Advanced #6

The structure of liquid water changes at high pressure

Under high pressure, the short-range order of liquid water changes fundamentally.

Scientific Explanation

Under ambient conditions, liquid water forms a loose, tetrahedral network: each molecule is surrounded on average by about four neighbors connected through hydrogen bonds. This open structure resembles that of ice Ih and is responsible for many of water’s unusual properties.

When the pressure is raised to several thousand atmospheres (the gigapascal range), this tetrahedral ordering collapses. Additional molecules are forced into the gaps of the network as so-called interstitial molecules. The number of nearest neighbors rises from about four to eight or even twelve. Hydrogen bonds become distorted and partially broken, and the structure begins to resemble that of a dense, ordinary liquid.

Remarkably, this transition does not happen abruptly but proceeds continuously. With increasing pressure, the equilibrium shifts from the open, tetrahedral arrangement toward the denser, less ordered structure. X-ray and neutron diffraction experiments reveal how the radial distribution functions — describing the probability of finding a neighboring molecule at a given distance — change systematically with pressure.

Water Structure Change Under High Pressure Schematic showing how the tetrahedral hydrogen bond network of liquid water collapses under high pressure, with the first coordination shell shifting from about 4 neighbors to about 8-12 neighbors as interstitial molecules fill the gaps. Low Pressure O ~4 neighbors tetrahedral High P High Pressure O ~8-12 neighbors collapsed network H-bonded neighbors Interstitial molecules Water Structure Under Pressure
Structural transformation of liquid water under pressure: from a tetrahedral network to a collapsed arrangement with interstitial molecules.

Everyday Relevance

This structural transformation explains why water loses many of its anomalies at high pressure — it increasingly behaves like a “normal” liquid. In the deep ocean, where pressure rises by about one atmosphere for every ten meters of depth, water properties begin to measurably change. For the geosciences, this behavior is important because water exists under extremely high pressures in the Earth’s crust and upper mantle, where it influences chemical reactions and mineralogy.