Thompson Helen, Soper Alan K, Buchanan Piers, Aldiwan Nawaf, Creek Jefferson L, Koh Carolyn A
ISIS Facility, Rutherford Appleton Laboratory, Didcot, Oxon OX11 0QX, United Kingdom.
J Chem Phys. 2006 Apr 28;124(16):164508. doi: 10.1063/1.2191056.
Neutron diffraction studies with hydrogen/deuterium isotope substitution measurements are performed to investigate the water structure at the early, medium, and late periods of methane clathrate hydrate formation and decomposition. These measurements are coupled with simultaneous gas consumption measurements to track the formation of methane hydrate from a gas/water mixture, and then the complete decomposition of hydrate. Empirical potential structure refinement computer simulations are used to analyze the neutron diffraction data and extract from the data the water structure in the bulk methane hydrate solution. The results highlight the significant changes in the water structure of the remaining liquid at various stages of hydrate formation and decomposition, and give further insight into the way in which hydrates form. The results also have important implications on the memory effect, suggesting that the water structure in the presence of hydrate crystallites is significantly different at equivalent stages of forming compared to decomposing. These results are in sharp contrast to the previously reported cases when all remaining hydrate crystallites are absent from the solution. For these systems there is no detectable change in the water structure or the methane hydration shell before hydrate formation and after decomposition. Based on the new results presented in this paper, it is clear that the local water structure is affected by the presence of hydrate crystallites, which may in turn be responsible for the "history" or "memory" effect where the production of hydrate from a solution of formed and then subsequently melted hydrate is reportedly much quicker than producing hydrate from a fresh water/gas mixture.
进行了氢/氘同位素取代测量的中子衍射研究，以探究甲烷笼形水合物形成和分解的早期、中期和后期的水结构。这些测量与同步气体消耗测量相结合，以追踪从气/水混合物中形成甲烷水合物，然后水合物的完全分解过程。使用经验势结构精修计算机模拟来分析中子衍射数据，并从数据中提取大量甲烷水合物溶液中的水结构。结果突出了在水合物形成和分解的各个阶段剩余液体的水结构的显著变化，并进一步深入了解了水合物的形成方式。这些结果对记忆效应也具有重要意义，表明在存在水合物微晶的情况下，形成阶段与分解阶段同等阶段的水结构存在显著差异。这些结果与之前报道的溶液中不存在所有剩余水合物微晶的情况形成鲜明对比。对于这些系统，在水合物形成之前和分解之后，水结构或甲烷水合壳没有可检测到的变化。基于本文提出的新结果，很明显局部水结构受到水合物微晶的存在的影响，这反过来可能是造成“历史”或“记忆”效应的原因，据报道，从形成然后随后融化的水合物溶液中生成水合物比从新鲜水/气混合物中生成水合物要快得多。
