Asymptotic Widom line of supercritical fluid within the scaling theory

The work is devoted to a theoretical study of fluid thermodynamic features in the supercritical region. The analysis is carried out within the framework of the scaling theory based on the general equation of state of a near-critical fluid. Analytical expressions for fluid characteristics lines on the pressure–temperature plane are obtained. It is shown that close to the critical point the characteristic lines come closer forming the so-called single Widom line. An analytical expression is formulated for the asymptotic Widom line, as a common part of fluid characteristics. These line tracks are found to depend on the initial scaling concept. Widom line turns out to be closest to the critical isochore and is directly related to the coexistence curve of the near-critical fluid. To illustrate the results, asymptotic lines of methane, n-pentane, carbon dioxide, and water are constructed based on the NIST simulator numerical data.