Supercritical fluids solutions

Phase Equihbria Models Two approaches are available for modeling the fugacity of a solute,, in a supercritical fluid solution. The compressed gas approach is the most common where ... 

Depolymerization, e.g., polyethylene terephthalate and cellulose hydrolysis Hydrothermal oxidation of organic wastes in water Crystallization, particle formation, and coatings Antisolvent crystallization, rapid expansion from supercritical fluid solution (RESS)... 

Tsutsumi, A., Nakmoto, S., Mineo,T., and Yoshida, K., A Novel Fluidized-Bed Coating of Fine Particles by Rapid Expansion of Supercritical Fluid Solutions, Proc. 1st Int. Particle Technol. Forum, pp. 452-455, Denver, CO (1994)... 

These Rh complexes have been the subject of intense interest due to their propensity for C-H activation of alkanes (Section 3.3.2.7). The noble gas complexes [CpRh(CO)L] and [Cp Rh(CO)L] (L = Kr, Xe) have also been studied in supercritical fluid solution at room temperature [120]. For both Kr and Xe, the Cp complex is ca. 20-30 times more reactive towards CO than the Cp analogue. Kinetic data and activation parameters indicated an associative mechanism for substitution of Xe by CO, in contrast to Group 7 complexes, [CpM(CO)2Xe] for which evidence supports a dissociative mechanism. 

Substantial evidence suggests that in highly asymmetric supercritical mixtures the local and bulk environment of a solute molecule differ appreciably. The concept of a local density enhancement around a solute molecule is supported by spectroscopic, theoretical, and computational investigations of intermolecular interactions in supercritical solutions. Here we make for the first time direct comparison between local density enhancements determined for the system pyrene in CO2 by two very different methods-fluorescence spectroscopy and molecular dynamics simulation. The qualitative agreement is quite satisfactory, and the results show great promise for an improved understanding at a molecular level of supercritical fluid solutions. 

Fluorescence Investigation of Cosolvent—Solute Interactions in Supercritical Fluid Solutions... 

Solvation in supercritical fluids depends on the interactions between the solute molecules and die supercritical fluid medium. For example, in pure supercritical fluids, solute solubility depends upon density (1-3). Moreover, because the density of supercritical fluids may be increased significantly by small pressure increases, one may employ pressure to control solubility. Thus, this density-dependent solubility enhancement may be used to effect separations based on differences in solute volatilities (4,5). Enhancements in both solute solubility and separation selectivity have also been realized by addition of cosolvents (sometimes called entrainers or modifiers) (6-9). From these studies, it is thought that the solubility enhancements are due to the increased local density of the solvent mixtures, as well as specific interactions (e.g., hydrogen bonding) between the solute and the cosolvent (10). 

Angus, S. Armstrong, B. de Reuck, K. M. eds., International Thermodynamic Tables of the Fluid State Carbon Dioxide. Pergamon Press, Oxford, 1976. Brennecke, J. F. Intermolecular Interactions in Supercritical Fluid Solutions from Fluorescence Spectroscopy. PhD Thesis, University of Illinois, Urbana, 1989. Brennecke, J. F. Eckert, C. A. AIChE J.. 1989, 35(9), 1409-1427. 

Matson, D. W. Petersen, R. C. Smith, R. D. The Preparation of Polycarbosilane Powders and Fibers during Rapid Expansion of Supercritical Fluid Solutions. Mater. Lett. 1986b, 4, 429 132. 

D.W. Matson, J.L. Fulton, R.C. Petersen, R.D. Smith, Rapid expansion of supercritical fluid solutions solute formation of powders, thin films and fibers, Ind. Eng. Chem. Res. 26 (1987) 2298-2306. 

A. Tsutsumi, S. Nakamoto, T. Mineo, K. Yoshida, A novel fluidized-bed coating of fine particles by rapid expansion of supercritical fluid solutions, Powder Technol. 85 (1995) 275-278. 

Kim, S. Johnston, K. P. "Molecular Interactions In Dilute Supercritical Fluid Solutions," Submitted to Ind. Eng. Chem. Fund. 1985. 

K Ohgaki, H. Kobayashi, T. Katayama, N. Hirokawa, Whisker formation from jet of supercritical fluid solution, J. of Supercriticl Fluids, 3 (1990), 103-107... 

