Solubility in supercritical fluid

Politzer, P., P. Lane, J. S. Murray, and T. Brinck. 1992a. Investigation of Relationships Between Solute Molecule Surface Electrostatic Potentials and Solubilities in Supercritical Fluids. J. Phys. Chem. 96, 7938. 

In terms of the solubilities of solutes in a supercritical phase, the following generalizations can be made. Solute solubilities in supercritical fluids approach and sometimes exceed those of liquid solvents as the SCF density increases. Solubilities typically increase as the pressure is increased. Increasing the temperature can cause increases, decreases, or no change in solute solubilities, depending on the temperature effect on solvent density and/or the solute vapor pressure. Also, at constant SCF density, a temperature increase increases the solute solubility (16). 

The Group 6 compounds glucose and glycine were not tested at the small-scale level because previous work suggested that they would not be soluble in supercritical fluid CO2. Additionally, chloroform was not tested at this level because it is undoubtedly extractable but would pose significant trapping problems because of its relatively high vapor pressure. 

Lemert, R.M., Johnston, K.P., "Chemical Complexing Agents for Enhanced Solubilities in Supercritical Fluid Carbon Dioxide," submitted to I EC Res., 1990. 

The NMR method we have developed gives a direct, in situ determination of the solubility and also allows us to obtain phase data on the system. In this study we have measured the solubilities of solid naphthalene in supercritical carbon dioxide along three isotherms (50.0, 55.0, and 58.5°C) near the UCEP temperature over a pressure range of 120-500 bar. We have also determined the pressure-temperature trace of the S-L-G phase line that terminates with the UCEP for the binary mixture. Finally, we have performed an analysis of our data using a quantitative theory of solubility in supercritical fluids to help establish the location of the UCEP. 

The prospect of using enzymes as heterogeneous catalysts in scC02 media has created significant interest. Their low viscosity and high diffusion rates offer the possibility of increasing the rate of mass-transfer controlled reactions. Also, because enzymes are not soluble in supercritical fluids, dispersion of the free enzymes potentially allows simple separations without the need for immobilization. 

The high solubility of solid substances in supercritical fluids compared to those in ideal gases (enhancement factors of lO -lO are common) allows their use as solvents in pharmaceutical, biomedical and food industries. Sections 2.4-2.7 are devoted to predictions of the entrainer effect, and of solubility in supercritical fluids with and without entrainer. Reliable predictive methods for solid solubilities in mixtures of a supercritical solvent -i- cosolvent were developed (2.4-2.6). These apply not only to the usual cosolvents such as organic liquids (2.4-2.5), but also to cases in which the cosolvent is a gas or another supercritical fluid (2.6). Our methods provided good agreement with experimental data in all of these cases (2.4-2.6). 

The enhancement factors for solubility of compounds, over ideal solubility, in supercritical fluids is typically... 

Paulaitis ME, McHugh MA, Chai CP. Solid solubilities in supercritical fluids at elevated pressures. Chemical Engineering at Supercritical Conditions. Ann Arbor, ME Ann Arbor Science, 1983 139-158. 

In parallel with the temperature variable, the operating pressure in SFC is maintained above the critical pressure (Pc) of the supercritical fluids. Pressure can exert a significant influence on the SFC process and the resulting material properties, since it is also an important determinant of drug solubility in supercritical fluids. In addition, having a direct bearing on the density (and supersaturation level) of the fluids, pressure can alter the crystallization mechanism and hence the crystal form produced. 

Investigation of relationships between solute molecule surface electrostatic potentials and solubilities in supercritical fluids, f Phys. Chem., 96, 7938-7943. 

The solubility of relatively nonvolatile components (including solids) in a gas may be measured by the gas saturation method mentioned in Section 1.8.5. The gas is passed through the solute, and the amount of solute collected by a given amount of gas is measured. This method is commonly used to measure solubilities in supercritical fluids, where the effects of pressure and vapor-phase nonideality make the partial pressure of the solute in the vapor much larger than its vapor pressure. The major challenges are analysis of the solute collected, avoiding condensation of the solute before it is collected, and ensuring that all the gas is saturated with the solute. 

Figure 2.1 Schematic diagram of the experimental apparatus used by Hannay and Hogarth to obtain solid solubilities in supercritical fluids. The glass tubing (a) is first connected to an air manometer and is then immersed in the constant temperature bath (b).

It is by now quite clear to the reader that the phenomenon of enhanced solubility in supercritical fluids has been known for more than a century. So why are there not scores of supercritical fluid extraction processes in operation today ... 

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