Nonaqueous solvents, dielectric

There is an extensive chemistry associated with the use of liquid ammonia as a nonaqueous solvent (see Chapter 10). Because it has a dielectric constant of 22 and a dipole moment of 1.46 D, ammonia dissolves many ionic and polar substances. However, reactions are frequently different than in water as a result of differences in solubility. For example, in water the following reaction takes place because of the insolubility of AgCl ... 

The properties of HF reflect the strong hydrogen bonding that persists even in the vapor state. As a result of its high polarity and dielectric constant, liquid HF dissolves many ionic compounds. Some of the chemistry of HF as a nonaqueous solvent has been presented in Chapter 10. Properties of the hydrogen halides are summarized in Table 15.9. 

Because of the small ionic radius of lithium ion, most simple salts of lithium fail to meet the minimum solubility requirement in low dielectric media. Examples are halides, LiX (where X = Cl and F), or the oxides Li20. Although solubility in nonaqueous solvents would increase if the anion is replaced by a so-called soft Lewis base such as Br , I , S , or carboxylates (R—C02 ), the improvement is usually realized at the expense of the anodic stability of the salt because these anions are readily oxidized on the charged surfaces of cathode materials at <4.0 V vs Li. 

The attempt to use these salts originated from the hope that their dissociation constants would be high even in low dielectric media, and the organic nature of perfluorinated alkyls would always assist the solubility of the salts in nonaqueous solvents. Because of the requirement for electrochemical stability, lithium carboxylates (RF-C02Li, where Rp- = perfluorinated alkyls) are excluded from consideration, because their oxidation still occurs at - 3.5 V vs lithium, which is similar to the cases of their non-fluorinated counterparts. Obviously, the electron-withdrawing groups do not stabilize the carboxylate anions sufficiently to alter their oxidative stability. 

One major drawback of these sulfonate salts is their poor ion conductivity in nonaqueous solvents as compared with other salts. In fact, among all the salts listed in Table 3, LiTf affords the lowest conducting solution. This is believed to be caused by the combination of its low dissociation constant in low dielectric media and its moderate ion mobil-ityi29 3 compared with those of other salts. Serious ion pairing in LiTf-based electrolytes is expected, especially when solvents of low dielectric constant such as ethers are used. ... 

Anastopoulos et al. [47] have analyzed interfacial rearrangements of triphenyl-bismuth and triphenylantimony at mercury electrode in nonaqueous solvents of high dielectric constant. These phenomena were detected as the peaks in the capacitance-potential curves at intermediate negative potentials for triphenyl-bismuth and triphenylantimony in N-methylformamide, A,A-dimethylforma-mide, dimethyl sulfoxide, propylene carbonate, and methanol solutions. 

The choice of electrode material is more critical in LCEC than in the usual electroanalytical experiment, primarily due to the mechanical ruggedness and long-term stability required. Carbon paste (an admixture of graphite powder and a dielectric material) remains a useful choice as an electrode material for LCEC. While carbon paste can be used in nonaqueous solvents if formulated... 

Nonaqueous Solvents. Many organic compounds are not soluble in water, and the investigator who desires to study their electrochemistry must resort to organic solvents. The solvents most often used are the so-called dipolar aptotic solvents that belong to Class 5a in the classification scheme of Table 7.5. These are solvents with moderately large dielectric constants and low proton availability. This aptotic character tends to simplify the electrochemical reactions often the primary product is a stable radical cation or anion that is produced by removal or addition of an electron. 

Dimethyl sulfoxide is an important solvent in nonaqueous electrochemistry due to its high polarity (dielectric constant of 47), its high donor number (29.8), and a relatively wide electrochemical window. The limiting cathodic voltages in which this solvent can be used depend on the cation used (as expected from the discussion on the cation effects on the reduction processes of the above nonaqueous solvents). Using salts of alkali metals (Li, Na, K), the cathodic limit obtained was around -1.8 — -2 V versus SCE [49], whereas with tetrabutyl ammonium, the cathodic limit was as low as -2.7 — -3 V versus SCE [49], There is evidence that in the presence of Na ions, DMSO reduction produces CH4 and H2 on plati-... 

Nonaqueous solvents can form electrolyte solutions, using the appropriate electrolytes. The evaluation of nonaqueous solvents for electrochemical use is based on factors such as -> dielectric constant, -> dipole moment, - donor and acceptor number. Nonaqueous electrochemistry became an important subject in modern electrochemistry during the last three decades due to accelerated development in the field of Li and Li ion - batteries. Solutions based on ethers, esters, and alkyl carbonates with salts such as LiPF6, LiAsly, LiN(S02CF3)2, LiSOjCFs are apparently stable with lithium, its alloys, lithiated carbons, and lithiated transition metal oxides with red-ox activity up to 5 V (vs. Li/Li+). Thereby, they are widely used in Li and Li-ion batteries. Nonaqueous solvents (mostly ethers) are important in connection with other battery systems, such as magnesium batteries (see also -> nonaqueous electrochemistry). 

