Solubility in a Supercritical Fluid

The prediction of analyte solubility in a SF is difficult it depends on the SF density and dielectric constant and on the analyte vapor pressure. In addition, the polarities of the SF and the analyte should be as similar as possible in order to improve the solubility. Effects of some variables on analyte solubility are  

The ideal matrix for SFE is a finely powdered solid with good permeability, allowing a large surface area for fluid-solid interaction. Typical examples are soils, particulates, and powdered dried plant materials. Intermediate in suitability are semipermeable solids, such as polymers, which can be partially penetrated but giving no quantitative extractions. The worst types of samples are wet body tissues, such as fish, solid wood, rocks and liquid samples.  

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Limited to analytes soluble in a supercritical fluid (usually scCCb)... 

Figure 2. Schematic representation of compound solubility in a supercritical fluid as a function of pressure.

The solubility parameter model has difficulty with temperature effects and also fails to predict solubilities in several instances, such as with silicones. However it is a good starting point for estimating the solubility characteristics of a SCF as a function of temperature and pressure. The most likely temperatures and pressures under which a material is soluble in a supercritical fluid are where the solubility parameters are within a value of unity of each other. See Fig. 1, taken from Fig. 2 of Ref 5 by Allada, for a graph of 6 versus T and P for CO2. This effect allows one to selectively remove a particular component from a material by tuning the 5 of the SCF using T and P. 

The solubility of a solute in a supercritical fluid can be quantitatively estimated using Giddings theory, which relies on differences between the Hildebrand solubility parameters for the SF and solute concerned. Solubility in a supercritical fluid can be understood by examining the Gibbs-Helmholtz equation ... 

As shown earlier, the solid solubility in a supercritical fluid solvent is... 

The similarity in P-x behavior near the UCEP for naphthalene-ethylene and biphenyl-carbon dioxide suggests that the location of the UCEP can be estimated solely from solubility data. Hence, it is possible to assume (incorrectly) that the solubility data of both the naphthalene-ethylene and the biphenyl-carbon dioxide systems represent solid solubilities in a supercritical fluid solvent. Notice, however, that the P-x behavior for these systems is very different at pressures greater than their respective UCEP pressures. At 55°C and at pressures greater than 465 bar, the solubility of biphenyl in supercritical carbon dioxide decreases dramatically for a small increase in pressure at 50°C and at pressures greater than 175 bar, the solubility of naphthalene in supercritical ethylene increases for a small increase in pressure until at higher pressures the solubility eventually reaches a limiting value. Obviously these two systems are not as similar as we initially conjectured. How can we explain these experimental observations ... 

Figure 4.1 Schematic diagram of a dynamic flows apparatus used to obtain liquid or solid solubilities in a supercritical fluid (Van Leer and Paulaitis, 1980).

The optical and scanning electron micrographs presented in this chapter show that the particle size of solid materials, such as polymers, monomers, and intermediate chemicals, can be altered by precipitation from a supercritical fluid solution. The only requirement for carrying out the SCF particle reduction process is that the compound must exhibit some solubility in a supercritical fluid. Because the pressure reduction rates are so rapid during the expansion of the solution, supersaturation ratios can be achieved that are much, much greater than can be achieved by thermal, chemical, or antisolvent precipitation processes. Furthermore, it is conjectured that such rapid nucleation rates can result in the particle formation of some materials with a size distribution or morphology that cannot now be achieved by any other process. 

Compounds not soluble in a supercritical fluid can be recrystallized in a process termed gas antisolvent (GAS) recrystailization. The process was first applied to the recrystailization of an explosive, RDX (cyclotrimethylenetrinitramine) into... 

The solubility in a supercritical fluid is related to both the density and the temperature of the fluid, in addition to the polarity of both the fluid and the analytes. Solubility increases with increasing density. Solubility also increases with increasing temperature, like in a liquid. 

In the RESS method, the solute of interest is solubilized in a supercritical fluid, which is then rapidly expanded through a nozzle. As the fluid expands, it loses its solvent capabilities and the solute precipitates out. While this technique has the advantage of not using any organic solvent, it is restricted by the generally poor solubility of most polymers in supercritical fluids. Indeed, polymers generally have to be below 10,000 MW in order to be eligible for this method of particle production [126]. 

Gas Antisolvent Recrystallizations. A limitation to the RESS process can be the low solubility in the supercritical fluid. This is especially evident in polymer—supercritical fluid systems. In a novel process, sometimes termed gas antisolvent (GAS), a compressed fluid such as C02 can be rapidly added to a solution of a crystalline solid dissolved in an organic solvent (114). Carbon dioxide and most oiganic solvents exhibit full miscibility, whereas in this case the solid solutes had limited solubility in C02. Thus, C02 acts as an antisolvent to precipitate solid crystals. Using C02 s adjustable solvent strength, the particle size and size distribution of final crystals may be finely controlled. Examples of GAS studies include the formation of monodisperse particles (<1 /zm) of a difficult-to-comminute explosive (114) recrystallization of p-carotene and acetaminophen (86) salt nucleation and growth in supercritical water (115) and a study of the molecular thermodynamics of the GAS crystallization process (21). 

The reaction mixture contains a dilute solution of benzophenone solute, a few mole percent isopropanol and the remainder supercritical carbon dioxide solvent. This is analogous to a solute dissolved in a supercritical fluid/cosolvent mixture. These types of systems are important because in many applications researchers have found that the addition of a small amount of cosolvent (such as acetone or an alcohol) of volatility intermediate between that of the solute and the SCF can greatly enhance the solubility of the solute (Van Alsten, 1986). 

