Gaseous and Liquid Nature of Supercritical Fluids



A study conducted by researchers from the National research council of Italy (Cnr) Iom and Ino Institutes, Sapienza University of Rome, University of Edinburgh, and Institut Laue Langevin (ILL) describes the "supercritical" phase of matter, which is still not well understood: it occurs when a substance, either liquid or gaseous, is at temperatures and pressures higher than its critical point, exhibiting behavior considered hybrid between that of a liquid and that of a gas. Understanding this phase has direct implications both in various industrial processes with a high degree of sustainability - for example, in the pharmaceutical and biotechnology fields - and in the study of planets: the supercritical phase characterizes planets such as Venus, Jupiter, and Neptune, and could also be the thermodynamic condition capable of favoring the existence of life on exoplanets (i.e., planets outside the solar system). In particular, the possible existence of thermodynamic regions in the supercritical phase where the nature of either liquid or gaseous type prevails has attracted considerable interest in recent years.

In the study published in Nature Communications, the group investigated the diffusion of molecules within a supercritical fluid and obtained experimental evidence that molecular diffusion changes, passing from a gaseous-like to a liquid-like behavior around the Widom line, a thermodynamic line that extends the curve of saturated vapor above the critical point. This result was obtained through a quasi-elastic neutron scattering experiment on supercritical methane conducted at the ILL neutron source. The neutron beam was directed at a cell containing methane under supercritical conditions, and the intensity of the neutron beam scattered by the sample was measured as a function of the energy exchanged in the quasi-elastic regime, i.e., in the energy region where molecular diffusion phenomena occur within the material. The measurements were carried out along an isothermal path at T=200 K (critical temperature=190 K), varying the methane pressure from a few bars up to about 3 Kbar (critical pressure=45 bar).

The authors of the research emphasize the unequivocal nature of the experimental evidence: "While at pressures lower than about 50 bar, we observe the signal of diffusive dynamics typical of gaseous systems, we could observe that as the pressure increases beyond 50 bar, the signal progressively evolves to assume the typical shape of liquids", explains Alessio De Francesco (Cnr-Iom and ILL), while Mario Santoro and Federico Gorelli (Cnr-Ino) clarify that: "Associating this peculiarity of the supercritical phase with a well-defined thermodynamic line emanating from the critical point provides an important interpretive key for modeling this elusive phase".

The result was made possible thanks to measurements performed at a high-flux neutron source such as ILL, with which Cnr has specific collaboration agreements: "These measurements are at the limits of current experimental possibilities and were unconceivable until a few years ago. As often happens in research, having opened a door means glimpsing new avenues to explore, and this objective can only be pursued thanks to access to large research facilities: we hope for the future that an increasing number of Italian users can access this type of instrumentation", concludes Ferdinando Formisano (Cnr-Iom)".

Per informazioni:
Ferdinando Formisano
Cnr-Iom and ILL

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