Phase Behavior of Different Forms of Ice Filled with Hydrogen Molecules (original) (raw)
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2014
A new, rigorous framework centered around the multi-scale GHC equation of state is presented for predicting bulk density and phase equilibrium for light gas–water mixtures at conditions where hexagonal ice and structure I hydrate phases can exist. The novel aspects of this new framework include (1) the use of internal energies of departure for ice and empty hydrate respectively to determine densities, (2) contributions to the standard state fugacity of water in ice and empty hydrate from lattice structure, (3) computation of these structural contributions to standard state fugacity from compressibility factors and EOS parameters alone, and (4) the direct calculation of gas occupancy from phase equilibrium. Numerical results for densities and equilibrium for systems involving ice and/or gas hydrates predicted by this GHC-based framework are compared to predictions of other equations of state, density correlations, and experimental data where available. Results show that this new GHC-...
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MATERIALS TRANSACTIONS, 2007
Molecular dynamics simulations were performed to investigate the possible role of ice shielding in the anomalous preservation of gas hydrates. Two cases of ice shielding were considered: immersion of hydrate particles into bulk ice Ih and wrapping of similar particles in a thin ice shell. For a microscopic-level model of methane hydrate clusters immersed in bulk ice the excess pressure in the hydrate phase at 250 K was found to be sufficient to shift the gas hydrate into the region of thermodynamic stability on the phase diagram. For the second model the temperature dependence of various properties of the hydrate/ice nanocluster was studied. The surface tension estimates based on the Laplace equation show non-monotonic dependence on temperature, which might indicate the possible involvement of hydrate/ice interfacial phenomena in the self-preservation of gas hydrates.
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Bulletin of the Chemical Society of Japan, 2006
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Crystallization in liquids is critical to a range of important processes occurring in physics, chemistry and life sciences. In this article, we review our efforts towards understanding the crystallization mechanisms, where we focus on theoretical modelling and molecular simulations applied to ice and gas hydrate systems. We discuss the order parameters used to characterize molecular ordering processes and how different order parameters offer different perspectives of the underlying mechanisms of crystallization. With extensive simulations of water and gas hydrate systems, we have revealed unexpected defective structures and demonstrated their important roles in crystallization processes. Nucleation of gas hydrates can in most cases be characterized to take place in a two-step mechanism where the nucleation occurs via intermediate metastable precursors, which gradually reorganizes to a stable crystalline phase. We have examined the potential energy landscapes explored by systems duri...
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Thermodynamic properties of water, in various families of hydroxides, oxihydroxides and hydrates (chlorides, chlorates, sulfates and sul®tes F F F), have been calculated by using a large number of data available in the literature. A phase diagram of water has been used to ®nd the ®rst complete set of thermodynamic properties at 298 K, 1 bar of 8 ice polymorphs, from Ih (hexagonal ice, the common polymorph) to IX (very low temperature and high pressure polymorph). These results are used to illustrate the concept of`ice-like water' available for a very large number of hydrated phases (noted X.H 2 O) in which water is attached to the corresponding anhydrous substrate (noted X) within a large spectrum of dierent enthalpies D f H8 or Gibbs free energies D f G8, but within a relatively small range of others properties. Heat capacity (Cp8), entropy (S8), and volume (V8) of hydration water (X.H 2 OÀX = H 2 O) appeared to be very close to those characterizing ice polymorphs such as ice II or ice VIII. This concept allows the authors to propose a classi®cation of minerals in terms of anity for water and to predict the relative stability of hydrated and dehydrated phases under climatic variations.