Supercritical solvation strength

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

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). 

Figure 20.6—Supercritical fluid extraction. A comparison of the solvation strength of CO2 with classical solvents (Hildebrand scale) as a function of temperature and pressure is shown.

Instead of a liquid, a supercritical fluid can be for the extraction of solid samples. Carbon dioxide is an ideal solvent. The solvation strength can be controlled via the pressure and temperature. The high volatility of CO2 enables concentration of the sample and easy removal of the extraction liquid. 

Figure 21.8 Supercritical fluid extraction. Comparison of the solvation strength of the COj with respect to the usual solvents (HUdehrand scale) as a function of the temperature and pressure. The polarity of carhon dioxide in the supercritical state is comparable with that of hexane (for 100 atm and 35 °C). SPE is a method for which automation becomes a justified investment when the sample throughput is large. Above, sample extractor by supercritical fluids (Model SFE-703 reproduced courtesy of Dionex).

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. ... 

Density is a factor in the solvating power of a supercritical fluid the more dense the fluid, the more powerful its solvent strength. Since changing the temperature and pressure within the supercritical phase changes the density, a supercritical fluid can be made to possess a wide range of solvent power. This property together with its increased diffusion and lower viscosity makes supercritical fluid an attractive extraction medium. 

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. 

Carbon dioxide (CO2) has a low supercritical temperature (31°C) and pressure (73 atm). It is nontoxic and nonflammable and is available at high purity. Therefore, CO2 has become the solvent of choice for most SFE applications. Being nonpolar and without permanent dipole moment, supercritical CO2 is a good solvent for the extraction of nonpolar and moderately polar compounds. However, its solvating power for polar solutes is rather poor. Moreover, when the solutes bind strongly to the matrix, the solvent strength of CO2 is often inadequate to break the solute-matrix bond. 

Mixed solvents The addition of a small amount of a co-solvent to a supercritical fluid can increase solubilities of certain substances from several percent to several orders of magnitude (32.33.34.35). Spectroscopic data, which have been obtained recently, indicate that preferential solvation by a co-solvent contributes to the large increases. The co-solvents acetone, methanol, ethanol, and n-octane, were investigated by Kim and Johnston(36) using the solute phenol blue as a solvatochromic indicator. In Figure 7, it is apparent that the red shift (solvent strength) exceeds the value which is obtained from linear behavior, i.e. the concavity is positive. This means that the local concentration of co-solvent near the solute... 

Supercritical fluids (SCFs) such as carbon dioxide have a "hydrocarbon-like solvent strength at typical conditions, so that they are appropriate solvents for lipophilic substances. The solvent strength may be raised significantly by the addition of small amounts of cosolvents such as ethanol to increase solubilities of moderately polar substances selectivelyQ), sometimes by several hundred percent(2,2 4). The solvent and cosolvent form clusters about solutes, in which the cosolvent concentrations are enhanced significantly( ,fi). The present objective is to explore the effects of considerably more powerful solvent additives, that is surfactants. Since very little is known about surfactants in SCFs, spectroscopic probes were used to measure polarities inside the reverse micelles. Polarity is a key indicator of the ability of a reverse micelle to solvate a hydrophile. Using the... 

One approach to increasing CO2 solvent strength relies on the fact that most volatile materials (such as alcohols, ketones, and hydrocarbons) are soluble in supercritical and near-critical CO2. This allows the employment of a wide range of cosolvents to enhance the C02 s solvation properties. The use of such cosolvent-modified CO2 as a solvent medium is most recognized in the Unicarb spray-coating process commercialized by Union Carbide (now with Dow Chemical, Midland, MI) in the early 1990s. In this process, the majority of traditional solvents used in the spray coatings are replaced by supercritical CO2. This process has been implemented in automotive and furniture industries. 

Unimolecular reactions that have been used to investigate the solvation properties of supercritical fluids include tautomeric reactions (67-71), rotational isomerization reactions (72-78), and radical reactions (79-83). O Shea et al. used the tautomeric equilibrium of 4-(phenylazo)-l-naphthol to examine the solvent strength in supercritical ethane, CO2, and fluoroform and to determine its dependence on density (67). The equilibrium is strongly shifted to the azo tautomer in supercritical ethane and the hydrazone tautomer in supercritical chloroform and... 

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