General Considerations
Determination of Solubility
Thereshold Density
Solubility in Modified Fluids




Knowledge of the solubility of solutes in SFs is critical. There are a number of methods available for estimating solubilities. It was found that the variation of the solute-solvent binary interaction parameter is often unpre­dictable for a series of solids in a given SF solvent owing to the large difference in the critical properties of the solute and the solvent. As a result, a large number of more reliable experimental solubility measurements have been made in recent years. High solubility is not required for many applications of analytical SFE, but some finite solubility is required to cause extraction.

Solubility can be defined as the concentration (w/w) or mole fraction of a substance in the supercritical phase at equilibrium with the pure fluid. It was determined the solubilities of 261 compounds in near-critical liquid CO 2 (900 psi, 25°C) and found that nearly half of the studied solutes were miscible with CO 2. It was studied many classes of organic compounds such as aliphatics, aromatics, heterocyclics, and com­pounds with a large variety of functional groups. Table 13 lists the solubility data for esters, alcohols, acids, amines, phenols, and nitriles. It was concluded from "Solubility of liquid CO 2 in substance in weight percent of solution. 'Solubility of the substance in liquid CO 2 in weight percent of solution. CM = miscible.

 


Table 13 Solubilities of Selected Compounds in Liquid Carbon Dioxide at 25°C

 

This study that the solubility of CO 2 in a liquid solute is usually much greater than that of the other liquid solute in CO 2, and that the ratio of these two solubil­ities is greatest for those liquid solutes of low miscibility with CO 2. Although Francis studied solubility behavior in liquid CO 2, the results are probably applicable to high-density supercritical CO 2 systems. Because the CO 2 molecule has no dipole moment, this fluid is usually regarded as nonpolar; however, as is evi­denced by Table 13, CO 2 has some affinity for polar solutes, probably because of its high quadrupole moment. Nevertheless, the presence of hydroxyl, amino, and nitro groups is known to diminish solubility, especially if two or more of these groups are present in the solute molecule.

Recently it is tabulated all the experimentally measured solubilities of single compounds of low volatility in pure SF CO 2 which had been published in tabular form up to the end of 1989 (e.g., in Chemical Abstracts) along with the temperature and pressure ranges of the experimental measurement and method employed to determine solubility. The experimental data for each isotherm (which was expressed in the form of the natural logarithm of the product of mole fraction and pressure) were fitted as a linear function of the density for pressures above 100 bar. For approximately 100 solutes, the saturated solutions in CO 2 were dilute (i.e., solute mole fraction typically below 0.1). This solubility data therefore corresponded only to particular solvent-rich boundaries on the phase diagram of the two-component system. Such substances were essentially those studied by Francis except at higher temperatures and pressures. All the data are characterized by a rise in solubility with increasing pressure at a fixed tempera­ture, owing to enhanced solvation arising from greater attractive forces between the fluid and solute molecules. It should be noted that in many of these studies, the binary solute-fluid mixture may or may not be supercritical, which calls into question whether one is measuring supercritical or near-critical phase solubili­ties.

Few solubility studies in other fluids have been performed. Fluoroform (CHF3) is an attractive solvent because of its polarity (1.6 D) and readily attain­able critical parameters (Tc = 21.6°C; Pc = 48.6 bar). The solubility of opium al­kaloids in SF CO 2, N2O, and CHF3 have been compared. With the exception of codeine (structure I), all other opium alkaloids tested (thebaine, pa-paverine, and noscapine, structures II—IV) exhibited the highest solubility in CHF3 (Table 14). The most polar of the alkaloids tested was morphine (structure V) and it exhibited little solubility in all three of the supercritical fluids (e.g., 5 jjug/g, SF CO 2—200 bar, 40°C).

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