Supercritical fluid solvents, critical parameters

Table 2 shows critical parameters of the fluids most used for SFE. When it comes to choosing a supercritical fluid, the critical pressure and the critical temperature are two important parameters. The critical pressure determines, from a first approximation, the importance of the solvent power of the fluid. Ethane, for example, which has a lower critical pressure than carbon dioxide, will not dissolve a moderately polar soluble in the same way as carbon dioxide. Similarly, fluids with a higher critical pressure are more able to dissolve polar compounds. The critical temperature has practical implications. Indeed, one should always consider the influence of the extraction temperature on the stability of the component to extract. 

A variety of equations-of-state have been applied to supercritical fluids, ranging from simple cubic equations like the Peng-Robinson equation-of-state to the Statistical Associating Fluid Theoiy. All are able to model nonpolar systems fairly successfully, but most are increasingly chaUenged as the polarity of the components increases. The key is to calculate the solute-fluid molecular interaction parameter from the pure-component properties. Often the standard approach (i.e. corresponding states based on critical properties) is of limited accuracy due to the vastly different critical temperatures of the solutes (if known) and the solvents other properties of the solute... 

The current state of analytical SPE was critically reviewed and no major changes of the technique have been observed. Overviews of the developments of the extraction technologies of secondary metabolites from plant materials refer to three types of conventional extraction techniques that involve the use of solvents, steam, or supercritical fluids. Each technique is described in detail with respect to typical processing parameters and recent developments. Eollowing the discussion of some technical and economic aspects of conventional and novel separation processes, a few general conclusions about the applicabilities of the different types of extraction techniques are drawn. ... 

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. 

Modified Mobile Phases. In addition to pure supercritical fluids, much research has been performed on the use of modifiers with supercritical fluids. That is, rather than switching to a completely different supercritical fluid for the mobile phase, a small percentage of a secondary solvent can be added to modify the mobile phase while (hopefully) maintaining the mild critical parameters of the primary fluid. 

Figure 11 illustrates the parameter space defined by the equilateral triangle. The initial pressure and conditions for the 3 vertices of the pressure gradient/ temperature triangle were determined arbitrarily from the critical conditions of the supercritical fluid (carbon dioxide), the retention characteristics of nitroaromatic compounds, and the following criteria (i) the first analyte should not co-elute with the sample solvent and (ii) the retention factor of the last analyte should not exceed 30. 

Microemulsions have the ability to partition polar species into the aqueous core or nonpolar solutes into the continuous phase (See Fig. 1). They can therefore greatly increase the solvation of polar species in essentially a nonpolar medium. The surfactant interfacial region provides a dramatic transition from the highly polar aqueous core to the nonpolar continuous-phase solvent. This region represents a third type of solvent environment where amphiphilic solutes can reside. Such amphiphilic species will be strongly oriented in the interfacial film so that their polar ends are in the core of the microemulsion droplet and the nonpolar end is pointed towards or dissolved in the continuous phase solvent. When the continuous phase is a near-critical liquid (7)j = r/7 > 0.75) or supercritical fluid, additional parameters such as transport properties, and pressure (or density) manipulation become important aids in applying this technology to chemical processes. 

Kistler was first to recognize that silica gel would dissolve if subjected to supercritical conditions when the gel pores contained water, so he exchanged the liquid by washing the gel with ethanol [147], It is customary to exchange solvents, but even evacuation of organic solvents (alcohols, ethers, etc.) would sometimes result in gel dissolution. The main reason for this lies in the fact that typical sol-gel solvents have high values of critical parameters (Pc, T ), which increases their solvent power. Critical parameters of certain typical sol-gel solvents (water, methanol, ethanol, and carbon dioxide) are shown in Table 24.1. A way to bypass this problem is to exchange sol-gel solvent with liquid carbon dioxide, which can be transformed into supercritical fluid at relatively moderate conditions [141,147]. 

In order to overcome some of these restrictions in the development of the impregnation processes Perman [56] suggests using a liquid solvent (preferentially water) to solubilize the drug. This solvent is insoluble in the supo critical fluid. The system is then pressurized with die supercritical fluid in order to swell the polymer and to allow rapid difi isional transport of the solute into the polymeric substrate. At the end, pressure is released and the liquid diffuses out of the matrix. The process proposed seems in some extent similar to the recrystallization processes since the impregnation of the solutes is partially enhanced by the decrease of the drug solubility in the liquid solvent. The most important parameters are ... 

Supercritical CO2 has been studied as an alternative polymerization medium (e.g., for ring-opening polymerization of lactones [35, 36]) for more conventional organic solvents due to its weU-known advantages [37]. Thus, it is environmentally friendly, nontoxic, nonflammable, and economical since its critical parameters are relatively easily obtained. Furthermore, its fluid/solvent properties can be tuned by small changes in temperature. 

---