Nonpolar solvents supercritical carbon

SFE is used mainly for nonpolar compounds [e.g. polychlorinated biphenyls (PCBs)]. Typically, small aliquots of soil (0.5-10 g) are used for extraction. The extraction solvent is a supercritical fluid, most commonly carbon dioxide, which has properties of both a liquid and gas. The supercritical fluid easily penetrates the small pores of soil and dissolves a variety of nonpolar compounds. Supercritical carbon dioxide extracts compounds from environmental samples at elevated temperature (100-200 °C) and pressure (5000-10 000 psi). High-quality carbon dioxide is required to minimize... 

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. 

SFE. SFE has been established as the extraction method of choice for solid samples. The usefulness of SFE for soil samples has been demonstrated for carbamate,organophosphorus and organochlorine pesticides. However, SFE is more effective in extracting nonpolar than polar residues. In order to obtain a greater extraction efficiency for the polar residues of imidacloprid, the addition of 20% methanol as modifier is required. Extraction at 276 bar and 80 °C with a solvent consisting of supercritical carbon dioxide modified with methanol (5%) for 40 min gives a recovery of 97% (RSD = 3.6%, n = 10). It is possible to use process-scale SFE to decontaminate pesticide residues from dust waste. ... 

To date most of the work which has been done with supercritical fluid extraction has concentrated on the extraction of analytes from solid matrices or liquids supported on an inert solid carrier matrix. The extraction of aqueous matrices presents particular problems [276-278]. The co-extraction of water causes problems with restrictor plugging, column deterioration, and phase separation if a nonpolar solvent is used for sample collection. Also, carbon dioxide isay have limited extraction efficiency for many water soluble compounds. 

The process employs the supercritical fluid carbon dioxide as a solvent. When a compound (in this case carbon dioxide) is subjected to temperatures and pressures above its critical point (31°C, 7.4 MPa, respectively), it exhibits properties that differ from both the liquid and vapor phases. Polar bonding between molecules essentially stops. Some organic compounds that are normally insoluble become completely soluble (miscible in all proportions) in supercritical fluids. Supercritical carbon dioxide sustains combustion and oxidation reactions because it mixes well with oxygen and with nonpolar organic compounds. 

Homogeneous molecular catalysts, which have far greater connol over selectivity than heterogeneous solid catalysts, are now being tested in SCFs, and early results show that high rates, improved selectivity, and elimination of mass-transfer problems can be achieved. Supercritical carbon dioxide may be an ideal replacement medium for nonpolar or weakly polar chemical processes. More than simply substitutes for nonpolar solvents, SCFs can radically change the observed chemistry (Jessop et al., 1995). 

Besides solvent extraction, supercritical carbon dioxide has been applied to extract isoflavones (Rostagno et al., 2002). Due to the hydrophobicity of carbon dioxide, supercritical fluid extraction is more suitable for extracting nonpolar aglycones than polar glycosides of isoflavones, and may not be quantitative. 

As shown in Figure 1.2, the solvent strength of supercritical carbon dioxide approaches that of hydrocarbons or halocarbons. As a solvent, C02 is often compared to fluorinated solvents. In general, most nonpolar molecules are soluble in C02, while most polar compounds and polymers are insoluble (Hyatt, 1984). High vapor pressure fluids (e.g., acetone, methanol, ethers), many vinyl monomers (e.g., acrylates, styrenics, and olefins), free-radical initiators (e.g., azo- and peroxy-based initiators), and fluorocarbons are soluble in liquid and supercritical C02. Water and highly ionic compounds, however, are fairly insoluble in C02 (King et al., 1992 Lowry and Erickson, 1927). Only two classes of polymers, siloxane-based polymers and amorphous fluoropolymers, are soluble in C02 at relatively mild conditions (T < 100 °C and P < 350 bar) (DeSimone et al., 1992, 1994 McHugh and Krukonis, 1994). 

Other extraction methods used in the lipid extraction include supercritical fluid extraction (SFE) and pressurized liquid extraction (PLE). With SEE, good extraction yields have been obtained for nonpolar lipids including ester-ified fatty acids, acylglycerols, and unsaponifiable matter. However, complex polar lipids are only sparingly soluble in supercritical carbon dioxide alone and polar modifiers, such as methanol, ethanol, or even water is required to improve the extraction of polar lipids (10). SFE has been used for the extraction of lipids especially from various food matrices, such as different nuts, edible oils, and seeds (11). The recoveries of lipids in SFE were on the same levels than with conventional solvent extraction methods (12,13), no significant differences between the fatty acids extracted were observed. PLE has also been used in lipid extraction, although only in very few applications (14). The elevated temperatures used in PLE can cause alteration of the lipid composition. 

