume in the cavity are represented by a set of harmonic oscillators, which makes it possible to account for lattice distortions as well as interactions between guest and surrounding waterand guest-molecules. Small molecules will move relatively freely in the cavities, in accordance with the original theory of van der Waals and Platteeuw, and may be more accurately represented by the single cavity integral that are commonly used. In this work we present some results for the changes of chemical potential due to guest inclusion using either the harmonic oscillator approach or the original theory. Simple spherical Lennard-. Jones models have been used for the guest molecules. Gas phase chemical potentials are calculated from the same sets of parameters using the virial-equation. Water is represented by the TIP4P potential. We have correlated these results into a convenient interpolation-equation.


Recently Tanaka and coworkers (1993a, 1993- b, 1993c) examined the thermodynamic stability of several hydrates of structure I and II. The basic technique was based on the observation that for the guests in consideration the potential energy between the guest and the watermolecules constituting the cavity had only one minimum. The free energy of the inclusion of the guest could thus be represented as a sum of this energy-minimum and the free energy of a set of harmonic oscillators representing displacements from this minimum.

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