SCF Solvent

The solubility (y) of a material in an SCF is usually expressed in terms of the overall mole fraction of the solute in the SCF phase. The ability of SCFs to dissolve many substances arises from the highly nonideal behavior of pure SCFs. The solubility of a component as predicted by the ideal gas law, decreases asymptotically with increasing pressure because the solubility is simply the ratio of the vapor pressure (p ) to the total pressure (p). Under supercritical conditions, however, the solubility is enhanced by several orders of magnitude above that predicted by the ideal gas law. 

The solubility enhancement of a component, particularly in the vicinity of the critical point, is driven primarily by the augmentation in the density of the SCF. The extent of solubility enhancement which occurs in the SCF phase is usually expressed in terms of an enhancement factor ( ) which is defined as the actual solubility divided by the solubility predicted from the vapor pressure of the solute and ideal gas considerations  

The enhancement factor measures the extent of solubility in excess of that generated from the vapor pressure of the pure solid. This provides a convenient way of comparing the solubilities of solids with different vapor pressures. As the actual solubility of a solid is heavily influenced by the SCF density, the enhancement factor displays a similar dependence on density. Values of the enhancement factor typically vary between 10 and 10, although enhancement factors as high as 10 have been reported for some systems [10]. 

The solubility of a given solute also depends on the type of SCF as shown in Table 1.2-4. Fluoroform has the highest mass density under the conditions shown but displays the lowest affinity for naphthalene. The variation in the solubility of naphthalene in different SCFs therefore suggests that there are varying degrees of intermolecular interaction between the solid and SCF. The different levels of intermolecular interaction can be explained in terms of solvent polarity. 

The overall effect of solvent polarity on the solubility of naphthalene follows the same general solubility rule in liquid extractions that like dissolves like . Naphthalene is a nonpolar solid and is most soluble in supercritical ethane. Carbon dioxide behaves as a nonpolar solvent but less so because of its quad-rupole moment [11]. Fluoroform is the most polar solvent because of the elec- 

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For a ternai y system, the phase diagram appears much like that in conventional liquid-liquid equilibrium. However, because a SCF solvent is compressible, the slopes of the tie lines (distribution coefficients) and the size of the two-phase region can vary significantly with pressure as well as temperature. Furthermore, at lower pressures, LLV tie-triangles appear upon the ternary diagrams and can become quite large. 

In 1981, Silvestri et al. [23] used supercritical HC1 and NH3 for studying the anodic dissolution of Fe and Ag. Since then, electrochemical studies in SCF solvents have been carried out to a considerable extent. Bard and his coworkers [24] carried out in supercritical water, ammonia, and acetonitrile a series of studies... 

Supercritical CO2 has been by far the most popular SCF solvent, both on grounds of cost and acceptability. However, SCCO2 is a comparatively poor solvent [10]. Broadly, it has a solvent power similar to a light alkane such as hexane. However, there is a number of compounds, particularly fluorinated organic compounds which are unexpectedly soluble in SCCO2. 

One of the main attractions of SCF solvents is the ease of separating products at the end of the reaction. For products which are liquids, phase separation can be achieved merely by reducing the pressure. However, it should be remembered that some of the SCF will still remain dissolved in the liquid phase. This may not be a problem in reactions carried out in SCCO2 but it could present difficulties for products isolated on a large scale from flammable SCFs, where outgassing of flammable vapour from the liquid product could occur. 

In order to verify that higher cyclohexenone concentrations increase the HH/HT ratio in SCF solvents as it does in liquids, the photodimerization was performed in high density SCF ethane at different concentrations. In this regime (P = 124 bar and T = 35°C) clustering is presumably minimal. The HH/HT ratio increases with monomer concentration, which is consistent with the trend observed in liquid solvents (30), see Table I. As bulk solute concentration increases by a factor of 3, the increase in HH/HT is relatively small compared with the HH/HT increases portrayed in Figure 7. This comparison suggests that the local solute concentrations are increased dramatically, perhaps by even an order of magnitude or more, in solute-solute clusters. 

To form a stable polymer-SCF solvent solution at a given temperature and pressure, the Gibbs energy, shown in eq. 7.1, must be negative and at a minimum. 

High-pressure processes have been widely applied in the polymer industry. Near-critical and supercritical fluids (SCFs) are in particular used owing to their easily tunable density, which enhances the control of polymer solubility and their good separability from polymer material [1], SCF solvents (e.g. scC02) offer a potential advantage for separation process. The solubility of different polymeric material in SCFs can be systematically varied by changing operating conditions. Several... 

