Influence of Critical Fluctuations on the Thermodynamic and Transport Properties of Supercritical Fluids: Nonclassical Crossover Equations of State: A Review

Based on the hypothesis of scale invariance (scaling), the review describes the main provisions of the modern nonclassical theory of critical phenomena in pure fluids. A detailed description is given to the main types (parametric and six-term or renormalized Landau expansion) of theoretically substantiated crossover equations of state (EoSs) of pure fluids and their application to describe the anomalous behavior of thermodynamic properties in sub- and supercritical fluids. It is shown that the crossover EoS model covers all the characteristic features of the scaling behavior of the thermodynamic properties of pure fluids in the asymptotic region of the critical point in the form of simple power laws with universal critical exponents and transforms into the classical EoSs (in particular, the Landau expansion) with distance from the critical (fluctuation) region. A detailed comparison is made between the predictions of the crossover EoS model and experimental data on the thermodynamic properties for a representative set of supercritical fluids in a wide range of temperatures and pressures. The crossover EoSs of pure fluids are used to quantitatively estimate the boundary of the region of influence of critical fluctuations on the thermodynamic properties, i.e., to evaluate the contribution of the fluctuation component to the experimentally observed anomalous enhancement of the thermodynamic properties of supercritical fluids. An interpretation of supercritical phase transitions (Widom lines) is given based on the concept of large-scale critical fluctuations and crossover theory. Dynamic crossover phenomena are also considered to describe the influence of fluctuations on the critical enhancement of transport properties (thermal conductivity, thermal diffusivity, and viscosity) in sub- and supercritical fluids.