Solubility in Supercritical Fluid Cleaning

Kenneth E, Laintz, L, Dale Sivils, and W. Dale Spall 

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The technique may be viewed as an alternative to the addition of cosolvents or modifiers (sometimes termed entraimrs) that are commonly used in supercritical fluid technology to enhance the polarity of the fluid. For cleaning processes, however, these cosolvents may be toxic or detrimental in various ways to the substrate. In addition, these modifiers are usually more difficult to separate downstream from the process due to their high volatility. In contrast, surfactants typically have very low volatility and thus interact to a much lesser degree with the substrate. Furthermore, they often dramatically improve the solubility of polar species, well beyond that of simple modifiers. 

Supercritical fluids also find application in the areas of pollution prevention and remediation, and supercritical carbon dioxide is used as a replacement solvent for many hazardous solvents in both extraction and separation processes and also as a reaction medium and in materials processing. Although carbon dioxide is considered as a greenhouse gas , there is actually no net increase in the amount of the gas if it is removed from the environment, used as the solvent instead of a hazardous substance, and returned to the environment. In this way, most of the uses of supercritical carbon dioxide may be considered as environmentally friendly. Because the solubilities of oils and greases in carbon dioxide are high, it is particularly suited to the cleaning of machinery 47 and, as discussed in the literature 48, it is used as a solvent in textile dyeing operations where it is used to treat any dye-laden... 

Suitable gases in the form of supercritical fluids represent clean solvents/ carriers, which neither leave residues nor impose an environmental load. A number of factors determine the solubility of polymers in supercritical carbon dioxides (SCCO2) and these are given in Table 3.1. 

Supercritical fluids have long been known for their abilities to dissolve organic contaminants. Their ability to display a wide range of solvent characteristics and the ability to tune solubility with small changes in temperature and pressure were identified early on in our search for alternative cleaning methods. The gas-like diffusivity and low surfece tension combined with liquid-like densities were important... 

The tunability of solvency with temperature and pressure as illustrated in Figs. 1 and 2 is a key advantage of cleaning with supercritical fluids. This allows optimization of conditions to extract a particular material from a part and then selection of other conditions in the recycle reactor to separate it from the SCF. As an example, hexane has a solubility much like CO2 near the critical conditions. At higher pressures, carbon dioxide acts like acetone, a more polar solvent. A good rule of thumb is that if low molecular weight materials are soluble in hexane, they are soluble in CO2 at pressures just above the critical point. As pointed out by DeSimone,t °l however, polymers exhibit a different behavior. 

Supercritical fluid extraction (SFE) clean-up Complete separation between the bulk matrix and the small organic contaminants (<500 Da) in situ. Removal (with a few exceptions) of aU soluble lipophilic material along with the trace organic contaminants Limitation and difficulties in selecting the optimum SFE parameters to obtain a lipid free sample 1,202... 

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