Advantages of using supercritical fluids

Supercritical solvents can be used to adjust reaction rate constants (k) by as much as two orders of magnitude by small changes in the system pressure. Activation volumes (slopes of In k vs P) as low as —6000 cm3/mol were observed for a homogeneous reaction (97). Pressure effects can also be pronounced on reversible reactions (17). In one example the equilibrium constant was increased from two- to sixfold by increasing the solvent pressure. The choice of supercritical solvent can also dramatically affect an equilibrium constant. An obvious advantage of using supercritical fluid solvents as a media for chemical reactions is the adjustability of the reaction kinetics and equilibria owing to solvent effects. 

Debenedetti and coworkers (88,89) provided one of the first examples of microencapsulation of a drug in the polymeric matrix. Richard and coworkers (90) provided a recent example of the microencapsulation process when they produced microparticles with the encapsulated model protein that showed sustained release. Foster and coworkers (87) also reported precipitation of copper-indomethacin by PVP with a 96-fold enhancement in the dissolution rate of indomethacin. These examples clearly demonstrated the advantages of using supercritical fluid processing for the preparation of polymer-drug formulations with potentially improved therapeutic properties. 

The advantages of using supercritical fluids over classical procedures, such as higher yields or selectivities, should be emphasized. 

One of the main advantages of using supercritical fluids as solvents is their abihty to dissolve large quantities of gases. Carbon dioxide and hydrogen, for example, mix in all proportions at temperatures above the critical temperature of CO2 (304.1 K). 

A major advantage of supercritical-fluid chromatography (SFC) is that it offers the advantage of liquid-like solubility, with the capability to use a nonselective gas-phase detector such as flame ionization detector. Other advantages of using supercritical fluids for extractions are that they are ine ensive, contaminant free, and less costly to dispose safely than organic solvents. 

Compressed gases (such as carbon dioxide) can also be used effectively as solvents for extraction of coal. Toluene, dodecane, p-cresol, etc. can be applied under supercritical conditions. Some advantages of using supercritical fluid as solvents are (1) an extract with low molecular weight (approximately 500) and higher hydrogen content can be obtained, (2) solvent recovery is easy, and (3) the reduction in the operating pressure or temperature precipitates the extract. 

SA system, the advantage of using supercritical fluids for polymer modification is less pronounced than expected. 

A second important property of many supercritical fluids is that analytes dissolved in them can be easily recovered by simply allowing the solutions to equilibrate with the atmosphere ai relatively low temperatures. Thus, an analyte dissolved in supercritical carbon dioxide, the most commonly used solvent, can be recovered by simply reducing the pressure and allowing the fluid to evaporate under ambient laboratory conditions. This property is particularly important with thermally unstable analytc,s. Another advantage of many supercritical fluids is that they are inexpensive, innocuous, and generally nontoxic substances. Supercritical fluid car-... 

Polymerization reactions in supercritical fluids have been studied extensively in the laboratory of DeSimone (59-61) at the University of North Carolina. DeSimone had demonstrated the ability to synthesize a variety of polymer types with several different monomeric systems. His methyl methacrylate polymer studies have demonstrated that there are pronounced advantages to using supercritical fluids as a solvent system compared to using conventional halogenated organic solvents. 

Supercritical Extraction. The use of a supercritical fluid such as carbon dioxide as extractant is growing in industrial importance, particularly in the food-related industries. The advantages of supercritical fluids (qv) as extractants include favorable solubiHty and transport properties, and the abiHty to complete an extraction rapidly at moderate temperature. Whereas most of the supercritical extraction processes are soHd—Hquid extractions, some Hquid—Hquid extractions are of commercial interest also. For example, the removal of ethanol from dilute aqueous solutions using Hquid carbon dioxide... 

Supercritical fluid extraction (SFE) is a technique in which a supercritical fluid [formed when the critical temperature Tf) and critical pressure Pf) for the fluid are exceeded simultaneously] is used as an extraction solvent instead of an organic solvent. By far the most common choice of a supercritical fluid is carbon dioxide (CO2) because CO2 has a low critical temperature (re = 31.1 °C), is inexpensive, and is safe." SFE has the advantage of lower viscosity and improved diffusion coefficients relative to traditional organic solvents. Also, if supercritical CO2 is used as the extraction solvent, the solvent (CO2) can easily be removed by bringing the extract to atmospheric pressure. Supercritical CO2 itself is a very nonpolar solvent that may not have broad applicability as an extraction solvent. To overcome this problem, modifiers such as methanol can be used to increase the polarity of the SFE extraction solvent. Another problem associated with SFE using CO2 is the co-extraction of lipids and other nonpolar interferents. To overcome this problem, a combination of SFE with SPE can be used. Stolker et al." provided a review of several SFE/SPE methods described in the literature. 

The most common and widely used supercritical fluid in SFC is carbon dioxide. It is inert, in that it is non-toxic and non-flammable, it also has mild critical parameters, a low critical temperature of 31.3°C and a critical pressure of 72.8 atm [1], Using pure, supercritical carbon dioxide eliminates organic solvent waste and with it waste disposal costs and concerns. This is extremely practical advantage in the industrial environment where the generation of waste requires special handling and significant cost. 

The use of supercritical fluids as mobile phases in chromatography can offer several advantages because their properties are between those of liquids and those of gases. In particular, the viscosity of a supercritical fluid is almost that of a gas (50 times lower than that of a solvent) while its solvation properties (governed by the distribution coefficients K) are similar to those of a nonpolar solvent such as benzene. 

Numerous processes for powder generation using supercritical fluids have been developed. The specific properties of dense gases allow obtaining fine dispersed solids, especially of substances with low melting point temperatures, high viscosities and very waxy or sticky properties. Economic evaluation of the process shows that these compounds cannot be efficiently and economically processed by conventioned mechanical processes and there is a big advantage of the use of supercritical fluids. 

---