The Supercritical State of Water: Between Liquid and Gas, a Fourth Phase?

A little-known but fundamental phase

The supercritical state of water, reached at a temperature of 374 °C and a pressure of 22.06 MPa (≈217.8 atm), is a thermodynamic domain in which water no longer distinguishes between liquid and gas. In this zone, it has both the density of a liquid and the diffusivity of a gas. This hybrid behavior leads to unique physical and chemical properties, making it highly reactive, excellent for dissolving organic and inorganic substances, and widely used in geochemistry, industrial processes, and even in the exploration of prebiotic conditions.

Physical properties of supercritical water

Unlike liquid water or steam, supercritical water exhibits significant density fluctuations on a small scale, which alters dielectric constants, ionic solvation, and thermal transport mechanisms. It can dissolve salts like a liquid but behave like a gas for non-polar species. Its thermal conductivity is high, its viscosity low, and its surface tension is zero. These characteristics make it an excellent working fluid or green solvent in sustainable chemistry processes.

Where to find supercritical water on Earth?

The supercritical state of water can only exist at pressures greater than 217 atm and temperatures above 374 °C. These conditions are naturally found in certain deep geothermal environments, particularly in the oceanic crust and near magma chambers. It can be found in the following areas:

• Deep hydrothermal systems: mid-ocean ridges (such as the Atlantic Ridge) host so-called "black smoker" chimneys, where the pressure of the water column and the proximity of magma allow water to cross its critical point.
• Ultra-deep geothermal drillings: projects like the Iceland Deep Drilling Project (IDDP) have reached zones where water is maintained in a supercritical state at more than 4 to 5 km deep below the Earth's surface.
• Active volcanic fields: in volcanic regions (such as Taupo in New Zealand or Yellowstone in the United States), very high geothermal gradients can generate supercritical zones at relatively shallow depths.

These extreme environments are of major interest both for fundamental research (origin of life, extremophiles, mineralization) and for emerging geoenergy technologies.

Scientific and industrial applications

Supercritical water is found in various fields:

• Deep geochemistry: it plays a role in metamorphism, volcanism, and the transport of heavy metals.
• Waste treatment: in supercritical water oxidation (SCWO), organic pollutants are efficiently destroyed.
• Pharmaceutical and food industry: it allows selective extractions without toxic solvents.
• Planetary exploration: supercritical oceans are suspected beneath icy crusts or on high-gravity exoplanets.

A key phase in the evolution of Earth

The supercritical state of water may have played a role in the origin of life on Earth. In deep hydrothermal environments, supercritical water may have favored complex organic reactions, precursors to the appearance of the first biological molecules. Furthermore, the exceptional solvation and catalysis properties may have supported the development of prebiotic chemistries on the surface or in the Earth's crust.

Comparative table: states of water

Source: NIST Chemistry WebBook, ScienceDirect - Supercritical water oxidation.