The most common sc-fluid for industrial processing and benchtop research is supercritical carbon dioxide, chosen because of its moderate and easily attained critical temperature and pressure and its non-toxicity. Reactions in SC-CO2 produce similar results as reactions in nonpolar organic solvents. Its solvent polarity, empirically determined using solvatochromic dyes as polarity indicators (see Section 7.4), corresponds to that of hydrocarbons such as cyclohexane [221, 222]. Carbon dioxide has no dipole moment and only a small quadrupole moment, a small polarizabihty volume, and a low relative permittivity (er = 1.4-1.6 at 40 °C and 108-300 bar) [221, 223]. Thus, SC-CO2 is only suitable as a solvent for nonpolar substances, which unfortunately imposes considerable limitations on its practical applications. To overcome this limitation, more polar co-solvents (modifiers) such as methanol can be added to SC-CO2. 

In experiments at the Agriculture Department s Northern Regional Research Center in Peoria, Illinois, scientists have found that supercritical carbon dioxide behaves as a very useful nonpolar solvent for removing fat from meat. At temperatures above 31°C (Tc for C02) and several hundred atmospheres of pressure, the carbon dioxide fluid can dissolve virtually all the fat from samples of meat. Even more important, the fluid also will dissolve any pesticide or drug residues that may be present in the meat. When the carbon dioxide fluid is returned to normal pressures, it immediately vaporizes, and the fat, drug, and pesticide molecules come raining out to allow easy analysis of the types and amounts of contaminants present in the meat. Therefore, this... 

Like supercritical carbon dioxide, supercritical water is a very interesting substance that has strikingly different properties from those of liquid water. For example, recent experiments have shown that supercritical (superfluid) water can behave simultaneously as both a polar and a nonpolar solvent. While the reasons for this unusual behavior remain unclear, the practical value of this behavior is very clear It makes superfluid water a very useful reaction medium for a wide variety of substances. One extremely important application of this idea involves the environmentally sound destruction of industrial wastes. Most hazardous organic (nonpolar) substances can be dissolved in supercritical water and oxidized by dissolved 02 in a matter of minutes. The products of these reactions are water, carbon dioxide, and possibly simple acids (which result when halogen-containing compounds are reacted). Therefore, the aqueous mixture that results from the reaction often can be disposed of with little further treatment. In contrast to the incinerators used to destroy organic waste products, a supercritical water reactor is a closed system (has no emissions). 

Carbon dioxide, water, ethane, ethylene, propane, ammonia, xenon, nitrous oxide, and fluoroform have been considered useful solvents for SEE. Carbon dioxide has so far been the most widely used as a supercritical solvent because of its convenient critical temperature, 304°K, low cost, chemical stability, nonflammability, and nontoxicity. Its polar character as a solvent is intermediate between a truly nonpolar solvent such as hexane and a weakly polar solvent. Moreover, COj also has a large molecular quadrupole. Therefore, it has some limited affinity with polar solutes. To improve its affinity, additional species are often introduced into the solvent as modifiers. For instance, methanol increases C02 s polarity, aliphatic hydrocarbons decrease it, toluene imparts aromaticity, R-2-butanol adds chirality, and tributyl phosphate enhances the solvation of metal complexes. 

Carbon dioxide is the supercritical solvent that is used most in homogeneous catalytic reactions. In addition to being environmentally acceptable (nontoxic, nonflammable), carbon dioxide is inert in most reactions, is inexpensive, and is available in large quantities. Its critical temperature is near ambient. Supercritical carbon dioxide dissolves nonpolar, nonionic, and low molecular mass compounds. However, addition of cosolvents enhances the solubility of many compounds in supercritical carbon dioxide. 

Carbon dioxide is, by far, the most attractive SCF for many reasons It is inexpensive and abundant at high purity (food grade) worldwide and it is nonflammable, non-toxic, and environment friendly moreover, its critical temperature T = 31 °C) permits operations at near-ambient temperature which avoids product alteration and its critical pressure (= 74 bar) leads to acceptable operation pressure, generally between 100 and 350 bar. In fact, supercritical carbon dioxide behaves as a rather weak nonpolar solvent, but its solvent power and polarity can be significantly increased by adding a polar cosolvent that is chosen among alcohols, esters, and ketones. Ethanol is often preferred because it is not hazardous to the environment, not very toxic, and available pure at low cost. Hydro fluorocarbons (HFCs) are very costly and their specific properties rarely justify their use in the replacement of carbon dioxide. 

The reverse micelles refer to the aggregates of surfactants formed in nonpolar solvents, in which the polar head groups of the surfactants point inward while the hydrocarbon chains project outward into the nonpolar solvent (Fig. 7) [101-126], Their cmc depends on the nonpolar solvent used. The cmc of aerosol-OT (sodium dioctyl sulfosuccinate, AOT) in a hydrocarbon solvent is about 0.1 mM [102]. The AOT reverse micelle is fairly monodisperse with aggregation number around 20 and is spherical with a hydrodynamic radius of 1.5 nm. No salt effect is observed for NaCl concentration up to 0.4 M. Apart from liquid hydrocarbons, recently several microemulsions are reported in supercritical fluids such as ethane, propane, and carbon dioxide [111-113]. 

