Solid solubilities in supercritical

The following experimental techniques were used to measure the pressures and temperatures for solid-liquid-gas equilibrium, phase compositions (bubble and dew points) for gas-liquid equilibrium, and solid solubilities in supercritical pentane. Experimental procedures and the apparatus are described in detail elsewhere (13). 

Measurements of solid solubilities in supercritical pentane were, in principle, identical to the bubble point or dew point measurements described above. The equilibrium pressure and corresponding solid solubility at a fixed temperature were determined from the measured pressure and known mixture composition in the cell when the last crystal of solid dissolved. These measurements were limited at low solubilities by the low porphyrin loadings, and at high solubilities by the dark purple color of the fluid phase which obscured observation of the solid phase. 

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. 

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).

Paulaitis, M. E., M. A. McHugh, and C. P. Chai. 1983. Solid solubilities in supercritical fluids at elevated pressures. In Chemical engineering at supercritical fluid conditions, ed. M. E. Paulaitis, J. M. L. Penninger, R. D. Gray, and P. Davidson, 139. Ann Arbor, MI Ann Arbor Science. 

By utilizing the rapid expansion of supercritical solutions, small-size particles can be produced from materials which are soluble in supercritical solvents. In this process, a solid is dissolved in a pressurized supercritical fluid and the solution is rapidly expanded to some lower pressure level which causes the solid to precipitate. This concept has been demonstrated for a wide variety of materials including polymers, dyes, pharmaceuticals and inorganic substances. 

Lamb, D. M., Barbara, T. M. Jonas, J. NMR Study of Solid Naphthalene Solubilities in Supercritical Carbon Dioxide Near the Upper Critical End Point. J. Phys. Chem. 1986, 90, 4210M215. 

Liu, G. T. Nagahama, K. Solubility and RESS Experiments of Solid Solution in Supercritical Carbon Dioxide. J. Chem. Eng. Jpn. 1997, 30, 293-301. 

The use of NMR spectroscopy as an analytical technique is well established ( 1 8). In order to quantitate our spin-echo height to the number of protons present, we performed an independent calibration using standard solutions of naphthalene in carbon tetrachloride. Concentrations for the standards were chosen to correspond to the anticipated supercritical C02 solubilities, and all calibration measurements were performed using a sample cell of the same dimensions as the solubility sample cell previously described. The response of our spectrometer to the standard solutions was linear over the concentration range. The reproducibility for independent measurements of the calibration curve was 3 . Throughout the experiment, all spectrometer conditions (pulse lengths, phases, receiver amplifier gain, etc.) were closely monitored, and frequent checks on the calibration of the spectrometer were performed. In this way we were able to obtain the molar solubility of solid naphthalene in supercritical carbon dioxide to an estimated experimental accuracy of 6%. 

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 experimental solubility data for solid naphthalene in supercritical carbon dioxide, given as moles naphthalene dissolved per liter, are shown in Figure 6. Qualitatively the three pressure-composition isotherms show characteristic behavior for a solid-supercritical fluid system. Each isotherm initially shows a large increase in solubility with increasing pressure, and then a limiting value is reached at higher pressures. 

Figure 6. Experimental solubilities for solid naphthalene in supercritical carbon dioxide expressed in moles naphthalene dissolved per liter solution.

The design and development of supercritical extraction processes depend on the ability to model and predict the solubilities of solid solutes in supercritical solvents. The prediction is usually difficult due to the large differences in sizes and molecular interactions between the solvent and solute molecules. 

Solubility calculation of solid compounds in supercritical CO2 by a group -contribution method... 

Kumik, R. T., and Reid, R. C., Solubility of Solid Mixtures in Supercritical Fluids, Fluid Phase Equilib., 8 93-105 (1982)... 

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 first supercritical fluid-based micronization process has been the rapid expansion of supercritical solutions (RESS) it is based on the solubilization of the solid to be micronized in the SCF and its subsequent precipitation by fast depressurization of the solution. However, the use of this technique is largely limited by the low solubility in supercritical carbon dioxide (SC-CO2) of many of the solids of interest. " ... 

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

Furthermore, one of the advantages of carbon dioxide-based SFE is that normally extractions are carried out at low temperatures. The resolution of this dilemma can be as simple as adding a short sub-step in the beginning to remove the ethyl alcohol in the extraction thimble. Recall that the capsaicin extraction is carried out at 40 C and a density of 0.70 g/mL. The ethyl alcohol can be readily removed at a density of 0.25 g/mL at 40 C in a short three-minute dynamic extraction step. This is well below the "threshold-density" at which the capsaicin would extract or be soluble in supercritical carbon dioxide. The mechanism of using the carbon dioxide to remove ethyl alcohol from the extraction thimble under supercritical conditions is quite different than the evaporation mechanism that would be taking place in the solid trap after expansion of the carbon dioxide plus ethyl alcohol to a mixture of gas plus condensing liquid. Note that the fact that ethanol can be pre-extracted ahead of the analyte of interest demonstrates that the role of the modifier in this application is mainly matrix modification had a more polar solvent (like that afforded by a mixture of ethanol in carbon dioxide) been necessary to solvate the capsaicin, the pre-extraction step to remove the ethanol would have been detrimental to the extraction of the capsaicin. 

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 1.7 Solubility behavior of solid naphthalene in supercritical ethylene (data of Tsekhanskaya, lom-tev, and Mushkina, 1962 and Diepen and Scheffer, 1948b).

Figure 2.2 Solubility of solid naphthalene in supercritical carbon dioxide (data assembled by Modell et al., 1979).

Figure 2.3 Solubility behavior of solid silica in supercritical water (Kennedy, 1950).

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).

Figure 13-5. Solubility behavior of solid naphthalene in supercritical ethylene.

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