Pressure-induced collapse of ice clathrate and hexagonal ice mixtures formed by freezing

We report thermal conductivity kappa measurements of the pressure-induced collapse of two mixtures of ice and tetrahydrofuran (THF) clathrate hydrate formed by freezing aqueous solutions, THF23 H(2)O and THF20 H(2)O, one containing twice as much excess water than the other. On pressurizing, kappa of the solid mixture first decreases at the onset pressure of approximately 0.8 GPa, as occurs for collapse of pure ice, reaches a local minimum at a pressure of approximately 1.0 GPa, and then increases as occurs for the collapse of the pure clathrate THF17 H(2)O. This shows that in the apparently homogeneous mixture, the ice and the clathrate collapse as if the two were in a mechanically mixed state. The manner in which the clathrate aggregate can arrange in the solid indicates that ice occupies the interstitial space in the tightly packed aggregates and H(2)O molecules belonging to the lattice of one form hydrogen bond with that of the other, a feature that is preserved in their collapsed states. On decompression, the original clathrate is partially recovered in the THF20 H(2)O mixture, but the collapsed ice does not transform to the low density amorph. We surmise that on irreversible transformation to the original clathrate, the aggregates expand. Any pressure thus exerted on the small domains of the collapsed ice with a hydrogen bonded interface with the clathrate aggregates could prevent it from transforming to the low density amorph. Measurements of kappa are useful in investigating structural collapse of crystals when dilatometry is unable to do so, as kappa seems to be more sensitive to pressure-induced changes than the volume.