Fluoroform, dielectric constant

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. 

They also obtained data indicating that in the Candida cylindracea lipase-fluorofbrm system the enzyme activity depends on fluoroform s dielectric con--stant. As the pressure rises from 59 bar to 280 bar the dielectric constant of fluoroform increases from 1 to 8 and the initial reaction rate decreases rapidly [50]. 

The majority of SCFs also show a sharp increase in the dielectric constant (e) with increasing pressure in the compressible region (around the critical point). This behavior reflects, to some extent, the change in density. The magnitude of the increase depends on the nature of the SCF whereas the dielectric constant varies little with pressure for non-polar substances such as SCCO2, dramatic increases are observed for more polar SCFs such as water or fluoroform (Figure 4.4). " ... 

The last point which can be evoked here is conceptually linked to the hot-spot theory. If the limit liquid layers around a bubble are in direct contact with the heated and pressurized bubble content, far above the critical point of the liquid, these layers should be in a supercritical state. 23 This attractive hypothesis (see p. 61) was used by Hoffmann et al to rationalize the sonolysis of nitrophenyl derivatives. Supercritical fluids are characterized by a very high flexibility of important parameters (density, dielectric constant, solubilizing power) as a function of pressure. Experts in the field distinguish gas-like and liquid-like media, in which the kinetics of a reaction can vary over a broad range. For instance, the conjugate addition of piperidine to methyl propiolate was studied in supercritical ethane or fluoroform (Fig. 5). ... 

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