Investigation concerning the uniqueness of separatrix lines separating liquidlike from gaslike regimes deep in the supercritical phase of water with a focus on Widom line concepts

Abstract

The supercritical phase of fluids has long been known to feature significantly different liquidlike and gaslike regimes. However, it is textbook knowledge that the supercritical state is a homogeneous fluid phase where properties change continuously. Nevertheless, there has been an increasing amount of evidence published that suggests that there might exist a unique line that rigorously separates different regimes in supercritical phases, particularly in the case of water. Here, we use the quasiexact IAPWS95 equation of state to rigorously assess the macroscopic thermodynamic properties of supercritical water without invoking any water model or related approximations. We focus on how these properties change deep in the supercritical phase, in particular if they allow one to introduce a unique "thermodynamic separatrix." Our rigorous thermodynamic analysis, which relies exclusively on accurate experimental data, makes clear that there is no unique separatrix in real supercritical water-such as the recently much-invoked "Widom line." A comparison to the van der Waals equation of state reproduces qualitatively all our findings for real water, thereby suggesting that our analysis should be transferable to other fluids and critical points. Topological analysis of the H-bond network structure of supercritical water, as obtained from molecular-dynamics simulations using a standard water model, demonstrates that also the percolation line does not provide a meaningful separatrix to rigorously distinguish liquidlike from gaslike regimes.

Polymorphic phase transition in liquid and supercritical carbon dioxide

“…The conclusion of Ref. 34 is consistent with the results of the present article, namely that the Frenkel line represents the borderline of the polymorphic phase transition. This observation may provide a new basis for a discussion of the Frenkel line in SCFs.…”

“…From the outline above, it might be not surprising to read in a recent article 34 that on the basis of thermodynamic and structural properties a “unique and sharp separatrix” which distinguishes liquid-like and gas-like phases is “not supported” in supercritical water and other SC fluids. “Percolation may be only relevant in water due to H-bond network clustering”.…”

Supercritical Water is not Hydrogen Bonded

“…These dynamical properties complement the analyses of intermolecular H‐bond vibrations and have been proven valuable to investigate SCW by FFMD and AIMD simulations, see for example, refs. . Taking into account the well‐known ambiguities in the selected H‐bond criterion, existing studies nevertheless broadly agree that the continuous H‐bond lifetime τ HB is more than one order of magnitude smaller compared to RTW and amounts to about 100 fs in SCW including long‐time tails.…”

Supercritical Water is not Hydrogen Bonded

“…These dynamical properties complement the analyses of intermolecular H‐bond vibrations and have been proven valuable to investigate SCW by FFMD and AIMD simulations, see for example, refs. . Taking into account the well‐known ambiguities in the selected H‐bond criterion, existing studies nevertheless broadly agree that the continuous H‐bond lifetime τ HB is more than one order of magnitude smaller compared to RTW and amounts to about 100 fs in SCW including long‐time tails.…”

“…SCW is even envisaged as a mediator in nuclear power plants . Moreover, SCW and supercritical fluids in general, attracted interest in fundamental physics in view of putative transitions from liquid‐like to gas‐like regions discussed in terms of separating Widom, Frenkel or percolation lines . In nature SCW occurs in the earth mantle where it takes an active part in hydrothermal formation processes and it could be discovered close to so‐called “black smokers” at the bottom of the deep sea .…”