Isodielectric solvent

Table VII collects the results for all monovalent ion systems for which spectroscopic data are available. Studies of preferential solvation are still at a stage comparable to the establishment of Raoult s and Henry s laws for binary nonelectrolyte solutions. Correlation with thermodynamic data is encouraging for isodielectric solvent systems, but further consideration of the electrostatic terms necessary in the discussion of other systems is required. It is hoped that this present work, which coordinates, correlates, and advances progress made by other workers (7, 18,19, 20, 45, 46, 61, 62, 66, 67, 68), will stimulate systematic experimental investigations of suitable systems by both spectroscopic and thermodynamic methods.

Mayer [22], the above correlations indeed work well and are quite useful for predicting values such as the free energy of salt solutions and complex formation in various solvents. Another typical example of the importance of the use of DN and AN as solvent parameters, instead of properties such as the dielectric constant, would be ion pair association constants in isodielectric solvents. For instance, as shown by Mayer [15], association constants of various perchlorates isocyanates, and halides (alkali metal, ammonium, and tetraalkyl ammonium cations) are very different in isodielectric solvents such as nitromethane (DN = 2.7), acetonitrile (DN = 14.1), and DMF (DN = 26.6), whose dielectric constant is around 26 at room temperature. 

This equation corresponds to Eq. (28) of Covington et (Part 2), who found that AG values from electrochemical and spectroscopic experiments agree satisfactorily in mixtures of isodielectric solvents. Relations between AG and 6 have also been treated which involve change of solvation number and nonstatictlcal distribution of the solvated species. ... 

Examples of the use of this procedure are shown in Table 5-20. These data for the Sn2 reaction (5-116) are taken from Parker et al. [6, 291], whose extensive compilation compares relative rates and solvent-transfer coefficients on transfer from the reference solvent methanol to the isodielectric solvent V,V-dimethylformamide at 25 °C. 

Non-Coulombic forces are those that are responsible for different degrees of association of electrolytes in isodielectric solvents. For example, one can see the importance of non-Coulombic forces in respect to ion-pair formation when one compares the temperature dependence of the association constants of alkali metai salts and the tetraalkylammonium salts in protic solvents. The association constants of aikali metal halides show a monotonicaily increasing when plotted against... 

Table IX shows six examples of reactions in almost isodielectric solvents. The increase of up to six orders of magnitude in the rate constant of the reactions 1) to 3) is the result of drastically decreasing anion solvation and weakly increasing solvation of the activated complex in the dipolar aprotic solvents with a minimiun effect for the soft anion, SCN , which is better solvated in AN or DMSO than hard anions. The stronger solvation of the polar activated complex in dipolar aprotic solvents and...

The reaction [9.104] also has been studied for isodielectric mixtures of alcohol-chlorobenzene with 8=20,2 (permittivity of pure n-propanol) and 8=17.1 (permittivity of pure n-butanol) to investigate flic relative effect of universal and specific solvation on the resolvation process. The mixtures were prepared by adding chlorobenzene to methanol, ethanol, and C1-C3 alcohol. Alcohol is a solvate-active component in these isodielectric solvents. K s data are given in Table 9.6. 

Table 9.6. Equilibrium constants of the process [9.104] in isodielectric solvents...

The expression for the standard molar Gibbs energy of transfer is similar to Equation 6.24, but in terms of the mole fractions rather than the volume fractions used by Cox et al. [75]. However, A G (I, A -> A + B) has an additional term that is related to the electrostatic effect of the transfer from the source solvent with permittivity to the solvent mixture with its different permittivity (unless transfer is between isodielectric solvents). 

A great variety of aqueous—organic mixtures can be used. Most of them are listed in Table I with their respective freezing point and the temperature at which their bulk dielectric constant (D) equals that of pure water. These mixtures have physicochemical properties differing from those of an aqueous solution at normal temperature, but some of these differences can be compensated for. For example, the dielectric constant varies upon addition of cosolvent and cooling of the mixture in such a way that cooled mixed solvents can be prepared which keep D at is original value in water and are isodielectric with water at any selected temperature (Travers and Douzou, 1970, 1974). 

When molecules of both solvents only weakly interact specifically, the last term of Eq. (66) can be neglected for isodielectric mixtures. Such a simplified equation was used [255] for the calculation of the AG of Ag(I) and Cu(I) from AN or DMSO to their mixtures with other solvents. A good agreement was found between the calculated and the experimental data. 

Adding a high-molecular-weight solvent (dimethylformamide) indeed raised the rate and the polymer molecular weight. On the other hand, when the reaction was conducted under isodielectric conditions, the second order of reaction was approached. An isolated polymer zwitterion can exist in solution in two conformations ... 

Table IX Solvent effect In (k /k ) in isodielectric scdvents. k° rate constant in methanol as the reference solvent...

In binary mixtures of MeOH, AN and NM having an almost constant Er, /-AmaBuN salts are dissociated but the variation of the Walden product with solvent composition suggests specific ion-solvent interactions. D Aprano and Fuoss observed specific effects on the conductance of the Pic", Br" and BPh of Bu4N in isodielectric mixtures of dioxan with water, MeOH, AN and / -nitroaniline (PNA). and the... 

As appears from the above, specific solvation of one component decreases equilibrium constants in comparison with equilibrium constants in isodielectric universal solvent. 

From [9.106], isodielectric solution formed by two mixed solvents I and II follows the relationship ... 

The thermodynamic description of a process in a solvent has to lead to defined values of AHt and ASx, which are called van t Hoff or original parameters. It follows from Figure 9.23 that one has to differentiate the equilibrium constant of polytherms corresponding to isodielectric values of the solvent on temperature (e.g., to equilibrium constant values of c-d dotted line). 

Attempts to correlate rate constants with solvent dielectric functions are not always successful. A marked lack of correlation between logk and D, IID, or (D — 1)/(2D + 1) has been reported for the addition of 2,2 -bipyridyl to [Ni(dtp)2] and for dissociation of the [Fe(phen)s] + cation. Interestingly, aquation rates for the [Fe(5-NOaphen)3] + cation in hydrogen peroxide-water mixtures vary markedly with solvent composition despite the almost isodielectric properties of these mixtures. ... 

A. M. Shkodin and T. P. Sogoyan, Elektrokhimiya, 6, 1643 (1970). Thermodynamic characteristics of hydrogen chloride in isodielectric mixed solvents. 

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