Krypton and the Fundamental Flaw of the Lennard-Jones Potential.

We have performed a series of neutron scattering experiments on supercritical krypton. Our data and analysis allow us to characterize the Frenkel line crossover in this model monatomic fluid. The data from our measurements was analyzed using Empirical Potential Structure Refinement to determine the short- and medium-range structure of the fluids. We find evidence for several shells of neighbors which form approximately concentric rings of density about each atom. The ratio of second to first shell radius is significantly larger than in any crystal structure. Modeling krypton using a Lennard-Jones potential is shown to give significant errors, notably that the liquid is overstructured. The true potential appears to be longer ranged and with a softer core than the 6-12 powerlaws permit.

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. The discovery of this dense mixture has once again highlighted the technical difficulties involved in accurately describing and sampling mixing phenomena both computationally and experimentally. Here, we present a systematic and critical study of the methods employed to characterize binary mixtures and their robustness. This study highlights the requirements needed to develop a quantitative understanding, and it proposes new and more accessible measures of miscibility to investigators, particularly for in silico analysis.

Structural Markers of the Frenkel Line in the Proximity of Widom Lines.

We have performed a neutron scattering experiment on supercritical fluid nitrogen at 160 K (1.27 TC) over a wide pressure range (7.8 MPa/0.260 g/mL-125 MPa/0.805 g/mL). This has enabled us to study the process by which nitrogen changes from a fluid that exhibits gaslike behavior to one that exhibits rigid liquidlike behavior at a temperature close to, but above, the critical temperature by crossing the Widom lines followed by the Frenkel line on pressure (density) increase. We find that the Frenkel line transition is indicated by a transition to a regime of rigid liquidlike behavior in which the coordination number remains constant within error, in agreement with our previous work at 300 K. The Frenkel line transition takes place at approximately the same density at 160 and 300 K. The data do not conclusively show an additional transition at the location of the known Widom lines. We find that behavior remains gaslike until the Frenkel line is crossed and our data support the hypothesis that Widom line transitions are density increase-driven.

Structure and nature of ice XIX.

Ice is a material of fundamental importance for a wide range of scientific disciplines including physics, chemistry, and biology, as well as space and materials science. A well-known feature of its phase diagram is that high-temperature phases of ice with orientational disorder of the hydrogen-bonded water molecules undergo phase transitions to their ordered counterparts upon cooling. Here, we present an example where this trend is broken. Instead, hydrochloric-acid-doped ice VI undergoes an alternative type of phase transition upon cooling at high pressure as the orientationally disordered ice remains disordered but undergoes structural distortions. As seen with in-situ neutron diffraction, the resulting phase of ice, ice XIX, forms through a Pbcn-type distortion which includes the tilting and squishing of hexameric clusters. This type of phase transition may provide an explanation for previously observed ferroelectric signatures in dielectric spectroscopy of ice VI and could be relevant for other icy materials.

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.

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 because of 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 (10-20 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.

A Chiral Gas-Hydrate Structure Common to the Carbon Dioxide-Water and Hydrogen-Water Systems.

We present full in situ structural solutions of carbon dioxide hydrate-II and hydrogen hydrate C0 at elevated pressures using neutron and X-ray diffraction. We find both hydrates adopt a common water network structure. The structure exhibits several features not previously found in hydrates; most notably it is chiral and has large open spiral channels along which the guest molecules are free to move. It has a network that is unrelated to any experimentally known ice, silica, or zeolite network but is instead related to two Zintl compounds. Both hydrates are found to be stable in electronic structure calculations, with hydration ratios in very good agreement with experiment.

When immiscible becomes miscible-Methane in water at high pressures.

At low pressures, the solubility of gases in liquids is governed by Henry's law, which states that the saturated solubility of a gas in a liquid is proportional to the partial pressure of the gas. As the pressure increases, most gases depart from this ideal behavior in a sublinear fashion, leveling off at pressures in the 1- to 5-kbar (0.1 to 0.5 GPa) range with solubilities of less than 1 mole percent (mol %). This contrasts strikingly with the well-known marked increase in solubility of simple gases in water at high temperature associated with the critical point (647 K and 212 bar). The solubility of the smallest hydrocarbon, the simple gas methane, in water under a range of pressure and temperature is of widespread importance, because it is a paradigmatic hydrophobe and occurs widely in terrestrial and extraterrestrial geology. We report measurements up to 3.5 GPa of the pressure dependence of the solubility of methane in water at 100°C-well below the latter's critical temperature. Our results reveal a marked increase in solubility between 1 and 2 GPa, leading to a state above 2 GPa where the maximum solubility of methane in water exceeds 35 mol %.

Pressure-induced localisation of the hydrogen-bond network in KOH-VI.

Using a combination of ab initio crystal structure prediction and neutron diffraction techniques, we have solved the full structure of KOH-VI at 7 GPa. Rather than being orthorhombic and proton-ordered as had previously be proposed, we find that this high-pressure phase of potassium hydroxide is tetragonal (space group I4/mmm) and proton disordered. It has an unusual hydrogen bond topology, where the hydroxyl groups form isolated hydrogen-bonded square planar (OH)4 units. This structure is stable above 6.5 GPa and, despite being macroscopically proton-disordered, local ice rules enforce microscopic order of the hydrogen bonds. We suggest the use of this novel type of structure to study concerted proton tunneling in the solid state, while the topology of the hydrogen bond network could conceivably be exploited in data storage applications based solely on the manipulations of hydrogen bonds. The unusual localisation of the hydrogen bond network under applied pressure is found to be favored by a more compact packing of the constituents in a distorted cesium chloride structure.

An improved method for calibrating time-of-flight Laue single-crystal neutron diffractometers.

A robust and comprehensive method for determining the orientation matrix of a single-crystal sample using the neutron Laue time-of-flight (TOF) technique is described. The new method enables the measurement of the unit-cell parameters with an uncertainty in the range 0.015-0.06%, depending upon the crystal symmetry and the number of reflections measured. The improved technique also facilitates the location and integration of weak reflections, which are often more difficult to discern amongst the increased background at higher energies. The technique uses a mathematical model of the relative positions of all the detector pixels of the instrument, together with a methodology that establishes a reproducible reference frame and a method for determining the parameters of the instrument detector model. Since all neutron TOF instruments require precise detector calibration for their effective use, it is possible that the method described here may be of use on other instruments where the detector calibration cannot be determined by other means.

High density amorphous ices: disordered water towards close packing.

The structure of amorphous ice under pressure has been studied by molecular dynamics at 160 K. The starting low-density phase undergoes significant changes as the density increases, and at rho=1.51 g/cm(3) our calculated g(OO)(r) is in excellent agreement with in situ neutron diffraction data obtained at 1.8 GPa and 100 K on very high density amorphous ice made at 150 K. As the system is further compressed, in the theoretical simulations, up to rho=1.90 g/cm(3), the structural modifications are continuous up to the highest density. The analysis of orientational distributions reveals that dense amorphous ice is characterized by major distortions of the tetrahedral geometry, and that the pressure structural changes, already observed experimentally at lower densities, can be interpreted as a trend towards a disordered closed-packed structure.