Modified Landau model for fluids: A rethink of pseudoboiling theory for supercritical fluids

A new model based on Landau theory is proposed that can quantitatively predict the density, isobaric specific heat capacity distortions of supercritical fluids. Unifying supercritical fluids into classical Landau gas-liquid transition theory, this model gives reasonable explanations for subcritical gas-liquid transition and critical/pseudo-critical phenomena. The results indicate that as inner fluctuations diminish away from the critical point, macroscale mean field approximations are valid, although microscale inhomogeneity exists. Based on the model, disputable pseudoboiling theory is reanalyzed. The thermodynamic stabilities of the supercritical and subcritical transitions are essentially different. Although transitions between the gas-like regime and liquid-like regimes exist, the separation and coexistence of these two regimes on a macroscopic scale never occur in supercritical fluids. For most applications, supercritical fluids can be regarded as single-phase fluids with anomalous properties. In the vicinity of the critical point, the classical description is invalid as the correlation length is on the macroscopic scale.