Critical size for guest molecules to occupy dodecahedral cage of clathrate hydrates

The five different structure-I hydrates were formed using the following five different guest substances: methane, fluoromethane, difluoromethane, trifluoromethane, and tetrafluoromethane, for the systematic investigation on the relation between the occupancy of the cages in the hydrates and the size of the guest molecules. The hydrate crystal samples were analyzed using Raman spectroscopy. The results revealed that the cage occupancy ratio between small and large cages of these guest molecules depended on their solvent accessible surface area (ASA). The critical ASA for the enclathration of small cages had a relatively narrow range: approximately 1.7 +/- 0.2 nm(2). It was found that clathrate hydrates formed with guest molecules having ASA in this critical range were expected to show high nonstoichiometric properties. The obtained relation between the cage occupancy ratio and ASA of guest molecules was generalized by comparison with the results on other structure-I hydrate-forming guest molecules reported in the literature.