Nonaqueous solutions dielectric constants

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. 

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. 

Lower Conductivity. The equivalent conductance of nonaqueous solutions a( infinite dilution is often comparable to that of aqueous systems, but it decreases with an increase in concentration more rapidly than the corresponding aqueous systems (the effect of the lower dielectric constant). Since the specific conductivity, K (that which determines the resistance between cathode and anode) is proportional to Ac, the equivalent conductance, the IR drop between the electrodes of a cell in which deposition from nonaqueous solutions is to lake place will be greater than that in aqueous solution (see Section 4.8.7). The electricity needed to deposit a given mass of metal is proportional to the total E between the electrodes, and this includes the IR between the electrodes, which is much greater in the nonaqueous than in the aqueous cases. Hence, nonaqueous deposition will be more costly in electricity (more kilowatt hours per unit of weight deposited) than a corresponding deposition in aqueous solution. The difference may be prohibitive. 

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). 

Polyimides have excellent dielectric strength and a low dielectric constant, but in certain electrolyte solutions they can electrochemically transport electronic and ionic charge. Haushalter and Krause (5) first reported that Kapton polyimide films derived from 1,2,4,5-pyromellitic dianhydride (PMDA) and 4,4 -oxydianiline (ODA) undergo reversible reduction/oxidation (redox) reactions in electrolyte solutions. Mazur et al., (6) presented a detailed study of the electrochemical properties of chemically imidized aromatic PMDA- derived polyimides and model compounds in nonaqueous solutions. Thin films of thermally... 

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. 

When numerical calculations are carried out with these equations, the essential conclusion that emerges is that in aqueous media, ion association in pairs scarcely occurs for Fl-valent electrolytes but can be important for 2 2-valent electrolytes. (However, see the results of post-Bjerrum calculations in 3.13.) The reason is that depends on through Eq. (3.156). In nonaqueous solutions, most of which have dielectric constants much less than that of water (r = 80), ion association is extremely important. 

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. 

Thus, because of the lower dielectric constant values, the effect of an increase of electrolyte concentration on lowering the equivalent conductance is much greater in nonaqueous than in aqueous solutions. The result is that the specific conductivity of nonaqueous solutions containing practical electrolyte concentrations is far less than the specific conductivity of aqueous solutions at the same electrolyte concentration (Table4.26 andFig. 4.107). 

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. 

Coates and Jordan, 1960 Herskovits et cU., 1961) suggest that the phosphate groups of the DNA molecule are extensively paired to counterions in methanol solution, such that a /D (Section IV,B,1) is only roughly one-sixth of its value in water. This applies to the disrupted conformation of the DNA molecule as it exists in methanol solutions, but it may be assumed that a /D is not greatly different for the hypothetical native helical conformation in methanol. It can be concluded, therefore, that there is a net reduction in electrostatic repidsive interactions, and in electrostatic free energy, for the native conformation of DNA in methanol compared to water. Similar considerations apply in ethanol, whose dielectric constant is still smaller than that of methanol. Therefore, electrostatic factors alone would tend to stabilize the helical conformation in these nonaqueous solvents, and in spite of this, the structure is disrupted. 

This combination of Equations [5] and [16] is called the Generalized Born/Surface Area model (GB/SA), and it is currently available in the Macro-ModeP computer package. The speed of the molecular mechanics calculations is not significantly decreased by comparison to the gas phase situation, making this model well suited to large systems. Moreover, the model takes account of some first-hydration-shell effects through the positive surface tension as well as the volume polarization effects. A selection of data for aqueous solution is provided later (Table 2), and the model is compared to experiment and to other models. Nonaqueous solvents have been simulated by changing the dielectric constant in the appropriate equations, 8 but to take the surface tension to be independent of solvent does not seem well justified. 

Our thinking in terms of acid strength is usually confined to aqueous solutions where we have considerable quantitative data, but in strongly basic or strongly acid nonaqueous solvents differences in acid or basic strength of weak acids or bases will be marked. As a specific example, let us consider the interaction of the methylbenzenes with the solvent anhydrous hydrogen fluoride. As this solvent has a high dielectric constant, the equilibrium constant for the reaction... 

In this chapter some aspects of the present state of the concept of ion association in the theory of electrolyte solutions will be reviewed. For simplification our consideration will be restricted to a symmetrical electrolyte. It will be demonstrated that the concept of ion association is useful not only to describe such properties as osmotic and activity coefficients, electroconductivity and dielectric constant of nonaqueous electrolyte solutions, which traditionally are explained using the ion association ideas, but also for the treatment of electrolyte contributions to the intramolecular electron transfer in weakly polar solvents [21, 22] and for the interpretation of specific anomalous properties of electrical double layer in low temperature region [23, 24], The majority of these properties can be described within the McMillan-Mayer or ion approach when the solvent is considered as a dielectric continuum and only ions are treated explicitly. However, the description of dielectric properties also requires the solvent molecules being explicitly taken into account which can be done at the Born-Oppenheimer or ion-molecular approach. This approach also leads to the correct description of different solvation effects. We should also note that effects of ion association require a different treatment of the thermodynamic and electrical properties. For the thermodynamic properties such as the osmotic and activity coefficients or the adsorption coefficient of electrical double layer, the ion pairs give a direct contribution and these properties are described correctly in the framework of AMSA theory. Since the ion pairs have no free electric charges, they give polarization effects only for such electrical properties as electroconductivity, dielectric constant or capacitance of electrical double layer. Hence, to describe the electrical properties, it is more convenient to modify MSA-MAL approach by including the ion pairs as new polar entities. 

The expression (28) has been successfully applied to the description of the osmotic coefficients for different nonaqueous electrolyte solutions with solvent dielectric constant in the range 20< e <36 [6, 14], when ion diameters are treated as adjustable parameters. Examples of the AMSA calculations of the osmotic coefficients for electrolyte solutions are given in Fig. 1 [14]. 

Dielectric constants cannot explain, quantitatively, most physicochemical properties and laws of solutions, and we shall soon see that they can become unimportant. The molecules of more polar solvents, which tend to cluster around the ions and dipole ions, produce a preferential or selective solvation that is reflected in measurements of such properties as solubility, acid—base equilibria, and reaction rates. Nonelectrostatic effects, such as the basicity of some solvents, their hydrogen-bonding, and the internal cohesion and the viscosity of mixtures, probably interfere with the electrostatic effects and thus reduce their actual influence. On the other hand, mixtures of water and nonaqueous solvents are enormously complicated systems, and their effective microscopic properties may be vastly different from their macroscopic properties, varying with the solute because of selective attraction of one of the solvents for the solute. 

---