Supercritical initial solvent

Several years ago we reported initial observations of reverse micelles and microemulsions in supercritical fluid solvents (JL) These studies suggested the possibility of creating a previously unsuspected broad range of organized molecular assemblies in dense gas solvents. Such systems are of interest due to potential applications which exploit the readily variable properties of supercritical fluids as well as the unique solvent environments of reverse micelles and microemulsions. These initial studies showed that even gram quantities of proteins, such as Cytochrome-c (Mwt. 12,842 dalton) could be solvated in a liter of supercritical ethane or propane due to the microemulsion solvent environment, something which is not achievable with "conventional"... 

The similarity in P-x behavior near the UCEP for naphthalene-ethylene and biphenyl-carbon dioxide suggests that the location of the UCEP can be estimated solely from solubility data. Hence, it is possible to assume (incorrectly) that the solubility data of both the naphthalene-ethylene and the biphenyl-carbon dioxide systems represent solid solubilities in a supercritical fluid solvent. Notice, however, that the P-x behavior for these systems is very different at pressures greater than their respective UCEP pressures. At 55°C and at pressures greater than 465 bar, the solubility of biphenyl in supercritical carbon dioxide decreases dramatically for a small increase in pressure at 50°C and at pressures greater than 175 bar, the solubility of naphthalene in supercritical ethylene increases for a small increase in pressure until at higher pressures the solubility eventually reaches a limiting value. Obviously these two systems are not as similar as we initially conjectured. How can we explain these experimental observations ... 

For biological evaluation larger quantities of component phenols and their constituents may be required and where different components such as alkylresorcinols and anacardic acids co-occur, an initial solvent separation may prove desirable. Mild extraction methods, such as supercritical fluid extraction with carbon dioxide, [1,219,220], followed by phase separation [221], preparative TLC [222], or column chromatography [223] may be valuable for complex mixtures. 

Supercritical anti-solvent and related processes (GAS/SAS/ASES/SEDS) In these processes, the SCE is used as an antisolvent that causes precipitation of the substrate(s) dissolved initially in a liquid solvent. This general concept consists of decreasing the solvent power of a polar liquid solvent in which the substrate is dissolved, by saturating it with carbon dioxide in supercritical conditions, causing the substrate precipitation or recrystallization. Depending on the desired solid morphology, various methods of implementation are available ... 

Watei has an unusually high (374°C) ctitical tempeiatuie owing to its polarity. At supercritical conditions water can dissolve gases such as O2 and nonpolar organic compounds as well as salts. This phenomenon is of interest for oxidation of toxic wastewater (see Waste treatments, hazardous waste). Many of the other more commonly used supercritical fluids are Hsted in Table 1, which is useful as an initial screening for a potential supercritical solvent. The ultimate choice for a specific appHcation, however, is likely to depend on additional factors such as safety, flammabiUty, phase behavior, solubiUty, and expense. 

We have also investigated the kinetics of free radical initiation using azobisisobutyronitrile (AIBN) as the initiator [24]. Using high pressure ultraviolet spectroscopy, it was shown that AIBN decomposes slower in C02 than in a traditional hydrocarbon liquid solvent such as benzene, but with much greater efficiency due to the decreased solvent cage effect in the low viscosity supercritical medium. The conclusion of this work was that C02 is inert to free radicals and therefore represents an excellent solvent for conducting free radical polymerizations. 

Initiator decomposition studies of AIBN in supercritical C02 carried out by DeSimone et al. showed that there is kinetic deviation from the traditionally studied solvent systems.16 These studies indicated a measurable decrease in the thermal decomposition of AIBN in supercritical C02 over decomposition rates measured in benzene. Kirkwood correlation plots indicate that the slower rates in supercritical C02 emanate from the overall lower dielectric constant (e) of C02 relative to that ofbenzene. Similar studies have shown an analogous trend in the decomposition kinetics ofperfluoroalkyl acyl peroxides in liquid and supercritical C02.17 Rate decreases of as much as 30% have been seen compared to decomposition measured in 1,1,2-trichlorotrifluoroethane. These studies also served to show that while initiator decomposition is in general slower in supercritical C02, overall initiation is more efficient. Uv-visual studies incorporating radical scavengers concluded that primary geminate radicals formed during thermal decomposition in supercritical C02 are not hindered to the same extent by cage effects as are those in traditional solvents such as benzene. This effect noted in AIBN decomposition in C02 is ascribed to the substantially lower viscosity of supercritical C02 compared to that ofbenzene.18... 

