Capturing Hydrate Formation Processes in Tetrahydrofuran/Water Mixtures with Temperature Resolved In Situ Synchrotron X-ray Diffraction and Infrared Spectroscopy.

Understanding the mechanisms behind clathrate hydrate formation is of both practical and fundamental interest. Here, we report the formation of a clathrate hydrate from a tetrahydrofuran (THF)-water mixture of "ideal" stoichiometry, deposited cryogenically in ultrahigh vacuum and studied with in situ synchrotron X-ray diffraction and infrared spectroscopy on slightly deuterated water samples. The experiments provide complementary information on the evolution of the system's local and long-range orders. They revealed distinctive structural transformations in different temperature regimes. The as-deposited sample showed a disordered diffraction pattern. Upon heating, THF segregated and crystallized, while solid water remained amorphous. Crystalline THF persisted until 110 K, then it melted and interacted with water to form type II clathrate without ice contamination. No new crystalline or amorphous phases were observed prior to the onset crystallization of the hydrate. This work provides atomic-level insight into molecular rearrangements during clathrate hydrate formation, highlighting molecular mobility even at low temperatures.