Fluid enhancement factor behavior

Solid-Fluid Equilibria The solubility of the solid is very sensitive to pressure and temperature in compressible regions, where the solvent s density and solubility parameter are highly variable. In contrast, plots of the log of the solubility versus density at constant temperature often exhibit fairly simple linear behavior (Fig. 20-19). To understand the role of solute-solvent interactions on sofubilities and selectivities, it is instructive to define an enhancement factor E as the actual solubihty divided by the solubility in an ideal gas, so that E = ysP/Pf, where P is the vapor pressure. The solubilities in CO2 are governed primarily by vapor pressures, a property of the solid... 

By factoring out solute volatility, the enhancement factor allows comparison of solvent and secondary solute effects. Empirically, there is a linear relationship between the log of the enhancement factor and solvent density. For nonpolar and polar solutes in supercritical carbon dioxide, plots of enhancement factor coincide, indicating that differences in solubility are primarily due to vapor-pressure differences. Nonlinear behavior is noted in the case of high solubilities. The enhancement m pure fluids is relatively independent of solute structure but is sensitive to solvent polarity and density. 

This chapter provides an introduction to supercritical fluid behavior. As a tutorial, qualitative fluid behavior is stressed rather than quantitative description. Solubilities in solid-fluid systems are interpreted with a simple fluid model. Enhancement factors are introduced to demonstrate the importance of repulsive forces. Intermolecular interactions in cosolvent systems are discussed. Liquid-fluid phase behavior and phase transitions in liquid-fluid and solid-fluid systems are briefly presented. Transport properties are briefly presented to stress their density dependence. 

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