Interactions between guest molecules and the water cages in clathrates are dominated by isotropic van der Waals forces at low temperatures because the cage structures satisfy the hydrogen bonding propensity of the water. However, above 200 K the water molecules become more labile and may interact strongly with the guests through hydrogen bonding. In this work we compare the dynamics of tetrahydrofuran (THF) and cyclopentane (CP) guests in the hydrate cages above 200 K. Since THF can form hydrogen bonds while CP cannot, the dynamics provide insight into host-guest hydrogen bonding. We use magic angle spinning (MAS) solid-state NMR to measure proton spin-lattice relaxation times (T1) of the guests as a function of temperature and find that the activation barrier to the motion of THF molecules is 4.7 kcal/mol (19.7 kJ/mol) at temperatures above 200 K. This is almost 5 times higher than the barrier at lower temperatures. In contrast, the barrier to guest motion in CP hydrate is found to be about 0.67 kcal/mol (2.8 kJ/mol), which agrees well with data at lower temperatures. These results demonstrate that hydrogen bonding interactions between the THF guest and the clathrate cage are significant above 200 K due to the host lattice mobility.

The role of methanol as both an inhibitor and a catalyst for the formation of clathrate hydrates (CHs) has been a topic of intense study. We report a new quantitative study of the kinetics of propane CH formation at 253 K from the reaction of propane gas with <75 μm ice particles that have been doped with varying amounts of methanol. We find that methanol significantly accelerates the formation reaction with quite small doping quantities. Even for only 1 methanol molecule per 10 000 water molecules, the maximum uptake rate of propane into CHs is enhanced and the initiation pressure is reduced. These results enable more efficient production of CHs for gas storage. This remarkable acceleration of the CH formation reaction by small quantities of methanol may place constraints on the mechanism of the inhibition effect observed under other conditions, usually employing much larger quantities of methanol.

Clay sediments are the main component of seabed sediment. Interactions between the nano-sized, disk-shaped and charged clay particles are complicated, as they control the sediment’s rheology. In this study, we studied the rheological behavior of the clay sediment modeled by laponite and bentonite suspensions experimentally using a rotational rheometer. The yield stress decreased when water content increased. Meanwhile, the yield stress of the laponite suspension first increased and then decreased with increasing salinity. It is considered that the face-to-face repulsive electrostatic interaction between clay platelets dominated the yield behavior. A yield stress model was developed to describe the change of the yield stress with both the water content and the salinity. When the system started to flow, the viscosity decreased with increasing shear. A master curve of viscosity is was from the viscosity-stress curves at different water contents if the applied shear stress was normalized by the yield stress and the viscosity normalized by a characteristic viscosity. This study provides a preliminary understanding of the clay sediment rheology and its mechanism for the investigation on the flowing of the sediment systems with strong interparticle interaction.

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