ABSTRACT
Unlike that of liquid solvents, the density of SCFs can be changed by altering the pressure and temperature of the fluid and therefore the dissolution power of a SCF can be enhanced. SCFs can have gas-like diffusivities, which have important implications for reaction kinetics, as well as liquid-like densities, which allow the deliverance for many compounds.[4]
The most widely used SCF is carbon dioxide because of its relatively low critical pressure and temperature (Tc of 31.11C and Pc of 7.38 104MPa, respectively) and because it is relatively non-toxic and inflammable.[7] Table 1 presents commonly used SCFs and their related critical values. Consideration of the properties of supercritical CO2 suggests that it is suitable for extraction of different materials and it can be used as a reaction medium for polymerization. Supercritical carbon dioxide is an excellent non-polar solvent for many organic compounds; it has solvent power similar to a light hydrocarbon for most solutes. However, fluorinated compounds are often more soluble in supercritical CO2 than in hydrocarbons. This increased solubility is important for polymerization reactions (such as biopolymers or scaffolds for tissue engineering). Organic compounds such as alkenes, alkanes, aromatics, ketones, and alcohols up to a relative molecular mass of around 400 dissolve in SCF CO2. Very polar molecules, such as amino acids, sugars, and most inorganic salts, are insoluble in this fluid.[4] However, there are some disadvantages in the usage of SCF technology as a reaction medium, such as expensive equipment and a complicated system thermodynamics.[5]
Another commonly used fluid in supercritical technology is water. Its critical temperature is 3741C and critical pressure is 220 bar. Water has been growing in importance as a medium for chemical reactions recently because of the potential for synthesis of different materials. The most important advantage of supercritical water technology is the possibility of varying the properties of the reaction medium over a wide range, basically by altering the pressure and temperature and optimizing the reaction in this way without changing solvent. Moreover, the reaction kinetics can be influenced in the supercritical region by varying the pressure. Additionally, many non-polar organic substances such as cyclohexane and different kinds of gases are highly soluble in supercritical water (critical pressure and temperature=374 and Pc=2.2 104MPa, respectively) so that mass transfer restrictions according to phase boundaries do not apply. However, there are disadvantages to working at high pressures, such as high investment and possibly energy costs. The other problem is related to corrosion, and expensive alloys frequently are used.
