Supercritical liquid–gas boundaries

According to textbook knowledge, it is possible to transform a liquid continuously into a gas, without undergoing a phase transition, by heating and compressing strongly enough to go around the critical point.

The Fisher–Widom line[1] is the boundary between monotonic and oscillating asymptotics of the pair correlation function

[2] However, it was first measured experimentally in 1956 by Jones and Walker,[3] and subsequently named the 'hypercritical line' by Bernal in 1964,[4] who suggested a structural interpretation.

The Frenkel line is a boundary between "rigid" and "non-rigid" fluids characterized by the onset of transverse sound modes.

The second criterion is based on the fact that at moderate temperatures liquids can sustain transverse excitations, which disappear upon heating.

The isochoric heat capacity per particle of a monatomic liquid near to the melting line is close to

Therefore at the Frenkel line, where transverse excitations vanish, the isochoric heat capacity per particle should be

[8][9][10] Anisimov et al. (2004),[11] without referring to Frenkel, Fisher, or Widom, reviewed thermodynamic derivatives (specific heat, expansion coefficient, compressibility) and transport coefficients (viscosity, speed of sound) in supercritical water, and found pronounced extrema as a function of pressure up to 100 K above the critical temperature.

supercritical gas-liquid boundaries in pT diagram