An overview of recent structural work focusing on the gas hydrates of methane and carbon dioxide is given. Both the crystal structure and the microstructure are considered. We report on the pressure-dependent molecular structure of methane clathrate hydrate using laboratory-made hydrogenous and deuterated samples investigated by neutron and hard-X-ray synchrotron diffraction experiments. The isothermal compressibilities are determined for hydrogenated and deuterated CH4 hydrate, and isotopic differences between both compounds are established for the first time. The cage filling of carbon dioxide and methane hydrate is determined and compared with predictions from statistical thermodynamic theory. In the case of small cages in methane hydrate, experimental results and predictions do not agree. Field-emission scanning electron microscopy reveals the meso- to macro-porous nature of gas hydrates formed with an excess of free gas. Furthermore, in situ measurements of the formation kinetics of porous hydrates are reported in which differences between methane and carbon dioxide are established quantitatively and the transient existence of a type II carbon dioxide structure is found. PACS Nos.: 82.75-z, 61.10Nz, 61.12Ld, 68.37Hk

Abstract. We present results of experimental studies on the formation of gas hydrates (clathrates) at conditions of geophysical interest. Clathrate hydrates formed by a reaction of gas at ice Ih surfaces are always found to be mesoporous to macroporous with pores sizes between 100 to 400 nm and pore volumes of approximately 25-40% for CH4, Ar and N2 hydrate, and smaller pores of 20 to 100rim with a porosity of approximately 10-20% for CO2 hydrate. The three-dimensional sponge-like microstructure occurs in single crystalline grains of typically a few •um size and was observed by field-emission scanning electron microscopy. It forms over a wide range of p-T conditions below the ice Ih melting. The porous microstructure is stable for at least several months, even close to the clathrate decomposition, and is proposed to be formed by local differences in the energy balance between hydrate formation and ice decompositon. The results presented are considered of potential major importance for the understanding of the behaviour of natural gas hydrates found e.g. in polar ice sheets and permafrost regions, and also in some celestial bodies.

We present a review of our work on the dynamics of clathrate hydrates (gas hydrates). The experimental results obtained with inelastic neutron scattering are compared with molecular-dynamics calculations. The vibrations of the guest molecules and their coupling to the cages is found to depend critically on the size, shape and electrostatic properties of the encaged guest. Atoms like xenon, that are large enough to fill the cages, show close-to-harmonic behaviour and couple strongly to the cage vibrations. Small atoms and molecules fully explore the anharmonicities of the potential within the cage, in particular at low frequencies and low temperatures. Their dynamic response is broad in energy and they couple weakly to the cage vibrations. The relevance of the microscopic dynamics for cage stability and the glass-like thermal conductivity is discussed. We equally place the observed dynamic peculiarities into the broader context of vibrations in disordered systems. Raman spectroscopic results on internal guest vibrations at high frequencies reflect also the influence of guest-host interactions and are discussed in the framework of the loose-cage tight-cage model.

We have studied the short-time dynamical behavior of polydimethylsiloxane (PDMS) bottlebrush polymers, PDMS-g-PDMS. The samples have similar backbone lengths but different side-chain lengths, resulting in a shape transition. Quasi-elastic neutron scattering was used to observe the dynamical changes inherent to these structural changes. The combination of data from three spectrometers enabled to follow the dynamics over broad frequency and temperature ranges, which included segmental relaxations and more localized motions. The latter, identified as the methyl group rotation, is described by a threefold jump model and shows higher activation energies compared to linear PDMS. The segmental relaxation times, τs, decrease with increasing molecular weight of the side chains but increase with momentum transfer, Q, following a power law, which suggests a non-Gaussian behavior for bottlebrush polymers.

Clathrate hydrates of N2 and CO2 were investigated in situ as a function of gas pressure by neutron powder diffraction. Gas pressure cells developed specifically for this work were employed.The pressure dependency of the filling of the different cages in these clathrate hydrates was established. In contrast to some earlier indirect evidence the small cages in CO2 clathrate hydrate are partly filled, however the generally accepted Langmuir-behaviour of the filling is violated in this case. Likewise, instead of the usually assumed filling of the large cages with only one N2 molecule a double occupancy of these cages is observed for N2 clathrate hydrate.[Clathrates, gas pressure, neutron diffraction, guest-host interaction]