Search for memory effects in methane hydrate: Structure of water before hydrate formation and after hydrate decomposition

Abstract

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.

Is it the shape of the cavity, or the shape of the water in the cavity?

“…from non-polar phases to aqueous phases by postulating a network of structured waters forms around the non-polar surfaces, although experimental programs employing neutron scattering, which is exquisitely sensitive to water structure, have repeatedly probed the structure of aqueous solutions containing non-polar molecules, and have not provided support for the notion that water in contact with these hydrophobic solutes is more "ordered" than water in the bulk. [34][35][36] B. "Lock and Key"…”

Some current challenges in clathrate hydrate science: Nucleation, decomposition and the memory effect

“…Since the formation of the solid hydrate from the gas and liquid is exothermic, there is considerable heat that must be dispersed, in addition to the mass transfer of methane that needs to be optimized. What is more, hydrate nucleation is a stochastic process which, when it occurs in a stirred solution, tends to give a sudden, catastrophic appearance of the hydrate phase, thus making detailed experimental studies and measurements very difficult [18]. One parameter that has been used consistently to study hydrate nucleation is the macroscopic induction time, that is, the time interval between the nucleation event and when the experiment first enters the pressure/temperature region of hydrate stability.…”

Influence of History of Solution in Crystal Nucleation of Fenoxycarb: Kinetics and Mechanisms

“…7 Interesting results on this effect have also been presented for the nucleation of clathrate hydrates. 8 In general, it is reported that increased preheating temperature and preheating time lead to slower crystallization kinetics. Recently, Nordström et al 9 gave a comprehensive summary of previous work in this area, as well as additional experimental evidence demonstrating that not only the rate of nucleation but also the polymorphic outcome in crystallization of m-hydroxy benzoic acid was influenced by the temperature and time of solution preheating.…”

“…Three different principles have been hypothesized: 8,9 1. Even after the solid phase has completely dissolved, the solution temporarily contains locally higher concentrations, and/or the molecules in the dissolved state retain, for some time, structural features from the solid and/or the dissolution process.…”