Dielectric mixed solvents

The bulk properties of mixed solvents, especially of binary solvent mixtures of water and organic solvents, are often needed. Many dielectric constant measurements have been made on such binary mixtures. The surface tension of aqueous binary mixtures can be quantitatively related to composition. ... 

Because the key operation in studying solvent effects on rates is to vary the solvent, evidently the nature of the solvation shell will vary as the solvent is changed. A distinction is often made between general and specific solvent effects, general effects being associated (by hypothesis) with some appropriate physical property such as dielectric constant, and specific effects with particular solute-solvent interactions in the solvation shell. In this context the idea of preferential solvation (or selective solvation) is often invoked. If a reaction is studied in a mixed solvent. 

Sn2 reactions with anionic nucleophiles fall into this class, and observations are generally in accord with the qualitative prediction. Unusual effects may be seen in solvents of low dielectric constant where ion pairing is extensive, and we have already commented on the enhanced nucleophilic reactivity of anionic nucleophiles in dipolar aprotic solvents owing to their relative desolvation in these solvents. Another important class of ion-molecule reaction is the hydroxide-catalyzed hydrolysis of neutral esters and amides. Because these reactions are carried out in hydroxy lie solvents, the general medium effect is confounded with the acid-base equilibria of the mixed solvent lyate species. (This same problem occurs with Sn2 reactions in hydroxylic solvents.) This equilibrium is established in alcohol-water mixtures ... 

As already indicated, ion exchange resins are osmotic systems which swell owing to solvent being drawn into the resin. Where mixed solvent systems are used the possibility of preferential osmosis occurs and it has been shown that strongly acid cation and strongly basic anion resin phases tend to be predominantly aqueous with the ambient solution predominantly organic. This effect (preferential water sorption by the resin) increases as the dielectric constant of the organic solvent decreases. 

When a solute is added to an acidic solvent it may become protonated by the solvent. If the solvent is water and the concentration of solute is not very great, then the pH of the solution is a good measure of the proton-donating ability of the solvent. Unfortunately, this is no longer true in concentrated solutions because activity coefficients are no longer unity. A measurement of solvent acidity is needed that works in concentrated solutions and applies to mixed solvents as well. The Hammett acidity function is a measurement that is used for acidic solvents of high dielectric constant. For any solvent, including mixtures of solvents (but the proportions of the mixture must be specified), a value Hq is defined as... 

Cohen et have also made some observations on the exchange in water-sucrose and water-ethylene glycol mixed solvents containing perchloric acid (0.106 M). Over a range of dielectric constant 68 to 88, no alteration in the exchange rate was observed. 

Mixed-solvent solutions of various cosolvent-water proportions are titrated and psKa (the apparent pKa) is measured in each mixture. The aqueous pKa is deduced by extrapolation of the psKa values to zero cosolvent. This technique was first used by Mizutani in 1925 [181-183]. Many examples may be cited of pKa estimated by extrapolation in mixtures of methanol [119,161,162,191,192,196,200], ethanol [184,188-190,193], propanol [209], DMSO [212,215], dimethylformamide [222], acetone [221], and dioxane [216]. Plots of psKa versus weight percent organic solvent, Rw = 0 — 60 wt%, at times show either a hockey-stick or a bow shape [119]. For Rw > 60 wt%, S-shaped curves are sometimes observed. (Generally, psKa values from titrations with Rw > 60 wt% are not suitable for extrapolation to zero cosolvent because KC1 and other ion pairing interferes significantly in the reduced dielectric medium [223].)... 

The cosolvent will lower the dielectric constant of the mixed solvent, independent of the properties of the solute molecule. The ionization constant of acids will increase and that of bases will decrease (see Sections 3.3.3 and 3.3.4), the result of which is to increase the fraction of uncharged substance in... 

