Evidence for phase separation during the crystallization of hyperquenched glassy clathrate hydrate forming solutions

Tetrahydrofuran, a water-soluble cyclic ether molecule, is well known for i ts ability to form clathrate hydrates (cubic structure II, melting point 27 7.5 K). An aqueous solution of THF (17:1 molecular ratio) was hyperquenched (cooled at a rate of 10(6) Ks(-1)) to 77 K to give recoverable samples of similar to 1 g of metastable amorphous material. The crystallization proces s was studied on three complementary length scales by x-ray powder diffract ion, quadrupole echo double resonance (QEDOR) NMR spectroscopy, and Raman s pectroscopy as the amorphous material was annealed at temperatures between 80 and 190 K. Results show that phase separation of the two components occu rred predominantly during the annealing process, resulting in clusters of c rystalline THF and ice Ic before clathrate hydrate crystallization was init iated. During the hyperquenching process, the decreasing molecular mobility of water molecules between room temperature and the fictive temperature (t he temperature below which the solution becomes structurally immobilized up on hyperquenching) inhibits growth of the crystal nuclei that normally woul d result in crystalline clathrate hydrate formation. Annealing the samples between 110-140 K showed that the local hydrogen bonded O-H--O structure of the water molecules changed toward the arrangement characteristic of cryst alline ice Ic. We propose that this process forces the THF out of the glass y solution, thus increasing the THF cluster size in the deposit so that the se become visible to diffraction. Further annealing, above 150-160 K caused large-scale growth of crystalline clathrate hydrate material, which then c ould be associated with a reduction in the THF cluster size, and ice Ic. (C ) 1999 American Institute of Physics. [S0021-9606(99)51113-6].