Dlepen and Scheffer ( 6) were the first to show that near either the lower or upper critical end point the solubility of a solid in a supercritical fluid is enhanced and also very sensitive to changes in temperature and pressure our solubility isotherms show this effect for both end points. First, the isotherms cross at about 140 bar so that the solubility at the lowest temperature (50.0°C) is largest at 120 bar. This is a result of approaching the lower critical end point region (which should be close to the critical point of pure C02 as previously mentioned). At temperatures and pressures near this LCEP the solubility enhancement results in lower temperature isotherms having the greater solubilities. The effect of the upper critical end point is also well shown by our data. The 58.5°C isotherm shows a large increase in solubility at about 235 bar the slope of the isotherm is near zero. As Van Welie and Diepen... 

Fundamental studies on the adsorption of supercritical fluids at the gas-solid interface are rarely cited in the supercritical fluid extraction literature. This is most unfortunate since equilibrium shifts induced by gas phase non-ideality in multiphase systems can rarely be totally attributed to solute solubility in the supercritical fluid phase. The partitioning of an adsorbed specie between the interface and gaseous phase can be governed by a complex array of molecular interactions which depend on the relative intensity of the adsorbate-adsorbent interactions, adsorbate-adsorbate association, the sorption of the supercritical fluid at the solid interface, and the solubility of the sorbate in the critical fluid. As we shall demonstrate, competitive adsorption between the sorbate and the supercritical fluid at the gas-solid interface is a significant mechanism which should be considered in the proper design of adsorption/desorption methods which incorporate dense gases as one of the active phases. 

The effect of temperature on retention has been described experimentally,(4-8) but the functional dependence of k with temperature has only recently been described.W A thermodynamic model was outlined relating retention as a function of temperature at constant pressure to the volume expansivity of the fluid, the enthalpy of solute transfer between the mobile phase and the stationary phase and the change in the heat capacity of the fluid as a function of temperature.(9) The solubility of a solid solute in a supercritical fluid has been discussed by Gitterman and Procaccia (10) over a large range of pressures. The combination of solute solubility in a fluid with the equation for retention as a function of pressure derived by Van Wasen and Schneider allows one to examine the effect of solubility on solute retention. 

The solubility of a solid in a supercritical fluid has been described by Gitterman and Procaccia.(lO) The region of interest chromatographically will be for infinitely dilute solutions whose concentration is far removed from the lower critical end point (LCEP) of the solution. Therefore the solubility of the solute in a supercritical fluid at infinite dilution far from criticality can be approximated as,... 

The effect of temperature on retention was studies using n-hexadecane on a 20 m, 50ji I.D. fused silica capillary column coated with an OV-17 phase using FID detection. The OV-17 was cross-linked in-situ to decrease its solubility in the supercritical fluid. The stationary phase film thickness was calculated to be 0.25pm. The... 

Termed Fractional Destraction, the method fractionates residuum according to the solubility of its constituent components in a supercritical fluid. The novel aspect of the approach is the incorporation of a system to promote reflux of less-soluble components onto a packed bed. 

The fraction of coal which dissolves instantaneously in the supercritical fluid increases with an increase in the density and temperature of the supercritical fluid. This effect is similar to that generally observed for the solubility of a solid in a supercritical fluid. With an increase in density and temperature... 

The solubility of a solid solute (component 2), in a supercritical fluid is calculated, if the solid phase is a pure component, by ... 

As indicated in Figures 5 and 6, there is a nearly linear relationship between the log[AOT] solubility and the fluid density over several order of magnitude of AOT concentration. This type of behavior would be expected for the solubility of a non-aggregate forming, solid substance in a supercritical fluid (XL). The solubility and phase behavior of solid-supercritical fluid systems has been described by Schneider (2H) and others, and such behavior can be predicted from a simple Van der Waal s equation of state. Clearly, this approach is not appropriate for predicting surfactant solubilities in fluids, because it does not account for the formation of aggregates or their solubilization in a supercritical fluid phase. 

The existence of a reverse micelle phase in supercritical fluids has been confirmed from solubility, conductivity and density measurements. The picture of the aggregate structure in fluids is one of a typical reverse micelle structure surrounded by a shell of liquid-like ethane, with this larger aggregate structure dispersed in a supercritical fluid continuous phase. 

A simple equation for the solubility of a solid in a supercritical fluid cosolvent with a gas or another supercritical fluid. 

The objective of this paper is to propose a predictive method for the estimation of the change in the solubility of a solid in a supercritical solvent when another solute (entrainer) or a cosolvent is added to the system. To achieve this goal, the solubility equations were coupled with the Kirkwood-Buff (KB) theory of dilute ternary solutions. In this manner, the solubility of a solid in a supercritical fluid (SCF) in the presence of an entrainer or a cosolvent could be expressed in terms of only binary data. The obtained predictive method was applied to six ternary SCF-solute-cosolute and two SCF-solute-cosolvent systems. In the former case, the agreement with experiment was very good, whereas in the latter, the agreement was only satisfactory, because the data were not for the very dilute systems for which the present approach is valid. 2001 Elsevier Science B.V. All rights reserved. 

A Simple Equation for the Solubility of a Solid in a Supercritical Fluid Cosolvent with a Gas or Another Supercritical Fluid... 

In a number of areas of modeling phase equilibria, the cubic equation of state (EOS) provided equal or even better results than the traditional approach based on the activity coefficient concept. In fact, for certain t5rpes of phase equilibria, the EOS is the only method that provided acceptable results. The solubility of solids in a supercritical fluid (SCF) constitutes such a case. For the solubility of a solid in a SCF [SCF (1) + solid solute (2)], one can write the well-known relation ... 

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