Superheated water is liquid water under pressure at above 100°C. It is much less polar than water at room temperature. As a result it can extract organic compounds.
The lower polarity combined with the higher temperature means that extracts are more concentrated and can contain more active compounds. At the same time water is the most benign of extraction media, both from the environmental impact and solvent residues in the product.
Superheated water is regarded as being liquid water under pressure above 100°C and below 374°C, which is its critical temperature, above which the distinction between liquid and gas disappears. It is sometimes called sub-critical water and sometimes pressurised hot water. The use of superheated water is not new. It has been used in the food industry for cooking a little above 100°C and the final extractions of instant coffee are sometimes up to 120°C. The processing of wood pulp is carried out sometimes above 100°C as is the hydrolysis of starch to polysaccharides, which has been carried out at high temperatures for some time. Recrystallisations in water contained in sealed tubes above 100°C have been carried out for 100 years or more. Superheated water has also been used for waste treatment, by the so-called wetair oxidation process.1 Chemical reactions have also been carried out in superheated water and this work has been thoroughly reviewed.2-4 However, in recent years there has been a renewed interest in superheated water as a green replacement for organic solvents in separations and related processes. Over 150 research papers on this subject have been published in the last 10 years. Much of this work has been restricted to the range of 100°C to 300°C. At these lower temperatures water is not highly compressible, and the pressure of the medium does not have much effect, as long as it is high enough to maintain the water in the liquid phase. This work on superheated water has been reviewed.5 Manipulation of water properties with temperature to achieve process ends has been made the subject of a patent.6 Most of the work that has been described is on a laboratory scale and some of it is directed towards analysis. However, some larger-scale processes have been initiated and others are under consideration.
Fall in polarity and enhanced solubility
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