Since then, the process has been extended to a wide variety of lactones of different size and to several lipases, as recently reviewed [93-96]. Interestingly, large-membered lactones, which are very difficult to polymerize by usual anionic and coordination polymerizations due to the low ring strain, are successfully polymerized by enzymes. Among the different lipases available, that fi om Candida antarctica (lipase CA, CALB or Novozym 435) is the most widely used due to its high activity. An alcohol can purposely be added to the reaction medium to initiate the polymerization instead of water. The polymerization can be carried out in bulk, in organic solvents, in water, and in ionic liquids. Interestingly, Kobayashi and coworkers reported in 2001 the ROP of lactones by lipase CA in supercritical CO2... 

It is worth noting that the enzyme can be withdrawn and recycled by using supercritical CO2. The success of the polymerizations carried out in organic solvents stems directly from the sustained activity of several lipases in organic solvents. In this respect, it must be noted that water has a manifold influence on the course of the polymerization. On the one hand, water can initiate the polymerization. On the other hand, a minimum amount of water has to be bound to the surface of the enzyme to maintain its conformational flexibility, which is essential for its catalytic activity [94]. Lipase-mediated polymerization cannot therefore be achieved in strictly anhydrous conditions. 

The electron will be solvated in a region where the solvent molecules are appropriately arranged. There must be a cluster of electrons of a size of 4-5 to support the formation of the solvated electron from the results of Gangwer et al., [23], Baxendale [24,25], and Kenney-Wallace and Jonah [16]. This behavior does not depend on the specific alcohol or alkane and even occurs in supercritical solutions, as has been shown in experiments done using mixtures of supercritical ethane-methanol mixtures [19]. Experiments have also shown that the thermodynamically lowest state might not be reached. For example, the experiments of Baxendale that measured the conductivity of the solvated electron in alcohol-alkane mixtures showed that when there was a sufficient concentration of alcohols to form dimers, there was a sharp decrease in the mobility of the electron [24,25]. This result showed that the electron was at least partially solvated. However, the conductivity was not as low as one would expect for the fully solvated electron, and the fully solvated electron was never formed on their time scale (many microseconds), a time scale that was sufficiently long for the electron-alcohol entity to encounter sufficient alcohols to fully solvate the electron. Similarly, the experiments of Weinstein and Firestone, in mixed polar solvents, showed that the electron that was observed depended on the initial mixture and would not relax to form the most fully solvated electron [26]. 

Some of the newer procedures use the same basic principles as the older extraction methods but provide fast and easy-to-use options and generally consume less organic solvent. For the most part, they have higher initial purchase price than the traditional methods. Examples include supercritical fluid extraction, accelerated solvent extraction, and automated solid-phase extraction and microextraction. Modular systems are now readily available that automate these proce-... 

The application of SCF as reaction media for enzymatic synthesis has several advantages, such as the higher initial reaction rates, higher conversion, possible separation of products from unreacted substrates, over solvent-free, or solvent systems (where either water or organic solvents are used). Owing to the lower mass-transfer limitations and mild (temperature) reaction conditions, at first the reactions which were performed in non-aqueous systems will be transposed to supercritical media. An additional benefit of using SCFs as... 

The pure oleoresin produced by solvents normally contains only pure curcumin, in a crystalline form. It is hardly soluble in liquid- and supercritical CO2- Even at an extraction pressure of 450 bar, and with 2 hours extraction time at 65°C, only 20% of the initial curcumin can be extracted. On the other hand, all the volatile oil and fatty oil is extracted, and a fat-free curcumin-starch mixture with a very low flavour-content can be produced. The total extraction yields are between 5 to 12%, with mostly fatty oil and volatile oil, and about 10% curcumin in the extract. 

The solvent is saturated with the substance to be powdered. This solution is diluted by contacting it with a supercritical gas. The solvent power of the classical solvent is reduced by the gas, initiating precipitation of the substance to be powdered. 

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