guests will also act as inhibitors, as accounted for by a modified van der Waals–Platteeuw equation. Thus, it is important to recognize that hydrate instability may have two origins – one from strong liquid water–guest interactions accounted for by the altered activity, the other by insufficiently strong guest–hydrate cage interactions as accounted for by the magnitude of potential function parameters. Note that the first of these effects can be “turned off” by eliminating the liquid aqueous phase and producing hydrate from an ice–guest molecule reaction. Clathrate hydrates of formaldehyde and ammonia were made this way. On the other hand, so far it has not been possible to produce a binary methanol hydrate. Another interesting observation was the catalyst‐like behavior when small quantities of methanol or ammonia were added to the reaction of methane and ice. These molecules, while being excluded from bulk ice, function as catalysts by disrupting the ice surface by hydrogen bonding to surface water molecules. This greatly enhances the rate of clathrate hydrate formation. This is not a true “catalytic” effect since a small amount of the methanol or ammonia may be incorporated into the hydrate phase.
Halogen–water interactions have proven to give chlorine and bromine hydrates unusual properties. Although the chlorine van der Waals diameter is far too large to fit into the CS‐I hydrate small D cage, cage occupancies of ∼30% have been measured. Compositional analysis by Cady has shown that chlorine hydrate is more stable than expected from applications of the van der Waals–Platteeuw equation.
Bromine by forming a unique hydrate structure illustrates the strong structure‐directing effect of the halogen–water interaction. One feature that stands out is the bromine guest in the tetrakaidecahedral (T) cage of the structure where, considering the accepted van der Waals diameter of Br2, this guest is far too large to fit into the T cage. Another feature is the presence of the pentrakaidecahedral (P) cage in Br2 hydrate, rarely observed in other hydrate structures. The halogen–water interaction in bromine hydrate is reminiscent of the electrophilic electron acceptor Br2 in the bromine‐p‐dioxane complex reported by Hassel many years ago.
Guest–guest interactions in hydrates with a single guest per cage rarely manifest themselves directly. The best‐known example is the guest dipole ordering of trimethylene oxide in the large CS‐I cage. From diffraction, the guest molecules are sterically constrained so that the guest dipoles lie along the symmetry axis of the T cages, but in disordered directions. Below ∼105 K the dipole directions order, as evident from dielectric relaxation, NMR, and calorimetry.
Computational work has shown that longer range interactions exist between THF guests in the large CS‐II cage and small guests in neighboring small cages. The number of Bjerrum defects generated by THF–water hydrogen‐bonding appears to depend on the electron donating properties and size of the small molecule in binary THF+small gas CS‐II hydrates. Most likely this interaction between guests is mediated by the intervening cage wall.
In the early 2000s, the US Department of Energy (DOE) set a target of 5 mass % as a target for onboard hydrogen storage in vehicles. In 2002, Mao and coworkers determined that multiple cage occupancies of the CS‐II clathrate hydrate cages with H2 molecules were possible at low‐temperature and high‐pressure conditions. In 2003, a paper published in the Proceedings of the National Academy of Sciences (PNAS) at the NRC by Patchkovskii and Tse examined the stability of the type II hydrogen clathrate with respect to hydrogen occupancy with a statistical mechanical model in conjunction with first‐principles quantum calculations. These works suggested that the required mass % of H2 gas in the hydrate phase was possible to attain, but at pressures and temperatures which would not be easily accessible to vehicles. To lower the pressure required to incorporate substantial amounts of H2 gas into a clathrate hydrate phase, a joint Korean Advanced Institute of Science and Technology (KAIST) and NRC study was carried out and published in Nature in 2005. This study showed hydrogen storage capacity is enhanced by composition tuning with THF. When aqueous solutions of THF with less than stoichiometric amounts of THF relative to the pure CS‐II phase were exposed to pressures of H2 gas, it was found that the H2 guests enter both the large and the small cages under milder conditions than the pure CS‐II hydrate. These experimental studies were followed by a series of molecular dynamics simulations where the energies of different H2 guest occupancies in pure and binary clathrate hydrates were determined.
A further development in this area was that it was experimentally realized that once pressure is relieved from an H2 containing clathrate hydrate, the hydrogen content gradually decreases, even though the crystal structure of the hydrate phase remained intact. In 2007, a quantum chemical study at the NRC, the energy barriers and diffusion rates of H2 molecules diffusing through the hexagonal and pentagonal faces of the CS‐II cages were determined. The barriers to diffusion from the hexagonal faces can be overcome at lower temperatures, making the CS‐II phase somewhat porous to the diffusion of this gas.
A unique method was described for producing high‐occupancy hydrogen hydrate in 2012. Replacement of the nitrogen guest in CS‐II nitrogen clathrate by hydrogen at high pressures was found to result in hydrogen clathrate where small cages are doubly occupied.
Following earlier work on the observation that organisms in cold climates use AFPs for protection against destructive freezing by a non‐colligative mechanism, A. R. Edwards and coworkers suggested that such proteins may well have a similar action on solid hydrate formation. Because of the difficulty in obtaining significant quantities of AFPs, much effort was expended in searching for polymers that might mimic the antifreeze behavior of AFPs. This led to a burgeoning research area focused on discovering and testing of low‐dosage kinetic inhibitors (LDKIs) and resulted in some early successes such as polyvinylpyrrolidone (PVP) and polyvinylcaprolactam (PVCap) by E. D. Sloan. With the greater availability of AFPs and related materials from V. Walker's (2005) group, a collaborative effort with the NRC group and later with P. Englezos' group was initiated to characterize the AFP function as a LDKI. This was carried out on a scale from the size of droplets to that of a stirred reactor with multiple techniques, including gas uptake and release, calorimetry, PXRD, solid‐state NMR spectroscopy and micro‐imaging, quartz crystal microbalance, and Raman spectroscopy. Some findings include there is no correlation between AFP function on ice or hydrate; some AFPs are as effective as polymers for antifreeze function; elimination of the memory effect for strong AFPs; and the presence of multiple decomposition temperatures for hydrate made in the presence of AFPs in calorimetric measurements. As explained in the chapter on kinetics of hydrate formation (Chapter 14), it is expected that the supercooling of hydrate forming solutions with inhibitor is mirrored by a superheating effect of the solid hydrate coated with inhibitor. What these effects have in common is that the processes are limited by the local radius of curvature of the advancing or retreating solid–liquid interface.
Molecular dynamics simulations were performed at the NRC and University of British Columbia to determine the mechanism of action of the winter flounder AFP as an inhibitor of methane hydrate nucleation and growth. These studies showed that the properly oriented dangling methyl groups on the amino acids of the AFP are incorporated into the half‐formed hydrate cages on the hydrate surface, thus inhibiting local growth of the hydrate and inhibiting global growth of the hydrate through pinning to the hydrate surface and the action of the Kelvin effect.
In the early part of the twenty‐first century, environmental concerns brought about an interest in the utilization of clathrate hydrates as a working medium for gas storage and transportation and separation of flue and fuel gases. The groups of P. Englezos and H. Lee had been active in the engineering research of the aforementioned processes and linked up with the NRC group to provide information on the molecular aspects using a multi‐technique approach, in some cases with in situ time resolution. Some early results were obtained on “guest‐swapping” where methane was recovered from methane hydrate by exposing the hydrate to CO2 and “composition tuning” where the composition of binary hydrates can be varied over a wide range.
