Structural determination of crystalline powders, especially those of complex materials, is not a trivial task. For non-stoichiometric guest-host materials, the difficulty lies in how to determine dynamical disorder and partial cage occupancies of the guest molecules without other supporting information or constraints. Here, we show how direct space methods combined with Rietveld analysis can be applied to a class of host-guest materials, in this case the clathrate hydrates. We report crystal structures in the three important hydrate crystal classes, sI, sII, and sH, for the guests CO 2 ,C 2 H 6 ,C 3 H 8 , and methylcyclohexane + CH 4 . The results obtained for powder samples are found to be in good agreement with the experimental data from single crystal X-ray diffraction and 13 C solid-state NMR spectroscopy. This method is also used to determine the guest disorder and cage occupancies of neohexane and tert-butyl methyl ether binary hydrates with CH 4 in the structure H clathrate hydrates. The results are found to be in good agreement with the results from the 13 C solid-state NMR and molecular dynamics simulations. It is demonstrated that the ab initio crystal structure determination methodology reported here is able to determine absolute cage occupancies and the dynamical disorder of guest molecules in clathrate hydrates from powdered crystalline samples.

Molecular dynamics simulations are used to compare microscopic structures and guest dynamics to macroscopic properties in structure II clathrate hydrates with cyclopentane, tetrahydrofuran (THF), 1,3-dioxolane, tetrahydropyran (THP), and p-dioxane as guests. Significant differences are observed between structural parameters and rotational dynamics for the different guests. The simulations show the formation of guest-host hydrogen bonds between the ether oxygen atoms of THF and THP and the cage water hydrogen atoms of the clathrate but the absence of similar hydrogen bonds in the clathrate hydrates of the other guests on the time scale of the calculations. This guest-host hydrogen bonding leads to the formation of Bjerrum L-defects in the clathrate water lattice where two adjacent water molecules have no covalently bonded hydrogen atom between them. Unlike Bjerrum defects of ice lattices, these guest-induced L-defects are not accompanied by the formation of a D-defect at an adjacent site in the water lattice. At the simulation temperature of 200 K, the guest-water hydrogen bonds in the THF clathrate are short lived (lifetime less than 1 ps) but in the THP they are longer lived (a minimum of 100 ps). A van't Hoff plot for the probability of defect formation in THF as a function of temperature gives an activation barrier of approximately 8.3 kJ/mol for guest-host defect formation in the THF clathrate. The consequences of the defect formation on the thermal expansivity, isothermal compressibility, dipole-dipole correlation function, and mechanical stability of the clathrate are discussed.

Recombinant antifreeze proteins (AFPs), representing a range of activities with respect to ice growth inhibition, were investigated for their abilities to control the crystal formation and growth of hydrocarbon hydrates. Three different AFPs were compared with two synthetic commercial inhibitors, poly-N-vinylpyrrolidone (PVP) and HIW85281, by using multiple approaches, which included gas uptake, differential scanning calorimetry (DSC) temperature ramping, and DSC isothermal observations. A new method to assess the induction period before heterogeneous nucleation and subsequent hydrate crystal growth was developed and involved the dispersal of water in the pore space of silica gel beads. Although hydrate nucleation is a complex phenomenon, we have shown that it can now be carefully quantified. The presence of AFPs delayed crystallization events and showed hydrate growth inhibition that was superior to that of one of the benchmark commercial inhibitors, PVP. Nucleation and growth inhibition were shown to be independent processes, which indicates a difference in the mechanisms required for these two inhibitory actions. In addition, there was no apparent correlation between the assayed activities of the three AFPs toward hexagonal ice and the cubic structure II (sII) hydrate, which suggests that there are distinctive differences in the protein interactions with the two crystal surfaces.

Methane-propane clathrate hydrate crystal growth within an enclosure partially filled with liquid water was examined under different undercooling conditions both with and without the presence of n-heptane. Water saturated with guest gas was rapidly undercooled and maintained at constant temperature. The growth of the hydrate phase always started with the formation of a film at the upper surface of the liquid water pool. The visual observations using a microscope revealed detailed features of crystal nucleation, migration, and growth occurring within the water pool. In all experiments, hydrate crystal growth was initiated by the formation of hydrate film at the interface between liquid water and the adjacent gas or n-heptane layer. Hydrate crystals were then observed to grow downward from the film. Undercooling was found to be a key parameter to control the morphology of hydrates growing underneath the hydrate film as it influences the growth rate and configurations of crystals. Moreover, a number of small crystals were seen in the water ascending toward the hydrate film. The evolution of the shape of these crystals was monitored. These crystals were octahedral or triangular or hexagonal platelets. Finally, the role of constitutional undercooling and the memory of the water to the observed crystal growth are discussed.

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The formation of guest-host hydrogen bonds in structure H (sH) clathrate hydrates is studied herein. We contrast the structure and guest dynamics of the tert-butylmethylether (TBME) and neohexane (NH) sH clathrates by performing molecular dynamics simulations on these two clathrates and measuring (1)H and (13)C NMR relaxation times of the guests. These two guests are isoelectronic and differ with respect to the presence of the ether oxygen atom in TBME and a CH(2) group in NH. The TBME guest forms long-lived hydrogen bonds with water molecules in the equatorial region of the large sH clathrate cage. These hydrogen bonds effectively tether the TBME guest to the side of the cage and restrict its rattling and rotational motions in the cage compared to NH, which does not become hydrogen bonded to the cage's water molecules.

Experimental data on the kinetics of formation of structure H gas hydrate obtained in a semibatch stirred vessel at pressures of 0.63−1.5 MPa above equilibrium are reported. Methane was used as a guest substance and neohexane, tert-butyl methyl ether, and methylcyclohexane were used as the large molecule guest substance (LMGS). The results indicate that the rates of hydrate formation and the induction times are dependent on the magnitude of the driving force and the type of LMGS. When tert-butyl methyl ether is used as the LMGS, rapid hydrate formation and a much smaller induction time can be achieved. Furthermore, the methane consumption rate for hydrate formation in the presence of tert-butyl methyl ether is 3 times greater than that for a pure methane−water system. It was also observed that, although the induction period was greatly shortened by the memory effect, the rate of gas consumption rate was not affected. Hydrate decompositions were also conducted at a pressure 20% below equilibrium. The system with tert-butyl methyl ether as LMGS exhibited the fastest decomposition rate.

in Wiley InterScience (www.interscience.wiley.com).The methane uptake and conversion rate to structure H (sH) hydrates was measured and compared to crystallization kinetics models. Three large molecule guest substances (LMGS) were used as sH hydrate formers: neohexane (NH), methylcyclohexane (MCH), and tert-butyl methyl ether (TBME). The initial crystallization occurred quickly at the LMGS liquid-ice interface until $20-30% of ice was converted into hydrate (hydrate growth stage I). Slower hydrate crystal growth was observed after a hydrate film covered the ice surface at a rate of 3-400 nm 2 /h (hydrate growth stage II). The TBME system showed the fastest kinetics at the beginning of the reaction followed by NH and MCH system. However the trend changed when the temperature was increased (''reaction'' stage III). Surprisingly, the conversion rate achieved with the TBME system upon melting the ice was the smallest. This was attributed to the strong interaction of TBME with water molecules that increased the energy barrier for water molecules to form hydrate cages. The conversion rates were well correlated with the Avrami equation and the shrinking core model. Finally, NH was found to be the best LMGS in this study to obtain full conversion within a short reaction time and achieving high methane gas storage in the hydrate.