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

## TL;DR

This study identifies solid-like behavior in dense vapors near the critical point by analyzing thermodynamic curvature, revealing correlations with molecular complexity and implications for fluid critical phenomena.

## Contribution

The paper introduces the concept of zero thermodynamic curvature lines as a new feature in the fluid phase diagram, systematically evaluating their presence across many fluids.

## Key findings

- 97 out of 121 fluids exhibit solid-like feature D.
- Feature D correlates with molecular complexity and sound anomalies.
- Solid-like properties may arise from spinning molecule correlations.

## Abstract

The phase diagram (pressure versus temperature) of the pure fluid is typically envisioned as being featureless apart from the presence of the liquid-vapor coexistence curve terminating at the critical point. However, a number of recent authors have proposed that this simple picture misses important features, such as the Widom line, the Fisher-Widom line, and the Frenkel line. In our paper we discuss another way of augmenting the pure fluid phase diagram, lines of zero thermodynamic curvature $R=0$ separating regimes of fluid solid-like behavior ($R>0$) from gas-like or liquid-like behavior ($R<0$). We systematically evaluate $R$ for the $121$ pure fluids in the NIST/REFPROP (version 9.1) fluid database near the saturated vapor line from the triple point to the critical point. Our specific goal was to identify regions of positive $R$ abutting the saturated vapor line ("feature D"). We found: a) $97/121$ of the NIST/REFPROP fluids have feature D. b) The presence and character of feature D correlates with molecular complexity, taken to be the number of atoms $Q$ per molecule. c) The solid-like properties of feature D might be attributable to a mesoscopic model based on correlations among coordinated spinning molecules, a model that might be testable with computer simulations. d) There are a number of correlations between thermodynamic quantities, including the acentric factor $\omega$, but we found little explicit correlation between $\omega$ and the shape of a molecule. e) Feature D seriously constrains the size of the asymptotic fluid critical point regime, possibly resolving a long-standing mystery about why these are so small. f) Feature D correlates roughly with regimes of anomalous sound propagation.