Supercritical solvation properties

Supercritical Fluid Extraction. Supercritical fluid (SCF) extraction is a process in which elevated pressure and temperature conditions are used to make a substance exceed a critical point. Once above this critical point, the gas (CO2 is commonly used) exhibits unique solvating properties. The advantages of SCF extraction in foods are that there is no solvent residue in the extracted products, the process can be performed at low temperature, oxygen is excluded, and there is minimal protein degradation (49). One area in which SCF extraction of Hpids from meats maybe appHed is in the production of low fat dried meat ingredients for further processed items. Its apphcation in fresh meat is less successful because the fresh meat contains relatively high levels of moisture (50). 

Solvation properties, of supercritical solvents, 14 80-81 Solvatochromic materials, 22 708t Solvatochromic probes, 26 853—855 Solvatochromic spectral shifts, 23 96 Solvatochromy, 20 517 Solvay, 7 641 Solvay process, 15 63... 

Supercritical regime, 11 756 Supercritical solvents, solvation properties of, 14 80-81... 

Supercritical fluid chromatography is the name for all chromatographic methods in which the mobile phase is supercritical under the conditions of analysis and the solvating properties of the fluid have a measurable effect on the separation. SFC has some advantages over GC and HPLC it extends the molecular weight range of GC, thermally labile compounds can be separated at lower temperatures, compounds without chromophores can be sensitively detected, and the use of open-tubular and packed columns is feasible. SFC can be employed in both the analysis of natural pigments and synthetic dyes, however it has not been frequently applied in up-to-date analytical practice. 

Supercritical fluid chromatography (SFC) is an intermediate chromatographic technique between GC and HPLC. It depends upon the fact that when a fluid becomes supercritical (both the temperature and pressure are at or above its critical point) it develops some of the solvating properties of a liquid whilst retaining the low viscosity of a gas. Hence, mass transfer (essential to efficient chromatography) is more akin to that of GC than HPLC, but many compounds can be chromatographed at temperatures much lower than what would be required by GC, so some thermally labile compounds are amenable to SFC where they would degrade under GC conditions [28]. 

The use of supercritical fluids as mobile phases in chromatography can offer several advantages because their properties are between those of liquids and those of gases. In particular, the viscosity of a supercritical fluid is almost that of a gas (50 times lower than that of a solvent) while its solvation properties (governed by the distribution coefficients K) are similar to those of a nonpolar solvent such as benzene. 

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). 

In order to investigate the effects of local density fluctuations on solvation properties, we decided to study two supercritical thermodynamic state points of the same density (5.7 at/nm3) but at different temperatures (295 and 153 K). The low temperature state point, close to the Ar critical point (Tc= 150.8 K, pc= 8.1 at/nm3), is expected to involve significant local density enhancements [5]. 

Supercritical fluids offer a convenient means to achieve solvating properties which have gas- and liquid-like characteristics without actually changing chemical structure. By proper control of pressure and temperature one can access a significant... 

In this chapter, we have reviewed some of our own work on solvation properties in supercritical fluids using molecular dynamics computer simulations. We have presented the main aspects associated with the solvation structures of purine alkaloids in CO2 under different supercritical conditions and in the presence of ethanol as co-solvent, highlighting the phenomena of solvent density augmentation in the immediate neighborhood of the solute and the effects from the strong preferential solvation by the polar co-solvent. We have also presented a summary of our results for the structure and dynamics of supercritical water and ammonia, focusing on the dielectric behavior of supercritical water as functions of density and temperature and the behavior of excess solvated electrons in aqueous and non-aqueous associative environments. 

P. B. Balbuena, Structure and Dynamics of Electrolyte Solutions from Ambient to Supercritical Conditions Effects on Solvation Properties and Chemical Reactions, Ph. D. Dissertation, University of Texas at Austin, 1996. 

One approach to increasing CO2 solvent strength relies on the fact that most volatile materials (such as alcohols, ketones, and hydrocarbons) are soluble in supercritical and near-critical CO2. This allows the employment of a wide range of cosolvents to enhance the C02 s solvation properties. The use of such cosolvent-modified CO2 as a solvent medium is most recognized in the Unicarb spray-coating process commercialized by Union Carbide (now with Dow Chemical, Midland, MI) in the early 1990s. In this process, the majority of traditional solvents used in the spray coatings are replaced by supercritical CO2. This process has been implemented in automotive and furniture industries. 

Since high solute dlffusivity, lower viscosity and excellent solvating properties can be obtained with supercritical fluids, higher chromatographic efficiencies and faster analysis time than liquid chromatography can be obtained with SFC (21). It is also possible to separate non-volatile high molecular weight compounds at relatively low temperatures. 

The formation of diphenylcarbene by laser flash photolysis of diphenyldiazomethane was studied in SC-CO2 and other supercritical media. [18] A laser-flash induced ring-closure reaction of a bipyridyl complex (Scheme 2) revealed solvation properties. [19] Laser-flash impact to metal carbonyl complexes activated hydrogen and simple alkanes like CH4, C2H4, C2H6, and further inorganic reactions in supercritical fluids have been reviewed. [20]... 

Supercrihcal solvent technology supercritical solvents, especially carbon dioxide, offer novel synthetic ophons owing to their unusual solvating properties... 

As with liquids, polar solutes are most soluble in polar supercritical fluids, although nominally nonpolar fluids can be remarkably good solvents for many moderately polar compounds. Carbon dioxide, for example, at higher pressures can exhibit solvating properties intermediate between pentane and aiethylene chloride. 

AlAough Ae unique solvating properties of supercritical C( may be Ae cause of Ae enantioselectivity enhancements observed, differences in selectivity can be caused by pressure in conventional solvents as well. For this reason, we carried out two hyctogenation reactions of 5 in hexane at 40°C wiA catalyst 1 as Ae BARF... 

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