This article is from the magazine Les Indispensables de Sciences et Avenir n°210 dated July/September 2022.
Drinking a decaffeinated coffee, delighting in a perfume, tasting a wine without the taste of a cork… These moments of pleasure would not exist without the surprising so-called “supercritical” fluids. To understand what the supercritical state of matter is, the easiest way is to look at the phase diagram of a pure chemical substance, which describes its state – liquid, solid or gas – depending ambient temperature and pressure.
By increasing these two parameters, we reach a “critical point” beyond which the boundary between liquid and gas ends up disappearing. “The body is then said to be supercritical. It has both the properties of a liquid and those of a gas”, explains Stéphane Sarrade, process engineering researcher and director of energy programs at the CEA.
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This article is from the magazine Les Indispensables de Sciences et Avenir n°210 dated July/September 2022.
Drinking a decaffeinated coffee, delighting in a perfume, tasting a wine without the taste of a cork… These moments of pleasure would not exist without the surprising so-called “supercritical” fluids. To understand what the supercritical state of matter is, the easiest way is to look at the phase diagram of a pure chemical substance, which describes its state – liquid, solid or gas – depending ambient temperature and pressure.
Phase diagram of a pure chemical substance, which describes its state – liquid, solid, gas or…both – depending on the ambient temperature and pressure. Credit: Bruno Bourgeois
By increasing these two parameters, we reach a “critical point” beyond which the boundary between liquid and gas ends up disappearing. “The body is then said to be supercritical. It has both the properties of a liquid and those of a gas”, explains Stéphane Sarrade, process engineering researcher and director of energy programs at the CEA. Dense like a liquid, but slightly viscous, it flows easily and can also diffuse like a gas, in a porous medium for example. A godsend for the chemical industry, which uses it in many applications.
“The CO2 supercritical energy interested chemists very early on”
The oldest is that of CO2 supercritical, whose critical point, at 31°C and 73 bars of pressure, can easily be reached. “The CO2 Supercritical energy interested chemists very early on, because it has a solvent power: from a mixture of molecules, it makes it possible to isolate the one that interests us. This is particularly the case with the caffeine in coffee., emphasizes Stéphane Sarrade. Until the 1970s, decaffeinated coffee was produced using organic solvents such as trichlorethylene or chloroform. Problem: their residues are toxic to the body. Industrialists then turned to CO2 supercritical.
For twenty years, it has also been used to extract a molecule present in cork, trichloroanisole, responsible for the unpleasant cork taste of certain wines. “By varying the temperature and pressure, and adding a little water or ethanol, we can control the solvent power of CO2 supercritical, and therefore its density, in order to extract a molecule given. This is how we obtain a number of active ingredients from plants in perfumery and how, more broadly, we do ‘green chemistry’ without organic solvents”, explains the researcher. CO2 supercritical is not only used to extract: molecules can also be dissolved in it to impregnate wood in order to reinforce its structure, or even used to dye clothes.
Naturally supercritical water
In the same way, water can also be brought to a supercritical state: it makes it possible, for example, to make the polyethylene in plastic bottles soluble in order to recycle them; or to process liquid radioactive waste by compacting it in mineral form. It is even found… in its natural state. Its critical point of 374°C and 220 bars is indeed reached at the bottom of the oceans, several thousand meters deep, where volcanic chimneys bring the water to high temperature.
These “black smokers” – so called because the effluents are darkened by dissolved minerals – are of great interest to biologists. In 2002, a team of Japanese researchers subjected a solution of glycine, an amino acid, to supercritical water in the laboratory. And find that the wisteria has come together in two or three copies. However, the assembly of amino acids is the first step in the synthesis of proteins, a fundamental phenomenon of life… Supercritical water could therefore have played a role as a solvent at the origins of life.
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