Stability in nonaqueous solvents with low dielectric constants and low surface charge densities,... 

The higher dielectric constant of NMA, compared to water, might be expected to promote dissociation of acids but studies in other nonaqueous solvents and in mixed solvents indicate that the dielectric constant is seldom the predominant factor controlling acid dissociation processes199). Instead, whether a particular acid is stronger in one solvent or another, will likely be quite dependent on the relative solvation of the acid, of the proton and of the conjugate base in the two solvents. 

The pure liquid (bp -10°C) is a useful nonaqueous solvent despite its low dielectric constant (—15), and lack of any self-ionization. It is particularly useful as a solvent for superacid systems. 

Nass et al. [47], Clasen [48], and Furonoet al. [49]. The deposition of aqueous dispersions of polymers onto dielectrics is discussed by Tikhonov et al. [50]. Deposition from nonaqueous solvents, frequently used for nitride and carbide ceramics, is discussed by Kolesov et al. [51] and Petrov [52]. Malov et al. [53] applied a correction to the deposition equations to take account of the nonimiformity of the electric field in the neighborhood of the deposit. Hein et al. [54] have developed an improved electrophoretic technique for the deposition of polycrystalline YBa2Cu307 (and Bi-Sr-Ca-Cu-0) layers 10-20 ixm thick on arbitrarily... 

Since A CD, these considerations must be modified appropriately in considering nonaqueous solvents of different dielectric than water. 

Studies of ionic solutions have been overwhelmingly aqueous in the hundred years or so in which they have been pursued. This has been a blessing, for water has a dielectric constant, s, of 80, about ten times larger than the range for most nonaqueous solvents. Hence, because the force between ions is proportional to 1/e, the tendency of ions in aqueous solutions to attract each other and form groups is relatively small, and structure in aqueous solutions is therefore on the simple side. This enabled a start to be made on the theory of ion-ion attraction in solutions. 

The Coulombic attractive forces given by z+z el/ > are large when the dielectric constant is small. When nonaqueous solvents of low dielectric constant are used, the... 

When one switches from water to some nonaqueous solvent, the magnitudes of several quantities in the Debye-Hiickel-Onsager equation alter, sometimes drastically, even if one considers the same true electrolyte in aU these solvents. These quantities are the viscosity and the dielectric constant of the medium, and the distance of closest approach of the solvated ions (i.e., the sum of the radii of the solvated ions). As a result, the mobilities of the ions at infinite dilution, the slope of the A versus... 

NO3 in acetonitrile has been obtained by Janz and Muller. Associated structures of ions have been studied in nonaqueous solvents over a wide range of dielectric constants. LiCNS in solvents of low dielectric constant, such as ethers and thioethers, gives rise to several different types of ion aggregates. Many different types of contact ion pairs or agglomerates have been identified, and the role the solvent has in this association—whether the solvent separates the ions or not—has been determined. The Bjerrum critical distance, that is, the distance at which the ion is able to interact with other ions to form ion-pair stmctures (see Section 4.8.8), is of great use in these types of studies. Table 4.25 shows some values for 1 1, 2 2, and 3 3 electrolytes in different solvents. 

So the question of the specific conductivity of nonaqueous solutions vis-a-vis aqueous solutions hinges on whether the dielectric constant of nonaqueous solvents is lower or higher than that of water. Table 4.23 shows that many nonaqueous solvents have fi s considerably lower than that of water. There are some notable exceptions, namely, the hydrogen-bonded liquids. 

In summary, it is the lower dielectric constants of the typical nonaqueous solvent that cause a far greater decrease in equivalent conductivity with an increase of concentration than that which takes place in typical aqueous solutions over a similar concentration range. Even if the infinite-dilution value A makes a nonaqueous electrochemical system look hopeful, the practically important values of the specific conductivity (i.e., the ones at real concentrations) are nearly always much less than those in the corresponding aqueous solution. That is another unfortunate aspect of nonaqueous solutions, to be added to the difficulty of keeping them free of water in ambient air. 

From the expression for q, it is clear that the lower the dielectric constant of the solvent, the larger is the magnitude of q. Hence, when one replaces water with a nonaqueous solvent, the likelihood of ion-pair formation inaeases because of the increasing q (assuming that a does not inaease in proportion to q). 

When the dielectric constant of the nonaqueous solvent goes below about 15, ions can associate not only in ion pairs but also in ion triplets. This comes about by one of the ions (e.g., M )ofan ion pairM -A Coulombically attracting a free ion A strongly enough to overcome the thermal forces of dissociation... 

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