Another type of ternary electrolyte system consists of two solvents and one salt, such as methanol-water-NaBr. Vapor-liquid equilibrium of such mixed solvent electrolyte systems has never been studied with a thermodynamic model that takes into account the presence of salts explicitly. However, it should be recognized that the interaction parameters of solvent-salt binary systems are functions of the mixed solvent dielectric constant since the ion-molecular electrostatic interaction energies, gma and gmc, depend on the reciprocal of the dielectric constant of the solvent (Robinson and Stokes, (13)). Pure component parameters, such as gmm and gca, are not functions of dielectric constant. Results of data correlation on vapor-liquid equilibrium of methanol-water-NaBr and methanol-water-LiCl at 298.15°K are shown in Tables 9 and 10. 

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). 

These results suggest that the crystallographic determination of the structure of a productive enzyme-substrate complex is feasible for lysozyme and oligosaccharide substrates. They also provide the information of pH, temperature, and solvent effects on activity which are necessary to choose the best conditions for crystal structure work. The system of choice for human lysozyme is mixed aqueous-organic solvents at -25°C, pH 4.7. Data gathered on the dielectric constant, viscosity, and pH behavior of mixed solvents (Douzou, 1974) enable these conditions to be achieved with precision. 

Finally, we note that all transfers to alcohol-water mixtures or additions of alcohol to crystal mother liquor involve changes in the proton activity of the solution. Care must be taken to ensure that the pH does not change too much, or the crystal may be disrupted. Worse still, the enzymatic activity may be abolished. Control of proton activity in mixed solvents is discussed in Section III,D. If dielectric effects are controlled and pH is properly adjusted, the microenvironment of a crystalline protein will correspond closely to that of aqueous solution at room temperature. Such correspondence is essential for temporal resolution of individual steps in a catalytic reaction. 

We have seen that crystals can be safely transferred to mixed solvents and that the percentage of organic solvent may often be increased to any desired level provided that its gradual addition is coupled with a gradual reduction in temperature so as to keep the dielectric constant of the medium as near as possible to the value for the normal mother liquor. Such a result deserves explanation and comment about the behavior of the dielectric constant in mixed solvents as a function of temperature. 

It might be useful in some cases to raise the dielectric constant of mixed solvents by addition of suitable substances and it is known that dipolar molecules such as amino acids do so in pure water. These amino acids are virtually insoluble in nonpolar solvents but they dissolve readily in aqueous salt solutions and in most mixed solvents according to their highly polar structure. Most of what is known about their dielectric behavior concerns aqueous solutions, in which they were studied up to concentrations near saturation. 

The addition of an amino acid to mixed solvents at selected temperatures can be a means to compensate even partially for the decrease of dielectric constant due to the solvent addition. Limitations are imposed by the solubility of the amino acid in such mixtures for instance, there is a salting-out effect in methanol-water 50 50 at 25°C when the concentration of glycine is about 0.5 Af (8 20). 

Figure 6. (a) Optimization of ion conductivity in mixed solvents 1.0 M LiC104 in PC/DME. (b) Dependence of dielectric constant (e) and fluidity (j ) on solvent composition. These plots are reconstructed based on the data reported in refs 187 and 194, respectively. 

Choquette et al. investigated the possibilities of using a series of substituted sulfamides as possible electrolyte solvents (Table 12). These compounds are polar but viscous liquids at ambient temperature, with viscosities and dielectric constants ranging between 3 and 5 mPa s and 30 and 60, respectively, depending on the alkyl substituents on amide nitrogens. The ion conductivities that could be achieved from the neat solutions of Lilm in these sulfamides are similar to that for BEG, that is, in the vicinity of 10 S cm Like BEG, it should be suitable as a polar cosolvent used in a mixed solvent system, though the less-than-satisfactory anodic stability of the sulfamide family might become a drawback that prevents their application as electrolyte solvents, because usually the polar components in an electrolyte system are responsible for the stabilization of the cathode material surface. As measured on a GC electrode, the oxidative decomposition of these compounds occurs around 4.3—4.6 V when 100 fik cm was used as the cutoff criterion, far below that for cyclic carbonate-based solvents. 

