Methanol, supercritical, organic

Summary Hydrophobic aerogels were prepared by base-catalyzed hydrolysis and condensation of RSi(OMe)3 (R = Me, Ph, PrI1)/Si(OMe)4 (1 4) mixtures in methanol, followed by supercritical drying of the obtained alcogels with methanol. The organic substituents also increase the elasticity of the aerogels. 

The extraction efficiency of supercritical fluids may be enhanced by mixing into it a small amount of a cosolvent such as acetone or methanol. Supercritical fluid extraction offers certain advantages over other extraction processes (1) it is relatively a fast process with greater extraction efficiency (2) sample concentration steps may be eliminated and (3) unlike LLE or Soxhlett extraction, a large amount of organic solvents is not required. 

The most common mobile phase for supercritical fluid chromatography is CO2. Its low critical temperature, 31 °C, and critical pressure, 72.9 atm, are relatively easy to achieve and maintain. Although supercritical CO2 is a good solvent for nonpolar organics, it is less useful for polar solutes. The addition of an organic modifier, such as methanol, improves the mobile phase s elution strength. Other common mobile phases and their critical temperatures and pressures are listed in Table 12.7. 

The principal solvents that have been used are alcohols such as ethanol, methanol, and propanol, and organic acids such as formic or acetic acid, but other solvents iaclude esters, ethers, phenols, cresols, and some amines. Even solvents such as CO2 and NH in the supercritical fluid state have been tried as solvents. 

Extraction from Aqueous Solutions Critical Fluid Technologies, Inc. has developed a continuous countercurrent extraction process based on a 0.5-oy 10-m column to extract residual organic solvents such as trichloroethylene, methylene chloride, benzene, and chloroform from industrial wastewater streams. Typical solvents include supercritical CO9 and near-critical propane. The economics of these processes are largely driven by the hydrophihcity of the product, which has a large influence on the distribution coefficient. For example, at 16°C, the partition coefficient between liquid CO9 and water is 0.4 for methanol, 1.8 for /i-butanol, and 31 for /i-heptanol. 

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 first use of supercritical fluid extraction (SFE) as an extraction technique was reported by Zosel [379]. Since then there have been many reports on the use of SFE to extract PCBs, phenols, PAHs, and other organic compounds from particulate matter, soils and sediments [362, 363, 380-389]. The attraction of SFE as an extraction technique is directly related to the unique properties of the supercritical fluid [390]. Supercritical fluids, which have been used, have low viscosities, high diffusion coefficients, and low flammabilities, which are all clearly superior to the organic solvents normally used. Carbon dioxide (C02, [362,363]) is the most common supercritical fluid used for SFE, since it is inexpensive and has a low critical temperature (31.3 °C) and pressure (72.2 bar). Other less commonly used fluids include nitrous oxide (N20), ammonia, fluoro-form, methane, pentane, methanol, ethanol, sulfur hexafluoride (SF6), and dichlorofluoromethane [362, 363, 391]. Most of these fluids are clearly less attractive as solvents in terms of toxicity or as environmentally benign chemicals. Commercial SFE systems are available, but some workers have also made inexpensive modular systems [390]. 

Interestingly, Qi, Smith, and co-workers reported that addition of an organic solvent such as acetone, DMSO, methanol, ethanol, ethylacetate, or supercritical carbon dioxide to BMIM Cl allowed the reaction to proceed at room temperature. For instance, in the presence of Amberlyst 15 as solid acid catalyst, authors showed that addition of 5 wt% of acetone to BMIM CF yielded, at room temperature, HMF with 86% selectivity at 90% conversion. Further investigations revealed that addition of an organic solvent to BMIM CF allowed one to overcome important mass transfer at room temperature due to the high viscosity of BMIM CD [96]. 

The solvent power of supercritical carbon dioxide is relatively weak and is strongly linked to its density (controlled by pressure and temperature), but it can be increased by adding a polar organic solvent (referred to as co-solvent) such as methanol or acetonitrile. 

At 16 000 kPa and 60 C, carbon dioxide has a density of 0.7 g/ml (see Fig. 6.2). Under these conditions, its polarity is similar to that of toluene and this is why the expression dense gas is used in order to indicate that it is not a classical gas. Because of its low polarity, it is often customary to add an organic modifier such as methanol, formic acid or acetonitrile to the supercritical fluid. 

Adding an organic solvent (such as methanol or acetone) to the supercritical fluid can modify its solvating properties. Since the polarity of C02 in its supercritical state (at 100 atm and 35 °C) is comparable to that of hexane, it can be altered by introducing a modifier. Nonetheless, isolating analyte from the matrix requires knowledge about the solubility and the transfer rate of solute in the solvent as well as chemical and physical interactions between matrix and solvent (Fig. 20.6). 

Bacterial mutagenesis tests have been conducted with distilled water solutions of the freeze-dried residues [concentrated up to 3000-fold (7)] and partially freeze-dried samples [concentrated 10-fold (49)]. High salt concentrations in such concentrates may cause toxicity problems in the bacterial tests. The use of dimethyl sulfoxide, methanol, or supercritical carbon dioxide to extract the organics from the freeze-dried residues for mutagenicity test purposes should be investigated. 

Supercritical fluid extraction uses a supercritical fluid (Box 25-2) as the extraction solvent.20 C02 is the most common supercritical fluid because it is inexpensive and it eliminates the need for costly disposal of waste organic solvents. Addition of a second solvent such as methanol increases the solubility of polar analytes. Nonpolar substances, such as petroleum hydrocarbons, can be extracted with supercritical argon.21 The extraction process can be monitored by infrared spectroscopy because Ar has no infrared absorption. 

The supercritical CO2 extraction of Capsicum annuum var. Scotch Bonnet gave 16.4% of extract, and it contained 3.2 and 0.5% capsaicin and dihydrocapsaicin, respectively, per dry weight of the raw material. Organic solvents (hexane, CHC13, and methanol) were used... 

Fahing et al. [176] studied the effect of the addition of modifiers such as methanol and water on the SCFE of organic solutes from soils and clays. Hawthorne et al. [177] compared the application of sub- and supercritical water in the extraction of organics from soil, and found that both were effective extractants. 

Ultrasonic extraction, methanol extraction [147] and supercritical fluid extraction have all been applied to the extraction of or the determination of volatile organic compounds [121,122] in soils. However, methods based on headspace analysis or on mass spectrometry are now the methods of choice. 

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