Supercritical fluids fundamental molecular properties

To understand any extraction technique it is first necessary to discuss some underlying principles that govern all extraction procedures. The chemical properties of the analyte are important to an extraction, as are the properties of the liquid medium in which it is dissolved and the gaseous, liquid, supercritical fluid, or solid extractant used to effect a separation. Of all the relevant solute properties, five chemical properties are fundamental to understanding extraction theory vapor pressure, solubility, molecular weight, hydrophobicity, and acid dissociation. These essential properties determine the transport of chemicals in the human body, the transport of chemicals in the air water-soil environmental compartments, and the transport between immiscible phases during analytical extraction. 

The unusual solvent properties of supercritical fluids (SCFs) have been known for over a century (1). Just above the critical temperature, Tc, forces of molecular attraction are balanced by kinetic energy and fluid properties, including solvent power, exhibit a substantial pressure dependence. Many complex organic materials are soluble at moderate pressures (80 to 100 atmospheres) and SCF solvent power increases dramatically when the pressure is increased to 300 atmospheres. The pressure responsive range of solvent properties thus attainable provides a tool for investigating the fundamental nature of molecular interactions and is also being exploited in important areas of applied research (2,3). 

Computer simulations are sometimes used in the geochemical literature with the sole objective to predict thermodynamic PVT properties of molecular fluids at high temperatures and pressures (e.g., Belonoshko and Saxena 1991, 1992 Duan et al. 1992 Fraser and Refson 1992). However, the ability to improve our physical understanding of the complex chemical behavior of geochemical fluids and to unravel fundamental molecular-scale correlations between the structural, transport, spectroscopic, and thermodynamic properties of supercritical aqueous fluids, seems to be a much more important feature of these techniques. 

Mechanistic and fundamental studies of reactions in liquid solvents often require a family of related compounds to cover the desired range in, e.g., solvent strength, viscosity a single suitable supercritical fluid covers a wide continuum of such properties without the need to vary molecular structure. This form of exploitation is discussed in section 3.1.2. 

On the topics of solute-solvent and solute-solute clustering, there is a significant number of publications by research groups from around the world, demonstrating the tremendous interest of the international research community. This chapter is a review of representative literature results, especially those based on molecular spectroscopy and related experimental techniques. Discussion of the fundamental properties of supercritical fluids will be within the context of enhanced solute-solvent and solute-solute interactions in supercritical fluid solutions, and the current understanding of the reasonably well-established solute-solvent clustering model and the somewhat controversial solute-solute clustering concept will be presented. 

---