From Atoms to Colloids: Does the Frenkel Line Exist in Discontinuous Potentials?

The Frenkel line has been proposed as a crossover in the fluid region of phase diagrams between a "non-rigid" and a "rigid" fluid. It is generally described as a crossover in the dynamical properties of a material, and as such has been described theoretically using a very different set of markers from those with which is it investigated experimentally. In this study, we have performed extensive calculations using two simple yet fundamentally different model systems: hard spheres and square well potentials. The former has only hardcore repulsion, while the latter also includes a simple model of attraction. We computed and analysed a series of physical properties used previously in simulations and experimental measurements, and discuss critically their correlations and validity as to being able to uniquely and coherently locate the Frenkel in discontinuous potentials

Pressure-induced Miscibility Increase of CH4 in H2O: A Computational Study Using Classical Potentials

Methane and water demix under normal (ambient) pressure and temperature conditions, due to the polar nature of water and the apolar nature of methane. Recent experimental work has shown, though, that increasing the pressure to values between 1 and 2 GPa (10 to 20 kbar) leads to a marked increase of methane solubility in water, for temperatures which are well below the critical temperature for water. Here we perform molecular dynamics simulations based on classical force fields – which are well-used and have been validated at ambient conditions – for different values of pressure and temperature. We find the expected increase in miscibility for mixtures of methane and supercritical water; however our model fails to reproduce the experimentally observed increase in methane solubility at large pressures and below the critical temperature of water. This points to the need to develop more accurate force fields for methane and methane-water mixtures under pressure

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. 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

Ammonia Mono Hydrate IV: An Attempted Structure Solution

The mixed homonuclear and heteronuclear hydrogen bonds in ammonia hydrates have been of interest for several decades. In this manuscript, a neutron powder diffraction study is presented to investigate the structure of ammonia monohydrate IV at 170 K at an elevated pressure of 3–5 GPa. The most plausible structure that accounts for all features in the experimental pattern was found in the P21/c space group and has the lattice parameters a=5.487(3) Å, b=19.068(4) Å, c=5.989(3) Å, and β=99.537(16) deg. While the data quality limits discussion to a proton-ordered structure, the structure presented here sheds light on an important part of the ammonia–water phase diagram

From atoms to colloids : does the Frenkel line exist in discontinuous potentials?

The Frenkel line has been proposed as a crossover in the fluid region of phase diagrams between a "non-rigid" and a "rigid" fluid. It is generally described as a crossover in the dynamical properties of a material, and as such has been described theoretically using a very different set of markers from those with which is it investigated experimentally. In this study, we have performed extensive calculations using two simple yet fundamentally different model systems: hard spheres and square well potentials. The former has only hardcore repulsion, while the latter also includes a simple model of attraction. We computed and analysed a series of physical properties used previously in simulations and experimental measurements, and discuss critically their correlations and validity as to being able to uniquely and coherently locate the Frenkel in discontinuous potentials

The effect of deuteration on the optical spectra of compressed methane

The in situ high pressure Raman spectrum of CD4 was found to be subtly different from its’ hydrogenous analog, CH4. High quality data were obtained for the first time for pressures between 12 and 20 GPa during both fast and slow compression. Similarly to CH4 in phase B, CD4 does exhibit peak splitting in the ν1 (symmetric stretch) and ν3 (antisymmetric stretch) modes, but having the emergent shoulders present on the high-frequency side of the peaks rather than the low-frequency one as in the case of CH4. The general aspect of the Raman spectrum was found to be very different from that of CH4, with modes ν1 and ν3 having comparable intensities and the latter being sharper and better defined, in stark contrast to how it appears in CH4

The re-entrant transition from the molecular to atomic phases of dense fluids : the case of hydrogen

A simple phenomenological thermodynamic model is developed to describe the chemical bonding and unbonding in homonuclear diatomic systems. The model describes the entire phase diagram of dimer-forming systems and shows a transition from monomers to dimers, with monomers favored at both very low and very high pressures, as well as at high temperature. In the context of hydrogen, the former region corresponds to hydrogen present in most interstellar gas clouds, while the latter is associated with the long sought-after fluid metallic phase. The model predicts a molecular to atomic fluid transition in dense deuterium that is in agreement with recently reported experimental measurements