The unusual solvent properties of supercritical fluids (SCFs) have been known for over a century (1). Just above the critical temperature, Tc, forces of molecular attraction are balanced by kinetic energy and fluid properties, including solvent power, exhibit a substantial pressure dependence. Many complex organic materials are soluble at moderate pressures (80 to 100 atmospheres) and SCF solvent power increases dramatically when the pressure is increased to 300 atmospheres. The pressure responsive range of solvent properties thus attainable provides a tool for investigating the fundamental nature of molecular interactions and is also being exploited in important areas of applied research (2,3). 

In contrast to liquid solvents, the properties of a single SCF solvent can be altered over a wide range through modest manipulations of pressure and/or temperature. At constant reduced temperature, (Tr=T/Tc), between 1.0 and 1.1, solvent properties are extremely responsive to very accessible changes in pressure (800 to 4000 psi) (14). SCFs thus provide an unprecedented opportunity to investigate the effects of solvent properties on chemical reactions without changing the solvent composition. 

This application of SCFs, while highly touted, has received scant experimental attention (15). Investigations have been limited to Diels-Alder reactions (11,16), electrochemical reactions (17,18), polymerizations (19,20), and high temperature processes (21,22). Recent semiempirical treatments of SCF solvent properties (2 3,24) have provided a basis for interpreting solvent effects in SCFs. 

Pentane and toluene were selected as SCF solvents because these hydrocarbons have critical temperatures convenient to the desired operating temperatures for processing petroleum residua and coal liquids, respectively. 

Absorption and fluorescence experiments observe the solvatochromic shift of molecular (usually electronic) transitions. Several authors have reported similar density dependences for the shift of fluorescence or absorbance spectral lines in different solute/SCF solvent systems (31-33). This density dependence is characterized by three regions. At low densities, the absorption/emission line red shifts strongly with density. Around pc/2, the slope changes and the line position becomes approximately independent of density. At higher densities, somewhat above pc, the slope changes again, and the spectral line shifts further to the red with increasing density. 

Figure 3 shows the lifetime data as a function of density in the three SCF solvents at 2 K above the critical temperature, Tc (upper panels), and at 20 K above Tc (lower panels) Tc = 32.2, 25.9, and... 

Many potential applications have been proposed which involve the desorption of solutes from matrix using SCF solvents at elevated pressure these include activated carbon regeneration [1,2,3,4,5] and soil remediation [6,7,8] using supercritical carbon dioxide. 

The enhancement of solvent power obtained by compressing a gas into its critical region can be demonstrated dramatically. An estimate of the solubility of a solid in an SCF solvent can be made using the following expression ... 

Table 3 (constructed from Refs. 21-24) lists various physical properties for a variety of candidate SCF solvents. A cursory inspection of the entries in this table shows that many hydrocarbons have a critical pressure close to 45 bar and the critical temperature of SCF solvents increases as the molecular weight of the solvent increases or as the polarity or intramolecular hydrogen bonding of the solvent increases. This means that the solvent s vapor pressure curve extends to very high temperature. 

In the present paper it is shown, on the basis of the KB theory of solution [15], that there are conditions under which the main parameters in Fqs. (9) and (10), namely, K22, K33 and K23, can be expressed in terms of the parameters for the two binary mixtures formed with the SCF solvent. Finally, the solubilities in ternary mixtures are predicted using solubility data for the above binary mixtures. 

The purpose of these scales is to provide a guide for choosing SCF solvents and co-solvents to achieve a desired solvent strength, for example in a separation or reaction process. An example of a solvatochromic scale is presented in Figure 1 for the UV-vis absorption of phenol blue in ethylene as a function of density at two temperatures (S). The scale is defined as the transition energy, Ej = he/ Xmax where Xmax is the wavelength of maximum absorption. 

With the use of solvatochromic probes, other non-specific forces (dispersion, dipole-induced dipole, and dipole-dipole) and specific acid-base forces have been explored in SCF solvents. In an effort to compare liquid and supercritical carbon dioxide, Hyatt(ll) measured UV-visible spectra of several solvatochromic probes. There was little difference between the Ex in the liquid and SCF states however, the data can not be interpreted fully since the density and the pressure were not given at the supercritical condition. The results indicated that the... 

Table I. Properties of SCF solvents versus n-hexane at a constant value of solvent strength as defined by Ex for phenol blue(f)...

The final application of solvatochromic solvent strength scales is the correlation of reaction rate and equilibrium constants in SCF solvents. Solvatochromic scales are often quantitative indicators of the solvent effect on rate constants for a variety of reaction mechanismsU) In a SCF, this solvent effect may be achieved conveniently with a single solvent using pressure. Based on solvatochromic data, it was predicted that an activation volume can reach thousands of mL/mol in a SCF(8). This prediction was confirmed for various types of reactionsClSbZl). For example, the solvatochromic parameter Ex for phenol blue... 

Reverse micelles have been investigated in SCF solvents only recently. It was observed qualitatively that Cytochrome-c forms a colored solution for AOT in... 

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