Supercritical fluid separations thermodynamic

Enantioselective separation by supercritical fluid chromatography (SFC) has been a field of great progress since the first demonstration of a chiral separation by SFC in the 1980s. The unique properties of supercritical fluids make packed column SFC the most favorable choice for fast enantiomeric separation among all of the separation techniques. In this chapter, the effect of chiral stationary phases, modifiers, and additives on enantioseparation are discussed in terms of speed and resolution in SFC. Fundamental considerations and thermodynamic aspects are also presented. 

The benefits from tuning the solvent system can be tremendous. Again, remarkable opportunities exist for the fruitful exploitation of the special properties of supercritical and near-critical fluids as solvents for chemical reactions. Solution properties may be tuned, with thermodynamic conditions or cosolvents, to modify rates, yields, and selectivities, and supercritical fluids offer greatly enhanced mass transfer for heterogeneous reactions. Also, both supercritical fluids and near-critical water can often replace environmentally undesirable solvents or catalysts, or avoid undesirable byproducts. Furthermore, rational design of solvent systems can also modify reactions to facilitate process separations (Eckert and Chandler, 1998). 

Interestingly, many hyperthermophiles also are highly salt tolerant. This is an adaptation to life involving aqueous systems that evolve with high-pressure liquid/vapor and supercritical fluid-phase separation of hydrother-mally heated seawater. Both psychrophiles and hyperthermophiles have large numbers of species that also require heavy-metal tolerance, due to the concentration of heavy metals by the thermodynamic phase-separation processes operative in both very cold and very hot aqueous systems (Breezee et al. 2004 Kaye and Baross 2002 Summit and Baross 1998). 

Gas flow processes through microporous materials are important to many industrial applications involving membrane gas separations. Permeability measurements through mesoporous media have been published exhibiting a maximum at some relative pressure, a fact that has been attributed to the occurrence of capillary condensation and the menisci formed at the gas-liquid interface [1,2]. Although, similar results, implying a transition in the adsorbed phase, have been reported for microporous media [3] and several theoretical studies [4-6] have been carried out, a comprehensive explanation of the static and dynamic behavior of fluids in micropores is yet to be given, especially when supercritical conditions are considered. Supercritical fluids attract, nowadays, both industrial and scientific interest, due to their unique thermodynamic properties at the vicinity of the critical point. For example supercritical CO2 is widely used in industry as an extraction solvent as well as for chemical... 

UV-visible, fluorescence, and IR spectroscopy have been used to characterize the solvent strength of pure and mixed supercritical fluid solvents, and to study solute-solvent interactions. The use of spectroscopic probes for the determination of clustering of pure and binary supercritical fluids about solutes is discussed. Spectroscopic studies of solvent strength and solute-solvent interactions are valuable for the development of molecular thermodynamic theory, engineering models, and for the molecular design of separation and reaction processes. 

Precipitation from supercritical fluids is of interest not only in relation to the production of uniform particles. The thermodynamics of dilute mixtures in the vicinity of the solvent s critical point (more specifically, the phenomenon known as retrograde solubility, whereby solubility decreases with temperature near the solvent s critical point) has been cleverly exploited by Chlmowitz and coworkers (12-13) and later by Johnston et al. (14). These researchers implemented an elegant process based on retrograde solubility for the separation of physical solid mixtures which gives rise to high purity materials. 

Batsanov SS (2009) Thermodynamic reason for delamination of molecular mixtures under pressure and detonation synthesis of diamond. Russ J Phys Chem A 83 1419-1421 Ree FH (1986) Supercritical fluid phase separations— implications for detonation properties of condensed explosives. J Chem Phys 84 5845-5856... 

Yaku K, Aoe K, Nishimura N, Morishita F (1999) Thermodynamic study and separation mechanism of diltiazem optical isomers in packed-column supercritical fluid chromatography. J Chromatogr A 848 337-345... 

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