No abstract

The structure of water around methane during hydrate crystallization from aqueous solutions of methane is studied using neutron diffraction with isotopic substitution over the temperature range 18 °C to 4 °C, and at two pressures, 14.5 and 3.4 MPa. The carbon–oxygen pair correlation functions, derived from empirical potential structure refinement of the data, indicate that the hydration sphere around methane in the liquid changes dramatically only once hydrate has formed, with the water shell around methane being about 1 Å larger in diameter in the crystal than in the liquid. The methane coordination number in the liquid is around 16±1 water molecules during hydrate formation, which is significantly smaller than the value of 21±1 water molecules found for the case when hydrate is fully formed. Once hydrate starts to form, the hydration shell around methane becomes marginally less ordered compared to that in the solution above the hydrate formation temperature. This suggests that the hydration cage around methane in the liquid may be different from that when hydrate is forming and from that found in the hydrate crystal structure. Methane–methane radial distribution functions show that methane molecules can adopt a range of separations during hydrate formation, corresponding to the more distorted nature of the methane–water correlations. There is noticeable ordering of the methane molecules with a monolayer of water molecules between them once hydrate has formed. The dipole moments of the hydrating water molecules lie mostly tangential to the methane–water axis, both before, during, and after hydrate formation.

We report the results on the structure of the binary dense CO 2 -water interface at 20 MPa and 318 and 338 K and 28 MPa and 318 K, as investigated by molecular dynamics computer simulations. Realistic potential models are used to describe the interactions, and the Ewald summation technique is employed to account for the long range electrostatic interactions. It is shown that the interface is molecularly sharp with distortions from a flat interface due to the presence of capillary waves induced by thermal fluctuations. The use of a local dynamic interface definition 1 provides a revealing density profile in which interfacial packing of fluids on both sides of the interface is observed. Atomic radial distribution functions, orientational probability distribution functions, and H-bond analysis are used to probe the nature of the bulk to interface transition. Specific attractive interactions between CO 2 and water due to Coulombic interactions are evident. The interfacial tension is determined from the pressure tensor analysis and from capillary wave theory, and the results are compared to the experimental values obtained in our laboratories.

Neutron diffraction with HD isotope substitution has been used to study the formation and decomposition of the methane clathrate hydrate. Using this atomistic technique coupled with simultaneous gas consumption measurements, we have successfully tracked the formation of the sI methane hydrate from a water/gas mixture and then the subsequent decomposition of the hydrate from initiation to completion. These studies demonstrate that the application of neutron diffraction with simultaneous gas consumption measurements provides a powerful method for studying the clathrate hydrate crystal growth and decomposition. We have also used neutron diffraction to examine the water structure before the hydrate growth and after the hydrate decomposition. From the neutron-scattering curves and the empirical potential structure refinement analysis of the data, we find that there is no significant difference between the structure of water before the hydrate formation and the structure of water after the hydrate decomposition. Nor is there any significant change to the methane hydration shell. These results are discussed in the context of widely held views on the existence of memory effects after the hydrate decomposition.

Molecular dynamics simulations of liquid squalane, C30H62, were performed, focusing in particular on the liquid-vacuum interface. These theoretical studies were aimed at identifying potentially reactive sites on the surface, knowledge of which is important for a number of inelastic and reactive scattering experiments. A united atom force field (Martin, M. G.; Siepmann, J. I. J. Phys. Chem. B 1999, 103, 4508-4517) was used, and the simulations were analyzed with respect to their interfacial properties. A modest but clearly identifiable preference for methyl groups to protrude into the vacuum has been found at lower temperatures. This effect decreases when going to higher temperatures. Additional simulations tracking the flight paths of projectiles directed at a number of randomly chosen surfaces extracted from the molecular dynamics simulations were performed. The geometrical parameters for these calculations were chosen to imitate a typical abstraction reaction, such as the reaction between ground-state oxygen atoms and hydrocarbons. Despite the preference for methyl groups to protrude further into the vacuum, Monte Carlo tracking simulations suggest, on geometric grounds, that primary and secondary hydrogen atoms are roughly equally likely to react with incoming gas-phase atoms. These geometric simulations also indicate that a substantial fraction of the scattered products is likely to undergo at least one secondary collision with hydrocarbon side chains. These results help to interpret the outcome of previous measurements of the internal and external energy distribution of the gas-phase OH products of the interfacial reaction between oxygen atoms and liquid squalane.