Unlocking the secrets of supercritical fluids

Unlocking the secrets of supercritical fluids
by Robert Schreiber
Berlin, Germany (SPX) May 23, 2024

A study now published in Nature Communications provides new insights into the behavior of supercritical fluids, a state of matter between liquids and gases, with applications ranging from the pharmaceutical industry to planetary science. The findings, achieved at the Institut Laue Langevin (ILL), were at the limits of current experimental possibilities.

A supercritical fluid is a substance that has been pushed beyond its critical point, where the distinction between liquid and gas disappears. These fluids can effuse like a gas and dissolve materials like a liquid, making them valuable in various industrial applications, including pharmaceutical processing and coffee bean decaffeination. They are also important for understanding giant planets like Jupiter and Neptune, where similar states of matter may exist.

An international research team from Sapienza University, Institute Laue Langevin, Ecole Polytechnique Federal, CNRS, and CNR has experimentally confirmed that molecular diffusion in a supercritical fluid transitions from gas-like to liquid-like behavior across the Widom line, a thermodynamic boundary extending the saturated vapor curve above the critical point. This transition occurs gradually within a narrow pressure range.

The team studied molecular diffusion in a supercritical fluid, focusing on whether there is a pressure-temperature region where the behavior shifts from gas-like to liquid-like. Although theoretical models proposed several transition boundaries, experimental validation was previously elusive.

The results were achieved through high-pressure, quasi-elastic neutron scattering (QENS) experiments on supercritical methane at the ILL. Neutrons were used to explore the material, and the intensity of the scattered neutron beam was measured as a function of the energy exchanged, revealing molecular diffusion dynamics. The experiments were conducted at a constant temperature of 200 K, varying the methane pressure from a few bars up to nearly 3 Kbar.

The study's authors highlight the clear experimental evidence: "While at pressures lower than approximately 50 bar the signal of the diffusion dynamics typical of gaseous systems is observed, we have been able to observe that as the pressure increases above that, the signal evolves progressively until it takes on the typical shape of liquids," explained Alessio De Francesco, a researcher at CNR and ILL.

The findings were made possible by the high flux neutron source and unique experimental support facilities at the ILL. "These measurements are at the limits of current experimental possibilities, and were unthinkable until a few years ago," added Ferdinando Formisano, a researcher at CNR and ILL. "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."

Research Report:Crossover from 'Gas-like' to 'Liquid-like' Molecular Diffusion in a Simple Supercritical Fluid