ABSTRACT
Supercritical fluids exhibit important characteristics such as compressibility, homogeneity, and a continuous change from gas-like to liquid-like properties. Lower pressures are important in practice also because the conditions are relevant to separation stages in supercritical processes. For the extraction and chromatography of more polar molecules, it is common to add polar modifiers, such as the lower alcohols. In the cases, it is important to be aware of the modifier-fluid phase diagram to ensure that the solvent is in one phase. The behaviour of density, as well as all other thermodynamic functions, as a function of pressure and temperature can be predicted by an equation of state. However, diffusion coefficients tend to zero at the critical point and fall in the critical region around it. An alternative approach, which has been used for both correlation and prediction, is to use the concept of solubility parameters as used in liquids.
