Molecular Dynamics Study of Clathrate-like Ordering of Water in Supersaturated Methane Solution at Low Pressure

Using molecular dynamics, the evolution of a metastable solution for “methane + water” was studied for concentrations of 3.36, 6.5, 9.45, 12.2, and 14.8 mol% methane at 270 K and 1 bar during 100 ns. We have found the intriguing behavior of the system containing over 10,000 water molecules: the formation of hydrate-like structures is observed at 6.5 and 9.45 mol% concentrations throughout the entire solution volume. This formation of “blobs” and the following amorphous hydrate were studied. The creation of a metastable methane solution through supersaturation is the key to triggering the collective process of hydrate formation under low pressure. Even the first stage (0–1 ns), before the first fluctuating cavities appear, is a collective process of H-bond network reorganization. The formation of fluctuation cavities appears before steady hydrate growth begins and is associated with a preceding uniform increase in the water molecule’s tetrahedrality. Later, the constantly presented hydrate cavities become the foundation for a few independent hydrate nucleation centers, this evolution is consistent with the labile cluster and local structure hypotheses. This new mechanism of hydrogen-bond network reorganization depends on the entropy of the cavity arrangement of the guest molecules in the hydrate lattice and leads to hydrate growth

Stability and Composition of Helium Hydrates Based on Ices I_h and II at Low Temperatures

The recently developed approach describing host lattice relaxation, guest–guest interactions and the quantum nature of guest behavior (Belosudov, R. V.; Subbotin, O. S.; Mizuseki, H.; Kawazoe, Y.; Belosludov, V. R. J. Chem. Phys. 2009, 131, 244510) has been used to derive the thermodynamic properties of helium hydrates based on ices I_h and II. The<i> p</i>–<i>T</i> phase diagrams of the helium hydrates in different ices are presented for a wide range of pressures and temperatures, and the structural transitions between pure ice I_h and ice II as well as between ice I_h-based helium hydrate and ice II-based helium hydrate have been found to be in agreement with the available experimental data. The “ice II-based helium hydrate–ice I_h-based helium hydrate” equilibrium shifts toward the higher pressures in comparison with the line of “ice II–ice I_h” equilibrium. The degrees of interstitial space filling by helium in ice I_h-based and ice II-based hydrates decrease with increasing temperature and lowering of pressure. It is demonstrated that the helium filling in ice I_h proceeds more slowly than in ice II. However, the mole fraction of helium in the hydrate based on ice I_h is significantly higher than that in the ice II-based hydrate. We predict that during the phase transition from the ice I_h-based hydrate to the ice II-based one a discharge of gaseous helium should be observed. This may serve as an indicator of the phase transition in experiment