Solid-like features in dense vapors near the fluid critical point

Area of Science:

• Thermodynamics
• Fluid Mechanics
• Physical Chemistry

Background:

• Pure fluid phase diagrams are typically simplified, often omitting complex behaviors beyond the liquid-vapor curve and critical point.
• Recent studies suggest additional features like the Widom, Fisher-Widom, and Frenkel lines exist.
• Thermodynamic curvature (R) offers a new metric to identify distinct fluid regimes.

Purpose of the Study:

• To investigate lines of zero thermodynamic curvature (R = 0) as a method to augment pure fluid phase diagrams.
• To identify regions of positive thermodynamic curvature (feature D) adjacent to the saturated vapor line.
• To correlate the presence and characteristics of feature D with molecular complexity and other thermodynamic properties.

Main Methods:

• Systematic evaluation of thermodynamic curvature (R) for 121 pure fluids using the NIST/REFPROP database (version 9.1).
• Analysis of R values near the saturated vapor line, from the triple point to the critical point.
• Correlation analysis between feature D and molecular complexity (number of atoms per molecule, Q), acentric factor (ω), and sound propagation regimes.

Main Results:

• Feature D was identified in 97 out of 121 fluids (97/121).
• The existence and nature of feature D correlate with molecular complexity (Q).
• Feature D significantly constrains the size of the asymptotic fluid critical point regime and correlates with anomalous sound propagation.

Conclusions:

• Lines of zero thermodynamic curvature provide a novel way to characterize pure fluid phase diagrams.
• Feature D, representing fluid solid-like behavior, is prevalent and linked to molecular structure.
• The findings may resolve mysteries regarding the small size of asymptotic fluid critical point regimes and explain anomalous sound propagation.