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Water and methane stay together at extreme pressures
Saturday, 2019/08/17 | 07:15:06

Christoph G. Salzmann

PNAS August 13, 2019 116 (33) 16164-16166

Figure: Prof. CG Salzman, Department of Chemistry, University College London, London WC1H 0AJ, United Kingdom

 

Large lakes of liquid methane nestle between mountain ranges of solid water ice in the polar regions of Jupiter’s moon Titan (1, 2). This strange world illustrates in a quite dramatic fashion that the isoelectronic CH4 and H2O molecules display profoundly different physical properties including a 182 °C difference in their melting points at ambient pressure. Unlike methane, water molecules form strong hydrogen bonds with up to 4 neighbors, which explains the high melting point of ice compared to that of solid methane. Rearrangements of those hydrogen bonds, which take place as temperature and pressure are varied, give rise to a large family of complex network structures beyond the “ordinary” hexagonal form of ice, ice Ih (3). However, the structural diversity of H2O does not end with the pure phases of ice. Water molecules can form cages around hydrophobic species such as methane to form clathrate hydrates (4, 5). These important inclusion compounds have been suggested as model systems for studying hydrophobic interactions (4), and they are also relevant for a wide range of industrial, geological, atmospheric, and cosmological settings (6, 7). Methane clathrate hydrate (MH) is one of the most thoroughly studied materials in this context with 3 distinct structural forms identified so far experimentally at different pressures (5). Schaack et al. (8) now report in PNAS the existence of a fourth hydrate of methane (MH-IV) that forms above ∼40 GPa and remains stable up to at least 150 GPa. Intriguingly, the water network of MH-IV takes on a very familiar form, that of ice Ih, but it is densely packed with methane molecules at a 2:1 H2O:CH4 ratio.

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