Vu Tuan Hoang, Gloesener Elodie, Choukroun Mathieu, Ibourichene Anaïs, Hodyss Robert
NASA Jet Propulsion Laboratory, California Institute of Technology , 4800 Oak Grove Drive, Pasadena, California 91109, United States.
J Phys Chem B. 2014 Nov 26;118(47):13371-7. doi: 10.1021/jp5042487. Epub 2014 Jul 1.
Clathrate hydrates, ice-like crystalline compounds in which small guest molecules are enclosed inside cages formed by tetrahedrally hydrogen-bonded water molecules, are naturally abundant on Earth and are generally expected to exist on icy celestial bodies. A prototypical example is Saturn's moon Titan, where dissociation of methane clathrates, a major crustal component, could contribute significantly to the replenishment of atmospheric methane. Ammonia is an important clathrate inhibiting agent that may be present (potentially at high concentrations) in Titan's interior. In this study, low-temperature Raman experiments are conducted to examine the dissociation point of tetrahydrofuran clathrates, an ambient-pressure analogue of methane clathrates, over a wide range of ammonia concentrations from 0 to 25 wt %. A phase diagram for the H2O-THF-NH3 system is generated, showing two main results: (i) ammonia lowers the dissociation point of clathrate hydrates to a similar extent compared to the melting of water ice and (ii) THF clathrate exhibits a "liquidus-like" behavior in the presence of ammonia, with a eutectic temperature of about 203.6 K. As temperatures higher than this estimated eutectic are anticipated within Titan's icy crust, these results imply that partial dissociation of clathrates can occur readily and may contribute to outgassing from the interior.
笼形水合物是一种类似冰的晶体化合物，其中小的客体分子被由四面体氢键连接的水分子形成的笼所包围。这种物质在地球上天然丰富，并且普遍认为在冰冷的天体上也存在。一个典型的例子是土星的卫星泰坦，其主要地壳成分甲烷笼形水合物的分解可能对大气甲烷的补充有显著贡献。氨是一种重要的笼形水合物抑制剂，可能存在于（潜在地以高浓度）泰坦的内部。在这项研究中，进行了低温拉曼实验，以研究四氢呋喃笼形水合物（甲烷笼形水合物的常压类似物）在0至25 wt%的宽氨浓度范围内的分解点。生成了H2O-THF-NH3系统的相图，显示出两个主要结果：（i）与水冰的熔化相比，氨将笼形水合物的分解点降低到类似程度；（ii）在氨存在下，四氢呋喃笼形水合物表现出“类液相线”行为，共晶温度约为203.6 K。由于预计泰坦冰冷地壳内的温度高于这个估计的共晶温度，这些结果意味着笼形水合物的部分分解可能很容易发生，并可能导致内部气体逸出。
