Theoretical study of phase transitions in Kr and Ar clathrate hydrates from structure II to structure I under pressure

Abstract

The theory developed in our earlier papers is extended to predict dynamical and thermodynamic properties of clathrate structures by accounting for the possibility of multiple filling of cavities by guest molecules. The method is applied to the thermodynamic properties of argon and krypton hydrates, considering both structures I (sI) and II (sII), in which the small cages can be singly occupied and large cages of sII can be singly or doubly occupied. It was confirmed that the structure of the clathrate hydrate is determined by two main factors: intermolecular interaction between guest and host molecules and the configurational entropy. It is shown that for guests weakly interacting with water molecules, such as argon or krypton, the free energy of host lattices without the contribution of entropy is the main structure-determining factor for clathrate hydrates, and it is a cause of hydrate sII formation at low pressure with these guests. Explicit account of the entropy contribution in the Gibbs free energy allows one to determine the stability of hydrate phases and to estimate the line of structural transition from sII to sI in P-T plane. The structural transition between sII and sI in argon and krypton hydrates at high pressure is shown to be the consequence of increasing intermolecular interaction and the degree of occupancy of the large cavities.

High Pressure Structural Transitions in Kr Clathrate-Like Clusters

“…3 The dissociation curve of the Kr-water clathrate systems has been measured by Dyadin et al 16,46 up to 1.5 GPa, and they have observed transitions from sII to sI and to sH structures. Such transitions have been also found from X-ray diffraction studies 14 15 Further, in a recent theoretical study by Subbotin et al 21 where the corresponding transitions occur.…”

Theoretical study of hydrogen storage in binary hydrogen-methane clathrate hydrates

“…This modification of the SPC/E water-water interaction potential significantly improves the agreement of the unit cell volume calculated in our model for ice I h with the experimental values as described in our previous works. 27,28 The long-range electrostatic interaction has been considered as the interaction between charges on both the hydrogen and oxygen atoms of different water molecules and has been computed by the Ewald method. 35 The charges on hydrogen (q H ¼ þ0.4238jej) and oxygen (q O ¼ À0.8476jej) within the SPCE model were not changed.…”

“…At low methane concentration, the second factor begins to play a more important role because in this case the chemical potential of the guests in the gas phase is significantly increased according to Eq. (28). Thus, the application of methane as a second guest may be preferable especially for stabilization of the CS-II structure due to weaker guest-host interactions in comparison with the commonly used THF molecules.…”

Transformation of hydrogen bond network during CO2 clathrate hydrate dissociation

“…It was chosen because it qualitatively describes the kinetics of dissociation [12,23,24]. The modified version was used according to good description of thermodynamics for these phases in our previous studies [43,44]. The short-range interaction was described by Lennard-Jones potential with parameter σ = 3.1556 Å and the energy parameter ε = 0.65063 kJ mol −1 .…”