Unlocking the secrets of supercritical fluids

A study now published in Nature Communications brings remarkable insights into the enigmatic behavior of supercritical fluids, a hybrid state of matter occupying a unique space between liquids and gases, and arising in domains that go from the pharmaceutical industry to planetary science. The obtained results are at the limit of current experimental possibilities and could only be obtained in a high flux neutron source such as the ILL.

A liquid or gaseous substance pushed beyond its critical point (i.e., beyond the temperature and pressure at which the distinction between liquid and gas can no longer be made) is called a supercritical fluid. Still little-known and defying conventional classifications, supercritical fluids possess the ability to effuse like a gas while dissolving materials like a liquid. This duality has made them invaluable in a myriad of industrial applications, from pharmaceutical processing to decaffeinating coffee beans. On the other hand, they are crucial to understand giant planets such as Jupiter and Neptune, where similar states of matter may reign.

In a study now published in Nature Communications, an international team of researchers from Sapienza University (Rome, Italy), Institute Laue Langevin (Grenoble, France), CNR (Italy), Ecole Polytechnique Federal (Lausanne, Switzerland), CNRS (France), University of Edinburgh (UK) and HPSTAR (Shanghai, China) obtained experimental proof that molecular diffusion in a supercritical fluid switches from gaseous-like behaviour to liquid-like behaviour across the so-called Widom line (a thermodynamic line that extends the saturated vapor curve above the critical point). The transition is gradual within a narrow pressure range.

Umbertoluca Ranieri (Università di Roma La Sapienza and University of Edinburgh) and co-workers measured the molecular diffusion coefficient of a supercritical fluid – a crucial parameter reflecting the mobility of molecules within the fluid – with a fundamental question in mind: can we pinpoint a physical observable and a region of pressure-temperature where the behaviour of a supercritical fluid goes from gas-like to liquid-like? While theoretical models have proposed various such transition boundaries (among them the Widom line) experimental validation had remained, until now, elusive.

The result was obtained through challenging, high-pressure, quasi-elastic neutron scattering (QENS) experiments on supercritical methane conducted at the Institut Laue Langevin, in Grenoble. At the ILL, neutrons are used to explore materials and processes in a very wide range of domains. In this study, a neutron beam was sent onto a cell containing methane in supercritical conditions at constant temperature T=200 K (above the critical T=190 K) varying the pressure from a few bars up to kbars (the critical pressure is P=45 bar). The intensity of the neutron beam scattered by the sample was measured as a function of the energy exchanged in the range of interest (i.e., in the energy range where molecular diffusion phenomena within matter occur, the so-called quasi-elastic regime).

The authors underline the striking observation: while at pressures lower than approximately 50 bar the signal of the diffusion dynamics typical of gaseous systems is observed, as the pressure increases it evolves progressively until it takes on the typical shape of liquids. The result was made possible thanks to the high flux neutron source and the unique experimental support facilities available at the ILL: "These measurements are at the limits of current experimental possibilities, and were unthinkable until a few years ago” adds Ferdinando Formisano (researcher at CNR and ILL), concluding: “As often happens in research, having opened a door means seeing new avenues to explore, and this objective can only be pursued thanks to access to large research facilities.”

As for the breadth of the results, Livia E. Bove (research director at CNRS and EPFL) explains: "Our findings not only advance our understanding of supercritical fluid dynamics but also hold significant implications for planetary science, where similar states of matter may govern the behaviour of gaseous giants and exoplanets. In gas giants, the existence of a non-homogeneous super critical state would indeed influence whether a boundary between the planets’ interior and their atmosphere can be defined. This would impact planetary properties such as the thermal conductivity and other heat-related physical phenomena like storm activity, as well as mass diffusion related properties like the ionic conductivities and the consequent generation of anomalous magnetic fields. By unraveling the complexities of these hybrid states, we move one step closer to understanding the farther planets of our Solar System and the fundamental properties of condensed matter."