Chemistry Reference
In-Depth Information
Solid
phase
Compressible
liquid
Supercritical
fluid
Critical point
Liquid
phase
Superheated
vapour
Triple point
Gaseous phase
Tp
Tcr
Temperature
Figure 7.8
Phase diagram.
the simplicity of concentrating and removing the extracting agent from the extract;
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the possibility of analysing the whole extract, not only a small part of it [77].
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7.3.3
Subcritical water extraction
Water is probably the most common chemical compound in human experience and also the most necessary. Water
is a very special solvent because of its unique structure, which includes vast numbers of hydrogen bonds. At room
temperature it has a very high boiling point for its mass, a high dielectric constant and a high polarity. But when
water is heated, its properties change markedly: the hydrogen-bonded lattice is disrupted with the increase in
thermal motion. As the temperature rises, permittivity drops markedly and systematically, the diffusion rate
increases, and viscosity and surface tension decrease. At elevated temperatures and moderate pressures the polarity
of water falls significantly, so that it acts like a typical organic solvent, for example methanol, ethanol or acetone;
water can therefore replace organic solvents for the extraction of semi- or poorly volatile compounds [81].
Superheated water is a general term for denoting the region of the condensed phase between 100°C and the
critical point. This phase is often described as subcritical water, but by analogy with supercritical fluid
extraction, the term should preferably be employed for the region close to the critical point. Superheated
water extraction is also called 'subcritical water extraction' (SWE), 'hot water extraction' (HWE) or
'pressurized water extraction' (PWE).
First reported by Hawthorne in 1994 as an environmentally friendly solvent, superheated water can be
effective for extracting a wide variety of organic compounds from diverse matrices but not for many other
organic compounds, which because of their polarity are very poorly soluble in water [82]. It has been used to
extract pesticides and polycyclic aromatic hydrocarbons, isoflavones [83], lignans [84], saponins [85]
anthocyanins [86], dioxins, PCBs and pesticides [87]. As the temperature of liquid water is raised under
pressure, the polarity decreases and it can be used as an extraction solvent for a wide range of compounds [88].
