Theoretically lower dielectric constants

Whereas in acetonitrile the rate limiting step is an opening of the solvent shell of a reactant, in benzonitrile the back reaction of (5) between the protonated acridine orange cation (BH ) and the 3-methyl-4-nitrophenolate ion (A ) to form the ion pair is diffusion controlled (although the overall reaction to the neutral molecules is an endothermic process). Because of its lower dielectric constant than acetonitrile, the electrostatic interactions between reactants in benzonitrile outweigh specific solvent effects. In other words, in benzonitrile a rate limiting coupling of proton transfer to the reorientation of solvent dipoles does not occur and the measured rates are very fast. The ion recombination (I) + (II) in benzonitrile has a diffusion controlled specific rate (theoretical) k = 9 -1 -1... 

Such a mechanism is open to serious objections both on theoretical and experimental grounds. Cationic polymerizations usually are conducted in media of low dielectric constant in which the indicated separation of charge, and its subsequent increase as monomer adds to the chain, would require a considerable energy. Moreover, termination of chains growing in this manner would be a second-order process involving two independent centers such as occurs in free radical polymerizations. Experimental evidence indicates a termination process of lower order (see below). Finally, it appears doubtful that a halide catalyst is effective without a co-catalyst such as water, alcohol, or acetic acid. This is quite definitely true for isobutylene, and it may hold also for other monomers as well. 

A number of eases of satisfactory agreement with theoretical requirements have been found in methyl alcohol solutions this is particularly the case for the chlorides and thiocyanates of the alkali metals. Other electrolytes, such as nitrates, tetralkyl-ammonium salts and salts of higher valence types, however, exhibit appreciable deviations. These discrepancies become more marked the lower the dielectric constant of the medium, especially if the latter is noii-hydroxylic in character. The conductance of potassium iodide has been determined in a number of solvents at 25 and the experimental and calculated slopes of the plots of A against Vc arc quoted in Table XXV,... 

Riande et al. (21,22) synthesized the cis and tmns isomers of PCDS as well as the trans isomer of poly(l,4-cyclohexylenedimethylene adipate) (PCDA), and studied their polarity by measurements of dielectric constants and dipole moments. They observed that the trans isomer exhibited lower polarity than the cis one, and that the dipole moment decreased with the number of methylene groups of the diacid unit. The configurational properties of the two PCDS isomers were comparatively examined, both experimentally and theoretically, using the rotational isomeric state model. 

The activation volumina for the solutions at the lower polarity are lacking because in this region the relaxation time is predominantly determined by triple-ion formation even at the lowest TBAP concentration measurable with the field modulation technique. If the ionisation equilibrium is treated as the association-dissociation of hard, charged, spheres subjected to Brownian motion in a continuous medium of dielectric constant D and viscosity n it is possible to give theoretical expressions for and k ... 

Fig. 9 Example of an operating electric field versus feeding rate diagram for electrospinning adapted from Hohman et al. [32,33]. The upper shaded area shows the theoretically predicted onset of bending instabilities the lower one shows the corresponding onset of axisymmetric instability. The lines represent experimental results on the instability thresholds for the two types of instabilities for PEO solutions for a given set of electric conductivity, viscosity, dielectric constant, and surface free energy values corresponding to the ones assumed in the theoretical treatment...

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