The behavior of methane-water mixtures under elevated pressures from simulations using many-body potentials.

Non-polarizable empirical potentials have been proven to be incapable of capturing the mixing of methane-water mixtures at elevated pressures. Although density functional theory-based ab initio simulations may circumvent this discrepancy, they are limited in terms of the relevant time and length scales associated with mixing phenomena. Here, we show that the many-body MB-nrg potential, designed to reproduce methane-water interactions with coupled cluster accuracy, successfully captures this phenomenon up to 3 GPa and 500 K with varying methane concentrations.

How to determine solubility in binary mixtures from neutron scattering data: The case of methane and water.

It has recently been discovered that, when subjected to moderate amounts of pressure, methane dissolves in water to form binary mixtures of up to 40% molar methane. No significant solubility of water in methane is known. In these mixtures, the water hydrogen-bond network is largely complete and surrounds the methane molecules.

Ionic Phases of Ammonia-Rich Hydrate at High Densities.

Mixtures of ammonia and water are major components of the "hot ice" mantle regions of icy planets. The ammonia-rich ammonia hemihydrate (AHH) plays a pivotal role as it precipitates from water-rich mixtures under pressure. It has been predicted to form ionic high-pressure structures, with fully disintegrated water molecules.

Squeezing Oil into Water under Pressure: Inverting the Hydrophobic Effect.

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.

Plastic and superionic phases in ammonia-water mixtures at high pressures and temperatures.

The interiors of giant icy planets depend on the properties of hot, dense mixtures of the molecular ices water, ammonia, and methane. Here, we discuss results from first-principles molecular dynamics simulations up to 500 GPa and 7000 K for four different ammonia-water mixtures that correspond to the stable stoichiometries found in solid ammonia hydrates. We show that all mixtures support the formation of plastic and superionic phases at elevated pressures and temperatures, before eventually melting into molecular or ionic liquids.

On the chain-melted phase of matter.

Various single elements form incommensurate crystal structures under pressure, where a zeolite-type "host" sublattice surrounds a "guest" sublattice comprising 1D chains of atoms. On "chain melting," diffraction peaks from the guest sublattice vanish, while those from the host remain. Diffusion of the guest atoms is expected to be confined to the channels in the host sublattice, which suggests 1D melting.

Novel phases in ammonia-water mixtures under pressure.

While ammonia and water readily form hydrogen-bonded molecular mixtures at ambient conditions, their miscibility under pressure is not well understood, yet crucial to model the interior of icy planets. We report here on the behavior of ammonia-water mixtures under extreme pressure conditions, based on first-principles calculations of 15 stoichiometries in the pressure range of 1 atm-10 Mbar. We show that compression facilitates proton transfer from water to ammonia in all relevant mixtures.

Stabilization of ammonia-rich hydrate inside icy planets.

The interior structure of the giant ice planets Uranus and Neptune, but also of newly discovered exoplanets, is loosely constrained, because limited observational data can be satisfied with various interior models. Although it is known that their mantles comprise large amounts of water, ammonia, and methane ices, it is unclear how these organize themselves within the planets-as homogeneous mixtures, with continuous concentration gradients, or as well-separated layers of specific composition. While individual ices have been studied in great detail under pressure, the properties of their mixtures are much less explored.