Gas Hydrates of Argon and Methane Synthesized at High Pressures:  Composition, Thermal Expansion, and Self-Preservation

Abstract

For the first time, the compositions of argon and methane high-pressure gas hydrates have been directly determined. The studied samples of the gas hydrates were prepared under high-pressure conditions and quenched at 77 K. The composition of the argon hydrate (structure H, stable at 460-770 MPa) was found to be Ar.(3.27 +/- 0.17)H(2)O. This result shows a good agreement with the refinement of the argon hydrate structure using neutron powder diffraction data and helps to rationalize the evolution of hydrate structures in the Ar-H(2)O system at high pressures. The quenched argon hydrate was found to dissociate in two steps. The first step (170-190 K) corresponds to a partial dissociation of the hydrate and the self-preservation of a residual part of the hydrate with an ice cover. Presumably, significant amounts of ice Ic form at this stage. The second step (210-230 K) corresponds to the dissociation of the residual part of the hydrate. The composition of the methane hydrate (cubic structure I, stable up to 620 MPa) was found to be CH(4).5.76H(2)O. Temperature dependence of the unit cell parameters for both hydrates has been also studied. Calculated from these results, the thermal expansivities for the structure H argon hydrate are alpha(a) = 76.6 K(-1) and alpha(c) = 77.4 K(-1) (in the 100-250 K temperature range) and for the cubic structure I methane hydrate are alpha(a) = 32.2 K(-1), alpha(a) = 53.0 K(-1), and alpha(a) = 73.5 K(-1) at 100, 150, and 200 K, respectively.

Monte Carlo study of sII and sH argon hydrates with multiple occupancy of cages

“…This qualitative finding agrees with the results from Molecular Dynamics simulations of Alavi et al [22]. Our GCMC results regarding the number of the enclathrated argon molecules in each type of cavity are in very good agreement with the available experimental data [18,21]. In specific, we estimate that the composition of the sH hydrate, at 760 MPa, is Ar·3.4H 2 O (all large cavities occupied by five Ar molecules) which is exactly the same with the measurements of Manakov et al [21] while Inerbaev et al reported a slightly higher Ar content in the hydrate (Ar·3.27H 2 O).…”

“…Grand Canonical Monte Carlo simulations were performed for sII and sH hydrates of argon that were identified to exhibit multiple occupancies of the large cages of the hydrate crystal [18][19][20][21][22][23]. All runs were carried out with a lattice parameter of 17.1423 Å [41].…”

“…Below 460 MPa, Ar forms a stable sII hydrate. For the pressure range between 460 and 770 MPa a transformation occurs resulting in a type sH hydrate, while above 770 MPa a tetragonal structure forms [18]. Note, however, that the pressure boundaries were the "sII-to-sH" structural transformations occur, are still under discussion and different values have been reported [19].…”

“…Manakov et al [21] studied neutron diffraction patterns of argon hydrates at pressures up to 1000 MPa and reported the filling of the large cavities of sH structure by five argon atoms. Ogienko et al [18] reported X-ray diffraction patterns of the sH structure argon hydrate at different temperatures. Their results confirmed the filling of the large cavities of the sH hydrate by five argon atoms.…”

Anomalous Preservation of CH4 Hydrate and its Dependence on the Morphology of Hexagonal Ice

“…Additionally, it was suggested that the nature of the ice crystals and the interaction between guest molecules and H 2 O molecules [6] both affect anomalous preservation phenomena for gas hydrates. We think that the gas hydrates which show self-preservation phenomena by the temperature-ramping method in earlier studies, [6,23] such as N 2 ,O 2 , Ar, and H 2 hydrate, will also show anomalous preservation by the rapid pressure-release method. Preservation phenomena for hydrates of these binary gas systems are also expected.…”