Dielectric pressure dependence

Many solvent properties are related to density and vary with pressure in a SCF. These include the dielectric constant (er), the Hildebrand parameter (S) and n [5], The amount a parameter varies with pressure is different for each substance. So, for example, for scC02, which is very nonpolar, there is very little variation in the dielectric constant with pressure. However, the dielectric constants of both water and fluoroform vary considerably with pressure (Figure 6.3). This variation leads to the concept of tunable solvent parameters. If a property shows a strong pressure dependence, then it is possible to tune the parameter to that required for a particular process simply by altering the pressure [6], This may be useful in selectively extracting natural products or even in varying the chemical potential of reactants and catalysts in a reaction to alter the rate or product distributions of the reaction. 

As for the rate constants above 300 °C, little experimental results have been reported. Again, not only temperature, but also pressure dependence was reported. One of the important issues is the decrease of the dielectric constant of water in supercritical water, where it is less than 10 and much lower than the value of 79 at room temperature. It was pointed out that the Coulombic interaction becomes important and the radiolysis of supercritical water resembles that of the organic liquids with low dielectric constant [88]. In addition, it should be noted that the solubility of the solute is quite low and ion pairing would have a significant role [89-91], which reflects the difficulties for the sample preparation in the actual experiment. 

The temperature-pressure dependency of the dielectric constant of water. 

Table 10.3 provides some examples of organic compounds and their respective dielectric constants. Many organic compounds become miscible in supercritical water because they behave almost as a nonaqueous fluid. For example, at 25°C, benzene is barely soluble in water (solubility, 0.07 wt%) however, at 260°C, the solubility is about 7 to 8 wt% and is fairly independent of pressure. At 287°C, the solubility is somewhat pressure dependent, with a maximum of solubility of 18 wt% at 20 to 25 MPa. In this pressure range and at 295°C, the solubility rises to 35 wt%. At 300°C, the critical point of... 

Dielectric monitoring has been used to determine the optimum time at which pressure can be applied on adhesives and composites that are very pressure-dependent. This is especially important for films and prepregs that have a high degree of retained solvent. Optimum pressure should not be applied until the solvent has had a chance to escape the adhesive when it is in the liquid state. Pressure is then applied when the solvent has escaped and the viscosity of the adhesive is high enough to prevent squeeze-out from the joint. 

J. G. Mavroides. J, Chem. Phys. 24, 398-9 (1956). Dielectric constant pressure dependence, water. 

Peercy PS, Fritz IJ, Samara GA (1975) Temperature and pressure dependences of the properties and phase transition in paratellurite (Te02) ultrasonic, dielectric and Raman and Brillouin scattering results. J Phys Chem Solids 36 1105-1122... 

The pressure dependence was explained by the fact that scCHFs is a more polar solvent (has a higher dielectric constant) at higher pressures than it is at lower pressures. The enantioselectivity was better in nonpolar CHF3 than in polar CHF3, consistent with the observation that the selectivity is better in nonpolar liquid solvents than in polar liquids. The pressure independence of the enantioselectivity in SCCO2 is due to the fact that the dielectric constant of that SCF does not vary significantly with pressure. Cyclopropanations of this type can be used in the synthesis of the antidepressant sertraline (124) and of cilastatin, a stabilizer of the antibiotic imipenem (125). 

Jessop et al. (127) studied the pressure dependence of the aminoalcohol-catalyzed alkylation of benzaldehydes (Eq. (29), R = H or CF3] in SCCHF3 because of the known variability of the dielectric constant of scCHFs (see Section 2.6) ... 

The general experimental fact of constant frequency dispersion (or time dependence of the correlation function) of the a-relaxation at constant Ta for different combinations of T and P has an immense impact on glass transition. Although the data were mostly obtained by dielectric relaxation, the same effect was found in some glass-formers by photon correlation spectroscopy. The primary concern of most theories, including those mentioned in the NY Times article, is to explain the temperature and pressure dependences of the structural relaxation time Tq.. In these theories, the dispersion of the structural relaxation is either not addressed, or else considered separately with additional input not involved in arriving at r . Consequently, the frequency dispersion is unrelated to the relaxation time of the structural a-relaxation in these theories, and they are unlikely to be consistent with the T, / -superpositioning property by happenstance. 

Corezzi, S., Lucchesi, M., Rolla P. A., Capaccioli, S., Gallone, G. (1999) Temperature and pressure dependences of the relaxation dynamics of supercooled systems explored by dielectric spectroscopy, Phil. Mag. B 79, 1953-1963... 

Table 2 predicts that, of the biphase processes, the aqueous version will attain particular importance because of the many advantages of water as the support. As a solvent, water has numerous anomalies (e.g., density anomaly, the only non-toxic and liquid hydride of the non-metals, pressure-dependence of the melting point, dielectric constant), and its two- or even three-dimensional structure is still not well understood (cf. Sections 2.1 -2.3). Some of the known properties are listed below ... 

I 70 Catalytic and Solvophobic Promotion of High Pressure Addition Reactions Tab. 10.15. Pressure dependence of the dielectric constant e (20 °C). 

State is highly sensitive to the medium. There is a change of polarity of the reaction states from initial to transition state with a volume decrease known as electro-strictive shrinkage. Electrostriction can be critical to rate enhancement under pressure. In terms of kinetic parameters it is related to the pressure dependence of the dielectric constant (dins/dp). Some representative values of this quantity are listed in Table 10.15 [50]. 

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