Position and orientation of water protons need to be specified when the molecular simulation studies are performed for clathrate hydrates. Positions of oxygen atoms in water are experimentally determined by X-ray diffraction analysis of clathrate hydrate structures, but positions of water hydrogen atoms in the lattice are disordered. This study reports a determination of the water proton coordinates in unit cell of structure I (sI), II (sII), and H (sH) clathrate hydrates that satisfy the ice rules, have the lowest potential energy configuration for the protons, and give a net zero dipole moment. Possible proton coordinates in the unit cell were chosen by analyzing the symmetry of protons on the hexagonal or pentagonal faces in the hydrate cages and generating all possible proton distributions which satisfy the ice rules. We found that in the sI and sII unit cells, proton distributions with small net dipole moments have fairly narrow potential energy spreads of about 1 kJ∕mol. The total Coulomb potential on a test unit charge placed in the cage center for the minimum energy∕minimum dipole unit cell configurations was calculated. In the sI small cages, the Coulomb potential energy spread in each class of cage is less than 0.1 kJ∕mol, while the potential energy spread increases to values up to 6 kJ∕mol in sH and 15 kJ∕mol in the sII cages. The guest environments inside the cages can therefore be substantially different in the sII case. Cartesian coordinates for oxygen and hydrogen atoms in the sI, sII, and sH unit cells are reported for reference.

This paper reports confirmation of structure II hydrate formation in a methane−2-propanol−water system, which was previously suggested by Østergaard et al. (Ind. Eng. Chem. Res. 2002, 41, 2064−2068) based on a comparison of the phase-equilibrium data with corresponding statistical-thermodynamics predictions. A hydrate crystal sample was prepared with a 16.4 mass % aqueous solution of 2-propanol pressurized with methane and then subjected to a powder X-ray diffraction analysis. The X-ray diffraction pattern thus obtained from the sample indicated that the crystallographic structure of the hydrate was structure II. The pressure−temperature data for aqueous liquid−hydrate−methane-rich vapor three-phase equilibrium in a temperature range from T = 273 to 283 K are also reported.

This paper reports the visual observations of the formation and growth of clathrate hydrate crystals on the surface of a water droplet exposed to gaseous methane, ethane, or propane. The hydrate crystals formed and grew at the guest-water interface. The nucleation of the hydrate first occurred at a random point on the water droplet and then grew to form a polycrystalline layer covering the surface. We observed the individual crystals that constitute the polycrystalline hydrate layer and classified the morphology of the hydrate crystals depending on the system subcooling ∆T sub , the difference between the system temperature and the guest-hydrate-water three-phase equilibrium temperature corresponding to the system pressure. As a general trend, at ∆T sub g 3.0 K, the shape of hydrate crystals is typically swordlike or triangular, and the shape changes to a polygon at ∆T sub from 2.0 to 3.0 K, and then to larger-sized polygons with one side of the polygon typically 0.5-1.0 mm in length. It may be concluded that the crystal morphology of the hydrate crystals formed at the interface between the liquid water and methane, ethane or propane gas can be classified using ∆T sub as the common criterion. The lateral growth rate of the hydrate crystal were also measured and presented.

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This paper reports on a visual study of formation and growth of clathrate hydrate crystals in liquid water saturated (prior to hydrate formation) and in contact with methane gas under the pressure of 6−10 MPa at a temperature of 273.5 K. Irrespective of the pressure set in the experimental system, in most of the experimental runs we observed that a hydrate film first formed to intervene between methane gas and liquid water, and then hydrate crystals grew in liquid water from the hydrate film. Distinct variations in the morphology of hydrate crystals grown in liquid water were observed depending on the pressure. At pressures of 6−8 MPa, hydrate crystals with skeletal, columnar morphology were observed. At the pressure of 10 MPa, the skeletal, columnar crystals were replaced by dendritic crystals. The dependency of the morphology on the degree of driving force for mass-transfer-controlled hydrate-crystal growth is discussed, comparing the present observations with those reported in the literature. Another category of hydrate formation and growth was observed in some experimental runs. The hydrate crystals first formed at the inner surface of the test cell in contact with liquid water instead of the methane−water interface. These crystals floated up to the methane−water interface, where they became a polycrystalline hydrate film, and continued to grow in liquid water.