Dielectric constants their explanation

The explanation of these results proposed by Kraus and Fuoss21 in a series of papers, is based on two assumptions. The first of these is that electrolytes that are completely dissociated in water or any other solvents of high dielectric constant will be more or less associated into ion pairs in solvents of low dielectric constants. Ion pairs, AB, are considered to form entirely by electrostatic forces from the charged ions A+ and B , and the complexes are assumed to take no part in the conduction. Though no sharp division has been made experimentally these ion pairs are considered to differ from the undissociated portion of a weak electrolyte in that no electron shift has occurred in their formation. The second assumption is that, as the concentration of the ion pairs increases, a proportion of them will combine with ions by electrostatic forces, to form triple ions. ... 

Equations (92) and (93) show that the presence of a solvent medium other than a free space much reduces the magnitude of van der Waals interactions. In addition, the interaction between two dissimilar molecules can be attractive or repulsive depending on refractive index values. Repulsive van der Waals interactions occur when n3 is intermediate between nx and n2, in Equation (92). However, the interaction between identical molecules in a solvent is always attractive due to the square factor in Equation (93). Another important result is that the smaller the n - nj) difference, the smaller the attraction will be between two molecules (1) in solvent (3) that is the solute molecules will prefer to separate out in the solvent phase which corresponds to the well-known like dissolves like rule. However there are some important exceptions to the above explanation, such as the immiscibility of alkane hydrocarbons in water. Alkanes have nx = 1.30-1.36 up to 5 carbon atoms, and water has a refractive index of n = 1.33, and very high solubility may be expected from Equation (93) since the van der Waals attraction of two alkane molecules in water is very small. Nevertheless, when two alkane molecules approach each other in water, their entropy increases significantly because of the very high difference in their dielectric constants and also the zero-adsorption frequency contribution consequently alkane molecules associate in water (or vice versa). This behavior is not adequately understood. 

The most trivial explanation for the effect of electrolytes on rate of proton dissociation is to consider the effect of salts on the dielectric constant of the solution (see also Equation 1). In concentrated salt solutions, a considerable fraction of the water molecules are oriented in an hydration shell around the ions thus, their dielectric constant is smaller than in pure water (Hasted et al., 1948). A decreased dielectric constant will accelerate ion-pair recombination and slow down ion-pair separation. 

However, in order for this explanation to be consistent with the observed monotonic increases of the products A and Dttc as the temperature is lowered toward A the breadth of the relaxation time distribution has to increase (or the Kohlrausch exponent, l-n, has to decrease) correspondingly. However, for two glassformers, ortho-terphenyl (OTP) and tni naphthylbenzene (TNB) which show the breakdown of the SE and DSE relations, Richert and coworkerss recently reported that their dielectric spectra are characterized by a temperature independent width (e.g. l-rid is constant and is equal to 0.50) from 345-417 K in the case of TNB. The Tg of TNB is 342 K. Photon correlation spectroscopic and NMR measurements all indicate a temperature-independent distribution of relaxation times. Thus, the data of TNB and OTP contradicts the explanation based on spatial heterogeneities. On the other hand, an alternative explanation based on intermolecular coupling (originating from many-molecule relaxation) continues to hold. ... 

Persistent reports indicate that the Stokes-Einstein equation may be valid in molten salts and that a suitable choice for the hydrodynamic friction constant a will be 4.67t, approximating to nearly complete slippage. The only explanation for this that has been offered so far is that the near-square potential energy wells in which ions reside in melts fortuitously resemble the Van der Waals interaction wells operating in the dielectric continuum envisaged in the original formulation in all respects except their strength. 

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