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

The phase behavior of the liquid CD 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 CD molecules above the gas-liquid line and in the supercritical fluid regime from just below the critical point for all temperatures at about Δ = 10 bar above the Widom line. This line is interpreted as the Frenkel line. These results are consistent with our previous studies on CO 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.