Clathrate Hydrates Coexisting Thiazole: Two Roles of Structure II Hydrate Former and Structure I Thermodynamic Inhibitor

In this study, we report a novel hydrate former that acts as a structure II (sII) former and thermodynamic inhibitor. Thiazole is an aromatic five-ringed molecule with two heteroatoms. Thiazole forms an sII hydrate crystal with small CH4 and CO2 molecules. The Rietveld refinement result of sII (CH4 + thiazole) hydrate positions the thiazole off the center of 51264 cages, and S and N atoms face O atoms at the apex of two pentagonal faces and one hexagonal face of the 51264 cage, respectively. Considering the nearest N-O/S-O distances between N/S atoms of thiazole and O atoms in the 51264 cage, both N and S atoms of thiazole exhibit guest-host interactions with water frameworks, such as O-H center dot center dot center dot N/O-H center dot center dot center dot S hydrogen bonds. Meanwhile, the thiazole molecule cannot form an sII hydrate with the C2H6 molecule. It functions as a thermodynamic inhibitor of the structure I (sI) C2H6 hydrate. The guest-host interaction of thiazole would not be so strong as that of tetrahydrofuran, which can form an sII hydrate without small molecules. Therefore, because the 51264 cage is difficult to form by the combination of thiazole and C2H6 molecules, the C2H6-thiazole-water system would form the sI C2H6 hydrate.