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Other factors to consider in selecting a cleaning medium include possible parts damage and the number of solvents needed. Carbon dioxide is superior for metals such as cast magnesium, which may be pitted by water contact and whose porous structure can be penetrated by the low viscosity, low surface tension SCF stated21 Adjusting the temperature and pressure of the SCF also provides tunability of the solvent to selectively remove undesirable materials. This effect is also the basis of a whole field of chromatography. t3l
A summary of common contaminants and their cleanability from various substrates using C02 is given in the chapter by Spall and Laintz. A wider range of materials can be solubilized if cosolvents or microemulsions are used, as described in Ch. 5 by Fulton and Jackson. With today’s concern for environmental factors, the recyclability of the solvent is also of key concern. Fractionation by pressure drop is often an attractive way of recycling supercritical fluids. (SeeCh. 10 by Huse and Smith.)
2.0 SOLUBILITY: THEORETICAL AND EMPIRICAL
Let’s consider in more detail why supercritical fluids are good solvents. The mole fraction of a liquid soluble in a SCF is shown in Eq. 1 from PrausnitzJ4!
(1 - xt) PW2z*v\Pp’"^!r Eq. 1 v =---------------------——
Óç Ô 2P
This equation is analogous to Eq . 5 of Ch. 1 for the solubility of a solid in a SCF. In this equation, the subscript 2 refers to the liquid component. The superscript s refers to saturation conditions at temperature T. Pi refers to the saturation vapor pressure of the liquid at temperature T. The variable u2 is the molar volume of the liquid, <|>2S is the fugacity coefficient at saturation pressure and ô2 is the fugacity coefficient in the high pressure gas mixture. For a detailed derivation of this equation, see PrausnitzJ4! As is stated in the derivation, it is the escaping tendency of the liquid into the supercritical fluid phase, as described by the fugacity coefficient, ô2, which is responsible for the enhanced solubility of liquids in compressed gases.
The coefficient <J>2 is expressed as a function of the second and third virial coefficients in Eq. 2 from Ref. 4. These coefficients depend on the magnitude of the pairwise and three-body interactions respectively. The value of ^becomes more negative as the pairwise attractive forces between molecules increase. These pairwise forces dominate the three-body interactions in the second term and the log of the compressibility factor, the third term of this equation. As the 1ïô2, which is proportional to By, becomes more negative, ô2 becomes «1 and, being in the denominator ofEq. 1, causes the mole fraction of the liquid in the SCF, y2, to increase.
People have taken other approaches to understanding the solubilizing characteristics of a SCF. One common method is solubility parameters. The solubility parameter, 5, is the square root of the internal pressure, or cohesive energy density of a liquid. This concept has been modified for a supercritical fluid to be the internal energy of the SCF relative to the isothermally expanded ideal gas state as shown inEq. 3byS. R. À1Ûàß
Eq. (3) 5 = J[(E*- E)/v]
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 8 of the SCF using T and P.
100 140 180 220
Figure 1. Solubility parameter of SCF C02 variation with temperature and pressure. (Reprinted with permission from Ref 5, Fig. 2, ©1984, American Chemical Society.)
In addition to the above theoretical approaches to solubility, there are various empirical tabulations of solubilities in CO2 which can be used. Although one can not find CO2 solubilities in such general references as the Chemical Rubber Co. Handbook!4! there are other good references. In 1954, A. W. Francis published tables of solubilities of 261 compounds and 464 ternary phase diagrams in liquid Ñ02Ë This data can be used to estimate the potential for cleaning in CO2 before any experiments are done. More recent work on this topic can be found in a 1985 article by Dandge et al. f 81 Polymer solubilities are discussed in articles by Krukonis[9l and DeSimone. ,101 Once it is determined that a particular material is a candidate for CO2 extraction, selection of appropriate temperatures and pressures must be made. As pointed out by Krukonis and McHugh, the solubilities of most materials in C02 or any other supercritical fluid follow a curve shown in Fig. 2.tnl Only the numerical values along the X and Y axis change with each solute and solvent. The foundation for this curve is contained in Eq. 1, where the different components of this equation have different temperature dependencies.