Solvent strength of supercritical fluids

Solvent strength of supercritical fluids based on solvatochromic scales... 

As shown in Figure 1.2, the solvent strength of supercritical carbon dioxide approaches that of hydrocarbons or halocarbons. As a solvent, C02 is often compared to fluorinated solvents. In general, most nonpolar molecules are soluble in C02, while most polar compounds and polymers are insoluble (Hyatt, 1984). High vapor pressure fluids (e.g., acetone, methanol, ethers), many vinyl monomers (e.g., acrylates, styrenics, and olefins), free-radical initiators (e.g., azo- and peroxy-based initiators), and fluorocarbons are soluble in liquid and supercritical C02. Water and highly ionic compounds, however, are fairly insoluble in C02 (King et al., 1992 Lowry and Erickson, 1927). Only two classes of polymers, siloxane-based polymers and amorphous fluoropolymers, are soluble in C02 at relatively mild conditions (T < 100 °C and P < 350 bar) (DeSimone et al., 1992, 1994 McHugh and Krukonis, 1994). 

Solvatochromic data, specifically absorption or transition energies (E s), have been obtained for the dye phenol blue in supercritical fluids as a function of both temperature and pressure. These data will be used to compare the "solvent strength" of these fluids with liquid solvents. He will use the terms "solvent strength" and "Et" synonymously in this paper such that they include the magnitude of the polarizability/volume as well as the dipole moment. The "solvent strength" has been characterized by the spectroscopic solvatochromic parameter, E, for numerous liquid solvents (9 JU, J7,JJ3). 

The K scale of solvent polarity/polarizability is based on the correlation between the experimentally observed absorption or emission shifts (vmax values) of various nitroaromatic probe molecules and the ability of the solvent to stabilize the probe s excited state via dielectric solute-solvent interactions (18). Since tt values are known for many commonly used liquid solvents, the k scale allows comparison of the solvation strength of supercritical fluids and normal liquid solvents. Several research groups have utilized the k probes to investigate solvent characteristics for a series of supercritical fluids (19-34). For example, Hyatt (19) employed two nitroaromatic dyes and the penta-ferf-butyl variation of the Riechardt dye (18) to determine the k values in liquid and supercritical CO2 (0.7 reduced density at 41°C). The experimental results were also used to calculate the t(30) solvent polarity scale (19), which is similar to the n scale. ... 

The solvent characteristics of supercritical fluids have been extensively investigated over the past two decades (2). Supercritical fluids have increased solvent strength versus gases due to their liquid-like densities. The pressure and temperature within the supercritical region can be adjusted to regulate the density and therefore the solvent strength of a supercritical fluid. In addition to the liquid-like density, supercritical fluids exhibit gas-like diffusivity and viscosity. 

Supercritical fluid chromatography (SFC) is a hybrid of GC and HPLC in which the mobile phase is a compound that is held above its critical temperature Tc) and critical pressure Pc)- By far the most common compound that is used as an SFC mobile phase is CO2, for which Tc 31°C and Pc 73 atm. The diffusivities of many compounds in supercritical CO2 are high enough that either packed-column or open-tubular columns may be used. The solvent strength of supercritical CO2 is roughly intermediate between those of n-hexane and benzene thus, many compounds that are too nonvolatile to be separable by GC can be separated by SFC. Detection of solutes separated by SFC is also easy, since GC detectors such as the FID and HPLC detectors such as UV absorption detectors can both be used. 

Thermodynamic Properties The variation in solvent strength of a supercritical fluid From gaslike to hquidlike values may oe described qualitatively in terms of the density, p, or the solubihty parameter, 6 (square root of the cohesive energy density). It is shown For gaseous, hquid, and SCF CO9 as a function of pressure in Fig. 22-17 according to the rigorous thermodynamic definition ... 

In general, the properties of supercritical fluids make them interesting media in which to conduct chemical reactions. A supercritical fluid can be defined as a substance or mixture at conditions which exceed the critical temperature (Tc) and critical pressure (Pc). One of the primary advantages of employing a supercritical fluid as the continuous phase lies in the ability to manipulate the solvent strength (dielectric constant) simply by varying the temperature and pressure of the system. Additionally, supercritical fluids have properties which are intermediate between those of a liquid and those of a gas. As an illustration, a supercritical fluid can have liquid-like density and simultaneously possess gas-like viscosity. For more information, the reader is referred to several books which have been published on supercritical fluid science and technology [1-4],... 