Radically different binary phase behavior is found for the methane-TMB and the methane-methanol systems. This suggests that TMB can be extracted from methanol. To verify this conjecture experimental information was obtained on the TMB-methanol-methane system to ascertain whether the weak TMB-methanol complex can be broken by nonpolar methane. Interestingly, carbon dioxide, ethane, and ethylene, all much better supercritical solvents than methane, dissolve both methanol and TMB to such a large extent that they are not selective for either component. But with methane, the interactions between methane and TMB are strong enough to maintain a constant concentration of TMB in the extract phase as TMB is removed from the methanol-rich liquid phase. This means that the distribution coefficient for TMB increases as the concentration in the liquid phase decreases. We know of no other system that exhibits this type of distribution coefficient behavior. 

The extraction techniques described in this book fulfill many of Anastas and Warner s principles. For example, the use of supercritical carbon dioxide (SC-CO2) as the sole extraction solvent results in a nonpolluting process (prevention of waste and safer solvents and auxiliaries). Other beneficial properties of supercritical CO2 include fast diffusivity and nearly zero surface tension, which lead to extremely efficient extractions. In Chapters 2-4, applications of SC-CO2 as an extraction solvent are described. Ethanol and water are also environmentally friendly solvents that can be used as extraction media in many applications (see Chapters 5-7). Pressurized hot water ( 100-200 °C) in particular is a safe and nonpolluting solvent that has a similar dielectric constant to polar organic solvents, such as ethanol or acetone. Hence, pressurized hot water is a viable green alternative to many current extraction processes that use toxic organic solvents. Similarly, pressurized hot ethanol is an excellent solvent for the extraction of most medium polar to nonpolar organic molecules. Some of the techniques, such as membrane-assisted solvent extraction, described in Chapter 10, use organic solvents but in much smaller amounts compared to classical extraction techniques. Other techniques, for instance solid-phase microextraction and stir-bar sorptive extraction, described in Chapter 11, use no solvents. 

By factoring out solute volatility, the enhancement factor allows comparison of solvent and secondary solute effects. Empirically, there is a linear relationship between the log of the enhancement factor and solvent density. For nonpolar and polar solutes in supercritical carbon dioxide, plots of enhancement factor coincide, indicating that differences in solubility are primarily due to vapor-pressure differences. Nonlinear behavior is noted in the case of high solubilities. The enhancement m pure fluids is relatively independent of solute structure but is sensitive to solvent polarity and density. 

The overall effect of solvent polarity on the solubility of naphthalene follows the same general solubility rule in liquid extractions that like dissolves like . Naphthalene is a nonpolar solid and is most soluble in supercritical ethane. Carbon dioxide behaves as a nonpolar solvent but less so because of its quad-rupole moment [11]. Fluoroform is the most polar solvent because of the elec-... 

SFE is a well-established process for the recovery of different organics, mainly nonpolar substances from various solid matrices. It allows the selective extraction of different chemicals without an additional clean-up step and uses small sample amounts. Supercritical carbon dioxide (SC-CO2) is a convenient solvent, due to its moderate constants (Tc = 31°C, Pc = 7.3 MPa), nontoxicity, nonflammability, sufficient solvation power, and availability in pure form. SC-CO2 is also used to extract various organic compounds from environmental aqueous samples. Quantitative removal requires the addition of low molecular organic modifiers such as alcohols to enhance the polarity of SC-CO2. 

Another very intere,sting and promising cleanup technique is supercritical fluid extraction (SFE). This method, which usually employs supercritical carbon dioxide as an extraction medium, shows a high selectivity toward nonpolar solutes. As a rule of thumb, if a solute is soluble in hydrophobic solvents such as heptane or sometimes even in more polar ones such as dichloromethane, it should also be soluble in supercritical carbon dioxide. Through addition of modifiers (e.g.. methanol, water, formic acid), even relatively polar molecules are extractable, although the in-... 

Nevertheless, SFC also has its drawbacks for example, only a limited number of samples are soluble in supercritical fluids. In supercritical carbon dioxide, for instance, only relatively nonpolar solutes are soluble. As a rule of thumb, solutes that are soluble in organic solvents having a polarity less than or equal to that of n-heptane are usually also soluble in supercritical carbon dioxide. Samples that are soluble in water are insoluble in CO2. 

In biodiesel production, methanol and lipid reaction products are immiscible and form two phases at room conditions. This results in low reaction effidency and lipase deactivation. Hydrophobic solvents can minimize this effect, however, they are toxic and require a separation unit, which further increases the overall production cost Supercritical carbon dioxide (SC-COj) has frequently been used to replace organic solvents in various chemical processes. Due to its properties— including the easy and complete removal of the solvent, an ability to manipulate the physical properties of the solvent by simply changing the pressure or tanperature, nontoxidty, nonflammability, and enhancement of substrate mass transfer properties— it was suggested as a green solvent in biocatalyst reactions. A chemical feature of SC-COj is its low critical temperature (below the denaturation temperature of lipase). This feature combines the good solubility of nonpolar compounds, such as lipids, and makes SC-CO2 the perfect medium for biodiesel production. 

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