Supercritical fluids polar SCFs

With traditional solvents, the solvent power of a fluid phase is often related to its polarity. Compressed C02 has a fairly low dielectric constant under all conditions (e = 1.2-1.6), but this measure has increasingly been shown to be insufficiently accurate to define solvent effects in many cases [13], Based on this value however, there is a widespread (yet incorrect ) belief that scC02 behaves just like hexane . The Hildebrand solubility parameter (5) of C02 has been determined as a function of pressure, as demonstrated in Figure 8.3. It has been found that the solvent properties of a supercritical fluid depend most importantly on its bulk density, which depends in turn on the pressure and temperature. In general higher density of the SCF corresponds to stronger solvation power, whereas lower density results in a weaker solvent. 

In addition to common organic solvents, supercritical fluids (scf s) can be used for a great variety of extraction processes [158 165], Supercritical fluid extraction (SFE), mostly carried out with SC-CO2 as eluant, has many advantages compared to extractions with conventional solvents. The solvent strength of a supercritical fluid can easily be controlled by the pressure and temperature used for the extraction at a constant temperature, extraction at lower pressures will favour less polar analytes, while extraction at higher pressures will favour more polar and higher molar mass analytes. As supercritical fluids such as CO2 and N2O have low critical temperatures (tc = 31 °C and 36 °C, respectively), SFE can be performed at moderate temperatures to extract thermolabile compounds. Typical industrial applications using SC-CO2 include caffeine extraction from coffee beans [158] as well as fat and oil extraction from plant and animal tissues [165]. For some physical properties of supercritical solvents, see Section 3.2. 

The tunability of solvency with temperature and pressure as illustrated in Figs. 1 and 2 is a key advantage of cleaning with supercritical fluids. This allows optimization of conditions to extract a particular material from a part and then selection of other conditions in the recycle reactor to separate it from the SCF. As an example, hexane has a solubility much like CO2 near the critical conditions. At higher pressures, carbon dioxide acts like acetone, a more polar solvent. A good rule of thumb is that if low molecular weight materials are soluble in hexane, they are soluble in CO2 at pressures just above the critical point. As pointed out by DeSimone,t °l however, polymers exhibit a different behavior. 

Supercritical fluids (SCF) have been used mainly for selective extraction of compounds the solubility of a compound in a given solvent is in many cases vastly different under ambient and supercritical conditions. Thus supercritical water dissolves both polar and nonpolar compounds, which may be explored in electrochemistry. When temperature and pressure approach the critical values, the internal structure of the solvent is loosened and the viscosity, the dielectric constant, and the density diminish the dielectricity constant e of water thus diminishes from 80 at 25°C to 5.2 at 647°C at 221 bar [441]. 

Supercritical fluids (SCFs) such as carbon dioxide have a "hydrocarbon-like solvent strength at typical conditions, so that they are appropriate solvents for lipophilic substances. The solvent strength may be raised significantly by the addition of small amounts of cosolvents such as ethanol to increase solubilities of moderately polar substances selectivelyQ), sometimes by several hundred percent(2,2 4). The solvent and cosolvent form clusters about solutes, in which the cosolvent concentrations are enhanced significantly( ,fi). The present objective is to explore the effects of considerably more powerful solvent additives, that is surfactants. Since very little is known about surfactants in SCFs, spectroscopic probes were used to measure polarities inside the reverse micelles. Polarity is a key indicator of the ability of a reverse micelle to solvate a hydrophile. Using the... 

Reactions using a PT catalyst can also be carried out in supercritical fluids (usually, sc CO2). In one of the few studies of this kind reported so far (Dillow et al., 1996), the two phases involved in the reaction were sc CO2 and a solid phase. The mechanism of PTC remains the same as in conventional PTC involving transfer of the reactant ion from the solid phase to the supercritical fluid by a quaternary ammonium salt or a crown ether. However, the choice of catalyst is restricted in this case by its solubility in the SCF phase. A polar or protic cosolvent is often necessary, and even small amounts of a solvent such as acetone can greatly increase the solubility. Thus, compared to traditional PTC, SCF-PTC requires much lower amounts of solvents. This combinatorial strategy has great potential for industrial application. 

Elucidation of solvation characteristics of supercritical fluids is indispensable to their utilization as media for separation or reaction. One powerful method for elucidating chemical equilibrium and solvation in SCFs is voltammetry. However, voltammetric measurement in pure supercritical CO2 is extremely difficult because CO2 is nonpolar. Electrochemical processes in several polar SCFs including acetonitrile, ammonia, and sulfur dioxide, were investigated by Bard and coworkers in the late 1980s (61-65). Dombro et al. reported the electrochemical synthesis of dimethyl carbonate from carbon monoxide and methanol in super-... 

---