In addition to linear carbonates, PC was also considered as a cosolvent that could help to improve the low-temperature performance of the electrolytes, mainly due to its wide liquid range and solvation ability to lithium salts. This latter property seems to be a merit relative to the linear carbonates, whose dielectric constants are generally below 10 and whose displacement of EC usually causes the solubility of lithium salts to decrease in such mixed solvents, especially at low temperatures. 

Even though Hildebrand theory should not apply to solvent systems having considerable solvent-solvent or solute-solvent interactions, the solubility of compounds in co-solvent systems have been found to correlate with the Hildebrand parameter and dielectric constant of the solvent mixture. Often the solubility exhibits a maximum when plotting the solubility versus either the mixed solvent Hildebrand parameter or the solvent dielectric constant. When comparing different solvent systems of similar solvents, such as a series of alcohols and water, the maximum solubility occurs at approximately the same dielectric constant or Hildebrand parameter. This does not mean that the solubilities exhibit the same maximum solubility. 

The spectra of substituted pyridinium iodides are characterized by charge transfer bands involving the interaction of pyridinium and iodide ions. Mukerjee and Rayt showed that this band is shifted about 90 nm toward the red for dodecyl pyridinium iodide, which forms micelles, compared to methyl pyridinium iodide, which does not. They measured max for the micelles in mixed solvents of variable relative dielectric constant and obtained the following results ... 

Methanol and Water. Methanol and water mixtures have been a popular choice for workers interested in free energies of transfer of ions from water into a mixed solvent. Such mixtures exhibit a drop in dielectric constant with increasing methanol content. Hence the electrical term must be estimated in order to compare spectroscopic and thermodynamic quantities. Feakins and Voice (28) have presented new data and revised earlier data for the alkali metal chlorides. In advance of carefully determined and extrapolated emf data for fluorides, using the solid state fluoride selective electrode based on lanthanum fluoride, some data of moderate accuracy have been presented (27). On the... 

This study was undertaken to determine whether or not the electrolytic conductance of the lithium bromide-bromosuccinic acid-acetone system can be described by the Fuoss-Onsager-Skinner equation (FOS equation)—Equation 2—by treating the system as lithium bromide in a mixed solvent, and to establish values for Ao and KA for lithium bromide in anhydrous acetone with the same equation. The equation requires knowledge of the concentration and corresponding equivalent conductance along with the dielectric constant and viscosity of the solvent and the temperature that is,... 

Procedure. Equation 1 indicates that it is necessary to determine the concentration, resistance, dielectric constant, viscosity, and temperature of the system. These data were acquired for five different solvent systems. A series of measurements, in which the concentration of lithium bromide was varied from about 10 5N to 10 3N, was made on each system. The solvents used were acetone (I), 0.02063m bromosuccinic acid in acetone (II), 0.05009m bromosuccinic acid in acetone (III), 0.09958m bromosuccinic acid in acetone (IV), and 0.05047m dimethyl bromosuccinate in acetone(V). Each solvent was used to prepare stock solutions of 10-2 and 10 3m lithium bromide. All mixed solvents and solutions were prepared in the dry box. 

The viscosities of the acetone-bromosuccinic acid mixed solvents were derived from the Jones-Dole (33) equation and data acquired by Muller, who used the special viscometer described by Tuan and Fuoss (34). The values used for the viscosities (in poise) of solvents I-V were 3.02 X 10 3, 3.05 X 10-3, 3.08 X 10-3, 3.13 X 10-3, and 3.02 X 10-3, respectively. The literature value for the dielectric constant of acetone, 20.7, was used as the dielectric constant for each solvent. This is justified because at the highest concentration of bromosuccinic acid its mole fraction is less than 0.004. 

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