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Beauty products and stardust

Supercritical fluid extraction with CO2 for very fine separation and precious substances

It may be named after the Roman goddess of love and beauty, but its surface is far from inviting. The planet Venus is extremely hot, and its surface pressure is akin to that at the bottom of the sea. Under these unpleasant conditions, carbon dioxide, which comprises 96 per cent of the Venusian atmosphere, takes on its “supercritical form”, in which it has both gaseous and liquid properties. While this does not help make Venus any more attractive, at least supercritical CO2 is a substance that is used to enhance beauty here on Earth.

The process is called supercritical fluid extraction, also known as SFE. It offers undreamt-of possibilities for the selective separation of substances without affecting the properties of the materials involved. The method was first used on a large scale for decaffeinating coffee. Coffee is a sensitive plant-derived product with countless constituents, of which only one has to be removed in decaffeination. Solvents which are damaging to health can be ruled out as they would make the product undrinkable. Other available methods would affect the complex aroma too much. The CO2 used in supercritical fluid extraction, on the other hand, can selectively remove only the caffeine without affecting any of the other components.

If a gas is heated beyond the so-called critical point and subjected to pressure in excess of the critical pressure, it is transformed into the supercritical state. In this state, it is no longer possible to determine whether it is gaseous or liquid, because supercritical fluids have the high density of a liquid and the low viscosity of a gas. The transition to this state entails a sharp increase in solvency by an order of magnitude.

More often than not, CO2 is used for SFE. The gas reaches its critical point at just 31 degrees Celsius and a pressure of 74 bar. The substance to be processed therefore only needs to be heated to slightly above room temperature – an important advantage of this gas, not just for sensitive plant products. After extraction, it is vaporised entirely and can subsequently be used again as a solvent in a closed-loop system.

Another advantage of the SFE method is that it can be adjusted with great precision for different substances, explains Professor Béla Simándi, who leads the supercritical fluid extraction research team at Budapest University of Technology. “With medicinal plants and herbs, for example, we can separate essential oils and other oily constituents from one another very easily. We only need to adjust the pressure and temperature to do this. The conventional method would require multiple changes of solvents.”

The production of liquid herbs and herbal extracts for cosmetics and natural remedies is one of the main areas of the Budapest SFE team’s work. “The extracts retain their properties – aroma, taste and colour – for a very long period of time,” explains the head of research. “They can be admixed more precisely and evenly and allow consistent results to be achieved, with formulations that are independent of the year of production.”

Supercritical fluid extraction even allows enantiomers to be separated. These are substances which, while they have an identical molecular structure, differ only in the mirror-image arrangement of their atoms, such as laevorotatory and dextrorotatory lactic acids. The Budapest research team is the first to have used the SFE method for separating enantiomers, which are also vitally important for the production of medicines. “Generally, only one of the two enantiomers of an active substance is responsible for the curative effect,” explains Dr. Edit Székely, a member of the SFE research team. “The other is at best neutral, at worst harmful.”

Supercritical fluid extraction is now used in many different fields, for example in the chemical and petrochemical industries, in biotechnology, paper production and environmental protection. It facilitates the regeneration of activated carbon filters, the treatment of waste oil or pharmaceutical wastewater and the decontamination of contaminated soils.

Another area of application is the production of so-called aerogels. In this process, the liquid component of a gel is replaced by a supercritical gas, which subsequently vaporises. This leaves highly porous solids in which up to 99.98 per cent of the volume consists of pores. They have the lowest density we have so far encountered in solids, making them extremely lightweight and very robust at the same time. Their fine structure makes them suitable as a collecting matrix for very fine dust particles, among other things. That is why they were used on board the Stardust space probe to capture comet dust. The dust particles and molecules can be trapped without being thermally destroyed thanks to the gradual deceleration that is made possible by the aerogel. For the first time, therefore, it has been possible to collect cometary material and bring it back to Earth intact.

Overview of the advantages of this method

  • Safe solvent with no health risks
  • Solvent properties can be varied by altering pressure and temperature
  • Selective separation even of nonvolatile substances possible
  • Aroma-preserving separation of natural substances
  • Gentle extraction of temperaturesensitive constituents
  • CO2 is inert and does not react with the product
  • Non-flammable, non-explosive
  • No environmental impact, no emission problems thanks to a closed-loop system

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