Enhanced-fluidity liquids (EELs) are mixtures that contain high proportions of liquefied gases, such as carbon dioxide [5]. Eluidity,/, is defined as the inverse of viscosity. EEL mixtures combine the positive attributes of commonly-used liquids, such as high solvent strength, with the positive attributes of supercritical fluids, such as low viscosity or high fluidity, low surface tension, high diffusivity. These attributes allow EELC to contribute to the quest for increased separation power. 

The unique feature of supercritical fluids as solvents is that their solvating strength is directly related to their densities, which can be easily varied as a function of pressure and temperature. Above the critical point, the densities of supercritical fluids increase with increased pressure and decrease with increasing temperatures. Their properties are similar to those of both liquids and gases. The densities and solvating power can approach that of a liquid, whereas the viscosity is intermediate and diffiisivity is much closer to properties of gases (19). 

A particularly attractive and useful feature of supercritical fluids is that these materials can have properties somewhere between those of a gas and a liquid (Table 2). A supercritical fluid has more liquid-like densities, and subsequent solvation strengths, while possessing transport properties, ie, viscosities and diffusivities, that are more like gases. Thus, an SCF may diffuse into a matrix more quickly than a liquid solvent, yet still possess a liquid-like solvent strength for extracting a component from the matrix. 

Steady-State Solvatochromism. The majority of the reports on supercritical fluid solvation have used steady-state solvatochromic absorbance measurements (21-28). The original aim of these experiments was to determine the solvating power of supercritical fluids for chromatography and extraction (SFC and SFE) (26,28). To quantify solvent strength, researchers (21-28) adopted the Kamlet-Taft x solvent polarity scale (50-55). This scale best correlates solvatochromic effects on a- x and x- x electronic absorption transitions. 

Vibrational spectroscopy, too, has been used to study supercritical fluid systems. Buback reviewed (59) this area however, much of his discussions are on fluid systems that are well removed from ambient conditions or difficult to handle easily (e.g., H20, HC1). In an early report, Hyatt (21) used IR absorbance spectroscopy to determine the influence of several solvent systems, including COz, on the vibrational frequencies ( ) of solute molecules. Specifically, he studied the vc=o of acetone and cyclohexanone and vs.H of pyrrole. The goal of this work was to determine the suitability of supercritical fluids as reaction solvent. Hyatt concluded that the ketones experienced an environment similar to nonpolar hydrocarbons in COz and that there were no differences between liquid and supercritical CO2. In contrast, the pyrrole studies indicated that the solvent strength of CO2 was between ether and ethyl acetate. This apparent anomalous result was a manifestation of the, albeit weak, degree of pyrrole hydrogen bonding to CO2. 

Nile Red was recently introduced as a solvatochromic dye for studying supercritical fluids (10). Although not ideal, Nile Red does dissolve in both nonpolar and polar fluids and does not lose its color in the presence of acids, like some previously used dyes. Major criticisms of Nile Red include the fact that it measures several different aspects of "polarity" simultaneously (polarizability and acidity (15)) yet it is insensitive to bases (10). However, in chromatography other single dimension polarity scales, like P, are routinely used. Measurements with Nile Red and other dyes indicate that the solvent strength of binary supercritical fluids is often a non-linear function of composition (10-14). For example, small... 

Supercritical CO2 is a particularly good choice in aroma studies since it has an extremely low boiling point and leaves no off-odor residue to interfere in either analytical work or sensory evaluation. The fact that the solvent strength of a supercritical fluid depends on density is an additional factor which may be useful. One can vary solvent properties by changing density, thereby obtaining an effective extraction of a broad range of aroma compounds. 

Finally, note that when products are soluble in organic solvents, the use of supercritical fluid as eluent can be of key interest. Moreover, substituing liquid by supercritical fluids enables us to introduce a pressure gradient and thus an eluent strength modulation in the system.54,55... 

In addition to common organic solvents, supercritical fluids (scf s) can be used for a great variety of extraction processes [158 165], Supercritical fluid extraction (SFE), mostly carried out with SC-CO2 as eluant, has many advantages compared to extractions with conventional solvents. The solvent strength of a supercritical fluid can easily be controlled by the pressure and temperature used for the extraction at a constant temperature, extraction at lower pressures will favour less polar analytes, while extraction at higher pressures will favour more polar and higher molar mass analytes. As supercritical fluids such as CO2 and N2O have low critical temperatures (tc = 31 °C and 36 °C, respectively), SFE can be performed at moderate temperatures to extract thermolabile compounds. Typical industrial applications using SC-CO2 include caffeine extraction from coffee beans [158] as well as fat and oil extraction from plant and animal tissues [165]. For some physical properties of supercritical solvents, see Section 3.2. 

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