Dielectric constant pressure variation

The dielectric constant of dense, supercritical water can range from 5 to 20 simply upon variation of the applied pressure. 

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. 

Figure 6.3 Variation in dielectric constant with pressure for scCHF3, scC02 and scH20 [7]...

A series of works by Matsuda et al. composed perhaps the first systematic study to explore the physical foundation for such a mixing effect. Using PC/DME as a model system, they investigated the dependence of vapor pressure, dielectric constant, and viscosity on solvent composition, and they correlated these variations with ion conductions. It was found that the dielectric constant varied with solvent composition by following an almost linear relation, with slight positive deviations, while viscosity always showed a pronounced negative deviation from what a linear relation would predict (Figure 6b). For such binary solvent systems, approximate quantifications... 

Some important dielectric behavior properties are dielectric loss, loss factor, dielectric constant, direct current (DC) conductivity, alternating current (AC) conductivity, and electric breakdown strength. The term dielectric behavior usually refers to the variation of these properties as a function of frequency, composition, voltage, pressure, and temperature. 

The inversion, with or without invertase or acid, has also been useful in studying the chemical effect of the deuterium ion, of high pressure, of sonic and supersonic energy of ultraviolet light " of high-frequency electric energy and of variation in the dielectric constant of the solvent. ... 

The basic properties of water such as viscosity, dissociation constant, dielectric constant, compressibility, and the coefficient of thermal expansion play a major role in determining optimal reaction conditions for obtaining maximum benefits in both SCWO and WAO processes. The properties of water change dramatically with temperature, particularly near the critical point [24-26]. A well-known example, the variation of pAw with temperature at the saturation pressure, is shown in Fig. 3. The dissociation constant of water goes through a maximum around 250°C (pAw minimum), and then undergoes a sharp decline as the temperature approaches the critical point. The density and the dielectric constant of water also show sharp changes close to the critical point, as shown in Fig. 4. 

Figure 4 Variation of density and dielectric constant with temperature at the saturation pressure. 

We now turn attention to a completely different kind of supercritical fluid supercritical water (SCW). Supercritical states of water provide environments with special properties where many reactive processes with important technological applications take place. Two key aspects combine to make chemical reactivity under these conditions so peculiar the solvent high compressibility, which allows for large density variations with relatively minor changes in the applied pressure and the drastic reduction of bulk polarity, clearly manifested in the drop of the macroscopic dielectric constant from e 80 at room temperature to approximately 6 at near-critical conditions. From a microscopic perspective, the unique features of supercritical fluids as reaction media are associated with density inhomogeneities present in these systems [1,4],... 

In the case of charged surfaces, Henderson and Lozada-Cassou pointed out that the physical origin of the hydration repulsion can be attributed to the presence of a layer of lower dielectric constant, e, in the vicinity of the interface. It was demonstrated that the DLVO theory complemented with such a layer correctly predicts the dependence of hydration repulsion on the electrolyte concentration. A further extension of this approach was given by Basu and Sharma, who incorporated the effect of the variation of e in the theory of electrostatic disjoining pressure. Their model provides quantitative agreement with the experimental data at low electrolyte concentration and pH, and qualitative agreement at higher electrolyte concentration and pH. 

The properties of a solvent also influence the rates of chemical reactions. During a reaction, the transition state may be of higher or lower polarity than the initial state, A high relative dielectric constant lowers the activation energy of the reaction with a transition state of higher polarity than the initial state. By variation of the relative dielectric constant, achieved by adjusting the temperature and pressure, the reaction rates may be controlled. As a consequence, these reactions show a high activation volume. 

The dielectric constant is an important property for chemical reactions and reaction theory. Although it is not technically a thermodynamic property, it will be discussed here. For supercritical fluids such as CHF3 and water, temperature and pressure can be adjusted to achieve large variations in the dielectric constant. For others, such as CO2, the dielectric constant changes little between ambient pressure to several times the reduced pressure. Figure 5 illustrates the large differences in the dielectric constant between CO2 and CHF3. Both have similar qualitative behavior... 

In conclusion, it appears that both the Redlich and Meyer equation or the Owen and Brinkley treatment would be much more satisfactory than the simple Masson equation. " Thorough tests of these new relationships, however, will require more accurate data on densities and also on the variation of dielectric constant of the pure solvents with pressure. 

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