Nonaqueous solvents, dielectric constants

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. 

Some physical properties of 3-propyl-4-ethylsydnone have been determined at various temperatures <1997BCJ315>. The dielectric constant (e = 64.6 at 25°C) is high compared to many organic solvents and close to that of propylene carbonate (e = 64.9), a typical nonaqueous polar solvent. 

The temperature coefficient of conductance is approximately 1-2 % per °C in aqueous 2> as well as nonaqueous solutions 27). This is due mainly to thetemper-ature coefficient of change in the solvent viscosity. Therefore temperature variations must be held well within 0.005 °C for precise data. In addition, the absolute temperature of the bath should be known to better than 0.01 °C by measurement with an accurate thermometer such as a calibrated platinum resistance thermometer. The thermostat bath medium should consist of a low dielectric constant material such as light paraffin oil. It has been shown 4) that errors of up to 0.5 % can be caused by use of water as a bath medium, probably because of capacitative leakage of current. 

In the nonaqueous organic solvent-based systems used for lithium batteries, the conductivities are of the order of 10 —10 S/cm k Compared to water, most organic solvents have a lower solvating power and a lower dielectric constant. This favors ion pair formation, even at low salt concentration. Ion pair formation lowers the conductivity as the ions are no longer free and bound to each other. Organic electrolytes show lower conductivities and much higher... 

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. ... 

The mixing of solvents in various ratios tinned out to be a powerful optimization tool in nonaqueous CE, since a favorable ratio of mixture dielectric constant to its viscosity enhances separation efficiency and analysis speed. 

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. 

Amirjahed, A.K. and Blake, M.l. Relationship of composition of nonaqueous binary solvent systems and dielectric constant),. Pharm. Sci., 63, 81-84, 1974. 

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. 

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