Simulation of methane-water mixtures at extreme conditions

Simulations and experiments carried out separately on methane and water, the main components of the middle layers of Neptune and Uranus, show that at those conditions methane disproportionates into carbon-rich species and water dissociates to form an ionic fluid. Water becomes electronically conducting only at the conditions found in the deepest layers of the planets. More recent simulations on water/methane mixtures suggest a pressure-induced softening of the methane-water intermolecular repulsion that points to an enhancement of mixing under extreme conditions. In the mixtures, ionized water causes the progressive ionization of methane and the mixture becomes electronically conductive at milder conditions than pure water. Calculations on the crystalline counterparts, methane hydrate clathrates, suggest however a different picture: mixtures at low temperature become increasingly unstable, with increasing pressure, towards phase separation, despite the prediction of a solid-solid phase transition between MH-III, the known high-pressure form of methane hydrate, and a new hypothetical phase.