Structures

Structure sI

  • Usually smaller molecules form sI hydrates. CH4, C2H6, CO2, H2S.

  • Made up of 8 polyhedral cages-6 large ones and 2 small ones.

  • 46 water molecules.

  • Body-centered cubic

  • 2 small cavities (512) and 6 large cavities (51262)

Structure sII

  • Larger molecules: C3H8, and iso-C4H10. Also N2, although relatively small.

  • Made up of 24 polyhedral cages--8 large ones and 16 small.

  • They are made up of 136 water molecules.

  • All face-centered cubic.

  • 8 small cavities (512) and 8 large cavities (51264)

Structure sH

  • Formed by larger molecules but only in the presence of a smaller one, such as methane.

  • Made up of 6 polyhedral cages - 1 large, 3 medium, and 2 small.

  • 34 water molecules.

  • Hexagonal crystal system.

  • 3 small cavities (512), 2 medium cavities (435663) and 1 large cavity (51266)

There are many differences between the structures; however, the essential differences are water/gas ratio, relative number and sizes of cages, and type of guest molecules. Pure hydrates are characterized thermodynamically by the phase boundary, free energy of formation and structure. Compound hydrates form in a similar manner to pure hydrates with four important distinctions:

  • Preferred hydrate forming materials can be completely consumed

  • Hydrate forming guests can be sequestered in compound hydrates outside of the stability field of the pure hydrate of one of the guests

  • Formation rates are faster and induction times shorter for compound hydrates

  • Composition of the reactant hydrate forming material may change with time as the proportion of the reactants changes.

In general, one gas of the mixture will be preferentially absorbed from the surrounding environment during hydrate formation. In many cases the absorption may be complete. This appears to be common in mixed hydrate systems where the higher temperature/lower pressure hydrate forming gas (the preferential hydrate former) is taken into the hydrate structure until it has been diminished to near zero concentration in the contributor phase.