What is supercritical steam? While a supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. So, supercritical steam is water at a temperature and pressure above its critical point (>647.096 K, >22.064 MPa), where distinct liquid water and steam phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid. In addition, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a supercritical fluid to be ‘fine-tuned’.
Above the critical point there is a state of matter that is continuously connected with (can be transformed without phase transition into) both the liquid and the gaseous state. It is called supercritical fluid. All differences between liquid and vapor disappear beyond the critical point.
This can be seen in the water phase diagram above, beyond the critical point (>647.096 K, >22.064 MPa) in the liquid-vapor space on the right. At the critical point there is about 30% free monomeric H2O molecules and about 17% hydrogen bonding. As with other supercritical fluids. supercritical water has no surface tension with its gas or liquid phase or any other supercritical phase as no such interfaces exist. Above 647.096 K, water vapor cannot be liquefied by increasing pressure.
The properties of supercritical water are very different from ambient liquid water. For example, supercritical water is a poor solvent for electrolytes. However, it is such an excellent solvent for non-polar organic molecules, due to its low relative permittivity (dielectric constant) and poor hydrogen bonding, that many are completely miscible. Viscosity and dielectric both decrease substantially whereas auto-dissociation increases substantially. The physical properties of water close to the critical point (near-critical) are particularly strongly affected by density. An extreme density fluctuation around the critical point causes opalescent turbidity. At the critical point, only one phase exists. The heat of vaporization is zero.
As pointed out in numerous studies, water’s properties exhibit dramatic changes under supercritical conditions: the fraction of hydrogen bound (HB) molecules greatly decreases with respect to ambient P and T, and there appears to be a consensus on the persistence of some hydrogen bounds up to at least 600 °C and 134 MPa (3). The hydrogen bound network, where present, is substantially distorted. Whether supercritical water is homogeneous (or composed of patches of clearly defined HB regions and non-HB ones), and over which length-scale possible density heterogeneities might appear, are still matters of debate. The presence of inhomogeneous patterns in the density has long been a contentious topic for the liquid at ambient conditions as well. Again, a tenuous consensus is forming in the scientific community around the idea that possible heterogeneities represent transient rather than equilibrium states of the liquid or supercritical fluid.