Ionic solutions, permittivity decreases

The present evidence is thus that kinetic effects may account for half or more of permittivity decreases of ionic solutions and this may be an important factor in determing the amplitude of the Y dispersion in conducting biopolymer solutions and lead to revisions in estimated nature and amount of bound water. The effect may also have some bearing on dielectric properties of cell interiors and membranes if these have appreciable conductances. It would seem premature to attempt definitive answers to such questions until the relative importance of static and kinetic effects in presumably simpler ionic solutions has been better established experimentally in comparison with theory which treats them self-consistently. 

The permittivity of ionic solutions, is less than that of the pure solvent and decreases linearly with an increase in concentration. The reason for this has already been discussed (Section 2.12.1) water dipoles held by the very strong local field of an ion cannot orient against the weak applied field used in measuring the dielectric constant. The average is therefore decreased. 

In aqueous electrolyte solutions the molar conductivities of the electrolyte. A, and of individual ions, Xj, always increase with decreasing solute concentration [cf. Eq. (7.11) for solutions of weak electrolytes, and Eq. (7.14) for solutions of strong electrolytes]. In nonaqueous solutions even this rule fails, and in some cases maxima and minima appear in the plots of A vs. c (Eig. 8.1). This tendency becomes stronger in solvents with low permittivity. This anomalons behavior of the nonaqueous solutions can be explained in terms of the various equilibria for ionic association (ion pairs or triplets) and complex formation. It is for the same reason that concentration changes often cause a drastic change in transport numbers of individual ions, which in some cases even assume values less than zero or more than unity. 

In the above sections, we considered electrolytes that are ionophores.10 Iono-phores, like sodium chloride, are ionic in the crystalline state and are expected to dissociate into free ions in dilute solutions. In fact, in high-permittivity solvents (er>40), ionophores dissociate almost completely into ions unless the solutions are of high concentration. When an ionophore is completely dissociated in the solution, its molar conductivity A decreases linearly with the square root of the concentration c (<10 2 M) ... 

With the decrease in permittivity, however, complete dissociation becomes difficult. Some part of the dissolved electrolyte remains undissociated and forms ion-pairs. In low-permittivity solvents, most of the ionic species exist as ion-pairs. Ion-pairs contribute neither ionic strength nor electric conductivity to the solution. Thus, we can detect the formation of ion-pairs by the decrease in molar conductivity, A. In Fig. 2.12, the logarithmic values of ion-association constants (log KA) for tetrabutylammonium picrate (Bu4NPic) and potassium chloride (KC1) are plotted against (1 /er) [38]. 

Debye and Falkenhagen [92] also predicted that the permittivity of electrolyte solutions should increase as c,/2 where c is the ionic concentration. According to Hasted [105], such an effect has not been demonstrated experimentally, probably because the high conductivity of such solutions can mask permittivity changes. On the contrary, the permittivity of electrolyte solutions decreases with concentration [106] by 25—50% at lmoldm-3. This is probably associated with the binding of dipolar solvent molecules to ions, thus reducing the solvent orientational contri-butional to the permittivity (dielectric saturation). 

Because the dielectric constant (relative permittivity) of TFP is lower than that of EC (e, 11 for TFP, 90 for EC), the ionic conductivity of LiPFg solution decreases when adding TFP to mixed EC h-EMC solvent in the wide temperature range [115]. Also the higher concentration of TFP tends to deaease the rechargeability of the graphite-based negative electrode. Thus, about 40 % of the TFP component in the electrolyte satisfies both the nonflammability and the reversibility of the electrode in the LIB system. 

Standard chemical handbooks list o values for specific solutions at certain (typically room) temperatures. The actual conductivity, however, may vary with the electrolyte concentration. At very low concentration conductivity decreases, as there are not enough ions to support the conduction. At very high concentrations the electrostatic interactions between the ions reduce their mobility if the ions are very tightly packed. For electrolytes of low permittivity the electrostatic forces between the particles increase and ionic pairs can be formed. Higher temperatures may lead to formation of a less coherent ionic atmosphere, resulting in higher conductivity. 

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