Understanding the Structure of High Pressure Water | Opinion

The liquid state has always been a mystery. The first correct theory describing it, proposed by Johannes Diederik van der Waals in his 1873 doctoral thesis, presented it as a modification of a gas. By incorporating intermolecular attraction and finite molecular volume into the gas laws relating pressure, temperature and density, he predicted an abrupt transition to a denser phase. From this point of view, the molecules in a liquid are more or less disordered, just like in a gas.

But the density of a liquid is much closer to the density of a solid. So might the solid phase be a better starting point when the liquid arises from a cluster of defects, as in glass? This perspective seems all the more appropriate for water, which is known (known?) to be a structured liquid whose three-dimensional network of hydrogen bonds (H-bonds) creates significant local order in the local environment of each H.₂O molecule, organizing them into a tetrahedral formation. It is because of this network that liquid water is actually denser than solid water: the constant formation and breaking of hydrogen bonds in the liquid allows molecules to drift into the voids of the network, which is held rigidly in the ice.

This attractiveness of the icy liquid water model teases many. In 1892, Wilhelm Roentgen proposed a two-phase model of water, in which microscopic clumps of ice floated in a more disordered environment; A century later, this two-state picture was supported by Wheels Robinson and his colleagues.¹ The spirit of these ideas can be captured in a sentence.^2.3 that water has two liquid states of different densities—a gas-like and an ice-like phase, one might say—that modeling predicts will separate into phases in the metastable low-temperature, high-pressure region of the phase diagram.

At the heart of this debate is the thorny question of how to capture the dynamic state of a fluid in a static structural model. How do you decide whether any given H₂The O molecule enters one phase or another, and are there any significant differences at all?

There is no perfect way to measure the structure of water.

The problem here has always been that there is no unique or ideal way to measure or quantify the structure of water. Neutron and X-ray scattering, infrared, Raman and terahertz spectroscopy, complemented by modeling, were used. Tse and his colleagues use X-ray Raman spectroscopy (XRS), in which changes in local hydrogen bonds are detected as subtle changes in scattering by oxygen atoms. Comparing their observations with XRS spectra calculated by ab initio methods based on molecular dynamics simulations, the researchers conclude that increasing pressure affects the structure of the liquid.

They concluded that the H-bond network itself was not significantly disrupted. Rather, compression causes some H₂O molecules sneak into the voids of the network and thus enter the first coordination shells of other molecules, giving them a fifth nearest neighbor without hydrogen bonds. Then in a real sense there is a difference in the types of molecules in the local environment of any given H.₂Molecule O: Some are part of its H-bond network, some are not (although they appear to be part of a network formed by other molecules).

This picture can be attributed to a situation where the ice is compressed. Ordinary ice has an extraordinary number of other structures, the possibilities of which are opened up by the many ways in which the relatively open network can respond to pressure by increasing its density. Interstitial voids can, for example, collapse, disrupting the H-bond; or, at very high pressures (in the ice-VI and ice-VII phases), the network can rearrange itself, forming two separate interpenetrating sublattices. A disordered version of this interpenetrating structure has been reported for high-density amorphous ice.⁶ In this story, if you want to understand what high pressure water is, start with ice.

There is a consequence to all this. Like most liquids, water is generally considered incompressible. But this ability to squeeze molecules into voids gives it much more freedom than others if the pressure is high enough. Indeed, it shows an unusually large increase in density of about 25% at 10 kbar. Waterjet cutting tools come close to this, operating at pressures of up to 6 kbar and creating a much denser fluid for the punch, which can cut through metal plates using suspended abrasive particles.