Abstract
The molecular structure of dense homogeneous fluid water–methane mixtures has been determined for the first time using high-pressure neutron-scattering techniques at 1.7 and 2.2 GPa. A mixed state with a fully H-bonded water network is revealed. The hydration shell of the methane molecules is, however, revealed to be pressure-dependent with an increase in the water coordination between 1.7 and 2.2 GPa. In parallel, ab initio molecular dynamics simulations have been performed to provide insight into the microscopic mechanisms associated with the phenomenon of mixing. These calculations reproduce the observed phase change from phase separation to mixing with increasing pressure. The calculations also reproduce the experimentally observed structural properties. Unexpectedly, the simulations show mixing is accompanied by a subtle enhancement of the polarization of methane. Our results highlight the key role played by fine electronic effects on miscibility and the need to readjust our fundamental understanding of hydrophobicity to account for these.
Highlights
The molecular structure of dense homogeneous fluid water− methane mixtures has been determined for the first time using high-pressure neutron-scattering techniques at 1.7 and 2.2 GPa
The evolution of hydrophobic interactions under different thermodynamic conditions is relevant to a wide range of science ranging from Earth and planetary sciences to biology and is a longstanding active field of research.[6−8] Methane−water mixtures are present at the bottom of oceans, where compression leads to the formation of solid methane hydrates;[9] they are major constituents of the middle layers of the ice giants Neptune and Uranus[10] and icy satellites like Titan and Triton.[11]
Combinations of water, ammonia, and methane are predicted to be widely present in the recently observed exoplanets, which have most commonly been of Neptune-like proportions,[12−14] and are likely to exist as either liquids or solids.[15−18] A microscopic understanding of how extreme pressures modulate the solubility of methane−water mixtures is critical to develop realistic models of the interior of planets.[19]
Summary
The molecular structure of dense homogeneous fluid water− methane mixtures has been determined for the first time using high-pressure neutron-scattering techniques at 1.7 and 2.2 GPa.
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