# Thermal Density Fluctuations and Polymorphic Phase Transitions of Ethane (C2D6) in the Gas/Liquid and Supercritical States

## TL;DR

This study uses neutron scattering to explore the phase behavior of ethane near and above its critical point, revealing universal patterns and a new interpretation of the Frenkel line.

## Contribution

The paper provides new insights into the Frenkel line as a polymorphic phase transition boundary, supported by experimental data on ethane.

## Key findings

- Thermal density fluctuations and droplet formation were observed in ethane near the critical point.
- The Frenkel line is interpreted as a polymorphic phase transition boundary, not just a dynamic process limit.
- Correlation lengths of density fluctuations suppress droplet formation until the Frenkel line is crossed.

## Abstract

The phase behavior
of the liquid C2D6 below
and above the critical point was investigated using small-angle neutron
scattering (SANS) in temperature and pressure ranges from 10 to 45
°C and 20 to 126 bar, respectively. The scattering of thermal
fluctuations of the molecular density was determined and thus the
gas–liquid and Widom lines. At the same time, we observed additional
scattering of droplets of more densely packed C2D6 molecules above the gas–liquid line and in the supercritical
fluid regime from just below the critical point for all temperatures
at about ΔP = 10 bar above the Widom line.
This line is interpreted as the Frenkel line. These results are consistent
with our previous studies on CO2 and thus indicate a universal
phase behavior for monomolecular liquids below and above the critical
point. The interpretation of the Frenkel line as the lower limit of
a polymorphic phase transition is in contrast to the usual interpretation
as the limit of a dynamic process. The correlation lengths (ξ)
of the thermal density fluctuations at the critical point and at the
Widom line are determined between 20 and 35 Å and thus in the
range of the droplet radius between 60 and 80 Å. These long-range
fluctuations appear to suppress the formation of droplets, which can
only form at about 10 bar above the critical point and the Widom line
when ξ becomes smaller than 10 Å.