Solvent strength critical

Solvent Strength of Pure Fluids. The density of a pure fluid is extremely sensitive to pressure and temperature near the critical point, where the reduced pressure, P, equals the reduced temperature, =1. This is shown for pure carbon dioxide in Figure 2. Consider the simple case of the solubihty of a soHd in this fluid. At ambient conditions, the density of the fluid is 0.002 g/cm. Thus the solubiUty of a soHd in the gas is low and is given by the vapor pressure over the total pressure. The solubiUties of Hquids are similar. At the critical point, the density of CO2 is 0.47 g/cm. This value is nearly comparable to that of organic Hquids. The solubiHty of a soHd can be 3—10 orders of magnitude higher in this more Hquid-like CO2. 

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],... 

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... 

Due to its compressibility in the liquid (near the critical point) and in the supercritical fluid state, the dielectric constant and density, and thus the solvent quality of C02, are tunable with pressure and temperature (Keyes and Kirkwood, 1930). As illustrated in Figure 1.2, this compressibility provides for control of the density and therefore solvent-dependent properties such as dielectric constant and overall solvent strength (Giddings et al., 1968). While supercritical C02 can have high liquidlike densities, it shares many of the... 

Solvent strength in the critical region. All of the experiments were performed with the dye phenol blue which has been well-characterized both experimentally and theoretically in liquid solvents (20,21,22). Since the dipole moment of phenol blue increases 2.5 debye upon electronic excitation (8), it is a sensitive probe of the local solvent environment. For example the absorption maxima occur at 550 and 608 nm in n-hexane and methanol, respectively. The excited state is stabilized to a greater extent than the ground state as the "solvent strength" is increased, which is designated as a red shift. 

In the solid state, II, III, and IV show 1 Ai ST2 spin equilibrium, while I is low-spin. In solution, II and III retain the spin equilibrium property, whereas IV is fully high-spin and I is low-spin over a temperature range of approximately 200 degrees. From the electronic spectra the critical field strength (crossover point) has been estimated to lie near 11700 cm-1. No solvent dependence has been observed. 

For temperatures superior to the critical temperature, it can be seen that the density increases with the pressure. The solvent strength can therefore be increased by increasing the pressure. 

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. 

The solvent strength of a SCF may be manipulated using pressure and/or temperature to adjust reaction rates by changing rate and equilibrium constants, or concentrations of reactants and products. The latter is due to the large changes in concentrations that occur in the critical region. 

The fluorescence data, specifically ratios of emission intensities, I1/I3, were used to show that solute-solvent clustering increases as the temperature approaches the critical temperature (I1/I3 is a measure of solvent strength). In liquid organic solvents, it is known that the ratio I1/I3 may be correlated linearly with another solvent strength parameter, it, by the relationship... 

Widespread application of near-critical and supercritical CO2 to process industries has been hindered up to now by a very weak solvent strength. In many applications where the physical and environmental properties of CO2 would otherwise be favorable, the fact that CO2 cannot dissolve or carry the required solute has blocked its application. One reason being the low dielectric constant (1.4-1.5) compared to the typical organic solvents (2.0. 0). The very hydrophobic nature of CO2 makes it a bad solvent for many solutes. Improving the solvent strength has been the focus of research in recent decades. 

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. 

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