Supercritical mixtures, attractive

Organic chemists have been attracted for a variety of reasons to supercritical media as an environment for performing reactions. These reasons include, especially for C02 and H20, the environmental friendliness of the medium. The fact that supercritical fluids can be removed without a residue is an advantage. Other advantages include the solubility of gases within supercritical mixtures, the high diffusion rates, and the variable and adjustable density, solvent power, and dielectric constant of the medium. Ordinary gases, such as 02 and H2, are miscible with... 

The addition of a small amount (usually less than 5 mol%) of a volatile cosolvent to a supercritical fluid (SCF) can lead to a very large enhancement in the solubility of a solute (up to several hundred percent) [1]. Similarly, the use of multiple solutes (usually binary) can enhance their solubilities [1]. These phenomena, often called the cosolvent and entrainer effects, have attracted attention both among experimentalists [2-7] and theoreticians [8-12]. Two types of ternary supercritical mixtures are of interest. 

Chialvo, A. A. and P. G. Debenedetti. 1992. Molecular dynamics study of solute-solute microstructure in attractive and repulsive supercritical mixtures. Industrial and Engineering Chemistry Research. 31, 1391. 

JW Tom, PG Debenedetti. Integral equation study of microstructure and solvation in model attractive and repulsive supercritical mixtures. Ind Eng Chem Res 32 2118-2128, 1993. 

The more obvious and consistent deviations from the hard sphere theory occur, at the low density values, due to the effects of attractive forces in the real system. We can attempt to correct for these effects using a method described previously (27-30) for the analysis of angular momentum correlation times in supercritical CFjj and CFjj mixtures with argon and neon. We replace the hard sphere radial distribution function at contact hs with a function gp (0) which uses the more realistic... 

One aspect of the last set of experiments on W(CO)6 in supercritical ethane that we have not yet discussed involves the possible role of local density enhancements in VER and other experimental observables for near-critical mixtures. The term local density enhancement refers to the anomalously high solvent coordination number around a solute in attractive (where the solute-solvent attraction is stronger than that for the solvent with itself) near-critical mixtures (24,25). Although Fayer and coworkers can fit their data with a theory that does not contain these local density enhancements (10,11) (since in their theory the solute-solvent interaction has no attraction), based on our theory, which is quite sensitive to short-range solute-solvent structure and which does properly include local density enhancements if present, we conclude that local density enhancements do play an important play in VER and other spectroscopic observables (26) in near-critical attractive mixtures. 

The presence of moisture in the soil was found to have no effect on the extraction of phenol using pure carbon dioxide. The effect of the presence of water may have been masked, since phenol distributes between water and supercritical carbon dioxide the same as it does between soil and supercritical carbon dioxide. However, the presence of water did have a dramatic impact on the effectiveness of the entrainers. The benzene/carbon dioxide mixture was able to remove essentially all of the phenol from the wetted soil. Since benzene is virtually insoluble in water, it highly favors the supercritical phase over the wetted soil phase. Hence the supercritical phase is able to attract almost all of the phenol, possibly due to chemical similarities between benzene and phenol. The methanol/carbon dioxide mixture, however, offered no enhancement over that of the pure carbon dioxide. As seen in the aqueous extractions, the methanol highly favors water and therefore is probably staying with the wetted soil. Therefore, the supercritical phase polarity is not increased as in the dry soil extractions and no solubility enhancement occurs. 

Although the relatively poor solvent power of carbon dioxide may be used to advantage for selective separations, as explained above, it is a key technical issue that is limiting its widespread use. It can, in fact, spoil the economics of otherwise highly attractive processes, namely integrated reaction-separation. The recent trend is to tackle this problem of solubility by using biphasic mixtures of COj and liquid compounds, where the liquid phase retains some of the advantages of supercritical fluids, but with enhanced solvent power. 

This chemical and physico-chemical behavior of the binary H2O-CO2 mixture [38] suggests that water is an attractive liquid to be combined with supercritical carbon dioxide in multiphase catalysis. CO2/H2O systems have adequate mass-transfer properties, especially if emulsions or micro-emulsions can be formed ([39] and refs, therein). The low pH of aqueous phases in the presence of compressed CO2 (pH ca. 3-3.5 [40]) must be considered and the use of buffered solutions can be beneficial in the design of suitable catalytic systems, as demonstrated for colloid-catalyzed arene hydrogenation in water-scC02 [41]. 

Popular supercritical solvent choices include pure carbon dioxide (CO2) and mixtures of CO2 with various co-solvents. These C02-based solvents have reasonably low critical temperatures, which makes them ideal for the synthesis of complex molecules that might be more thermally labile at higher temperatures such as in supercritical water. Furthermore, the low costs of both CO2 and H2O, their ease in handling, environmental benignness and the ability to adjust solubility by changing density with pressure to induce phase separation are all attractive characteristics. 

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