The morphology of the matrix on which we wish to make a SFE can have an enormous influence on the efficiency of the extraction rate. Generally a rapid and complete extraction depends upon the relative size of the matrix particles, the smaller being the better. This is due principally to the short internal distance that the solute must cover in order to attain the core of the supercritical fluid solution. Some studies have shown that the geometrical form can also have an influence on the rate and efficacity of the extraction. As in the case of an extraction solid-liquid, an increase in the porosity of the matrix will lead to an efficient and rapid extraction. 

Gas-phase studies have not been restricted to the group VI hexacar-bonyls. Fu and co-workers (54) have used TRIR to study the coordina-tively unsaturated species CpMn(CO) (x = 1 and 2) generated by 266-and 355-nm laser photolysis of CpMn(CO)3 in the gas phase. In the presence of noble gas L (L = He, Ar, or Xe), they were able to measure the rate constant for reaction of the noble gas complex CpMn(CO)2L with CO. Interestingly, they foimd that only Ar significantly perturbed the rate fi om that observed in the absence of noble gas. This was thought to be because He has too high an ionization potential and Xe is too bulky to interact with the Mn center. In light of recent TRIR experiments conducted in supercritical fluid solution, the conclusion that Xe is unable to coordinate is incorrect. 

The effect of cyclopentadienyl-ring substituents on the reactivity of the group VII half-sandwich complexes (t7 -C5R5)M(CO)2L[M = Mn and Re R = H, Me, and Et (Mn only) L = Kr and Xe] toward CO in supercritical fluid solution at room temperature has been investigated (70). The reactivity of the corresponding alkane complexes ( 7 -C5R5)Mn(CO)2(n-heptane) (R = H, Me, and Et) toward small molecules such as CO, N2, and H2 in n-heptane solution steadily increased in the order H < Me < Et (71). These results indicated that steric rather... 

Reverse micelle and microemulsion solutions are mixtures of a surfactant, a nonpolar fluid and a polar solvent (typically water) which contain organized surfactant assemblies. The properties of a micelle phase in supercritical propane and ethane have been characterized by conductivity, density, and solubility measurements. The phase behavior of surfactant-supercritical fluid solutions is shown to be dependent on pressure, in contrast to liquid systems where pressure has little or no effect. Potential applications of this new class of solvents are discussed. 

An understanding of the phase behavior of surfactant-supercritical fluid solutions may be relevant to developing efficient secondary oil recovery methods because oil displacing fluids, such as a C02/surfactant mixture, may be supercritical at typical well conditions. In addition, the original oil in the well may contain dissolved gases such as ethane, propane, or butane, which may effect the phase behavior of the surfactant solution used to sweep out remaining oil. 

There have been a number of modeling efforts that employ the concept of clustering in supercritical fluid solutions. Debenedetti (22) has used a fluctuation analysis to estimate what might be described as a cluster size or aggregation number from the solute infinite dilution partial molar volumes. These calculations indicate the possible formation of very large clusters in the region of highest solvent compressibility, which is near the critical point. Recently, Lee and coworkers have calculated pair correlation functions of solutes in supercritical fluid solutions ( ). Their results are also consistent with the cluster theory. 

Figure 7. Excimer formation in dilute supercritical fluid solutions.

This suggests that the inclusion of density dependent local composition mixing rules can improve models of phase behavior in highly compressible polar supercritical fluid solutions. 

The magnetic metals were also prepared by a method [25] based on the rapid expansion of supercritical fluid solutions (RESS) coupled with chemical reduction to produce nickel, cobalt, iron, and iron oxide nanopartides of reasonably narrow size distribution. Under the protection of a polymer stabilization agent, the largely amorphous metal nanopartides form stable suspensions in room-temperature solvents. 

Meziani, M. J. and Sun, Y. P. (2003). Protein-conjugated nanoparticles from rapid expansion of supercritical fluid solution into aqueous solution. J. Am. Chem. Soc. 125, 8015-8018. Meziani, M. J., Pathak, P., Hurezeanu, R., Thies, M. C., Enick, R. M., and Sun, Y. P. (2004). Supercritical fluid processing technique for nanoscale polymer particles. Angew. Chem. Int. Ed. Engl. 43, 704—707. 

Supercritical chemistry a review of chemical reactions in supercritical fluids solutions [63]... 

Poliakoff, M. Howdle, S.M. Supercritical chemistry a review of chemical reactions in supercritical fluids solutions. In Third International Symposium on Supercritical Fluids, International Society for the Advancement of Supercritical Fluids Nottingham, UK, 1999. 

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