Superheated alcohols are liquid alcohols under pressure above their boiling points. They are much less polar than they are at room temperature. As a result they can extract lipid materials from biomass. This is often carried out by industrial organic solvents such as hexane. Instead the extraction can be carried out using benign solvents, both from the point of view of the environmental impact and solvent residues in the product. In addition novel compounds of interest can be obtained from a particular biomass.
The motivation behind this development was to find environmental solvents, which can extract lipid materials from plant materials. We have experience in using superheated water (liquid water under pressure above 100˚C) and wrote an article for Personal Care in February 2012.1 Molecular association by hydrogen bonding increases the polarity of a solvent. This is particular the case in liquid water where there is an extensive intermolecular structure. When water is heated, this hydrogen-bonded structure breaks up causing a reduction in polarity as measured by the dielectric constant strictly called nowadays the relative permeability. We will call it here by the older term: dielectric constant.
The dielectric constant of water is 78 at room temperature and falls to 35 at 200˚C. This enables it to dissolve organic molecules if they are slightly polar or polarizable. However, it will not dissolve completely non-polar molecules. We therefore decided to carry out research on superheated alcohols.
Alcohols form dimers by hydrogen bonding and these break up at about 125˚C.2
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