Properties of the supercritical region

Some properties of supercritical fluids endow them with improved extraction capabilities relative to liquids. Their individual influence is discussed below. 

The diffusion coefficients of solutes in supercritical fluids are in between those they possess in liquids and gases. Because diffusion coefficients in SFs are higher than those in liquids, mass transfer is usually more favourable in the former. The diffusion coefficient can also be altered to advantage as diffusivity in a supercritical fluid decreases with increasing pressure and increases with increasing temperature, especially in the vicinity of the critical point. 

Supercritical viscosity lies in between the viscosity of liquids and gases. For this reason, supercritical fluids exhibit more favourable hydrodynamic properties than do liquids. Also, the low surface tension of SFs allows them to readily penetrate porous solids and packed beds. The viscosity of SFs, like that of conventional fluids, is temperature-dependent however, while pressure has little effect on the viscosity of fluids, it exerts a strong influence on that of SFs. As a result, increased pressures lead to increased supercritical viscosity and hence to diminished solute diffusivity and hindered transport phenomena, but also, most often, to increased solubility through decreased density. 

The dielectric constant is one of the most relevant physico-chemical parameters with a view to defining solubility in fluids. Table 7.2 lists the constants for the solvents most 

Substance Dielectric constant (C-/NM=) Substance Dielectric constant (C-/NM-) 

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