Mixed solvent bulk dielectric constants

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). 

Figures 2 and 3 represent typical plots of E (= Eobs + 2RT/F In m) vs. ra in mixed solvents with a low (8.68%) and a high (89.00%) proportion of organic component (10,11,12,13). The pronounced increase in the curvature of the plots with increased organic component (decreased bulk dielectric constant) is quite obvious.

Incidentally, such plots (Figure 4) may be used to compute approximate values of the bulk dielectric constants of water-organic mixed solvents from the emf measurements provided that the bulk dielectric constant is higher than 40. In fact, for auto-ionizing solvents this may be a convenient method, and our calculations have shown that such computed values are within 5-7% of the experimental values. 

Our primary objective was to develop a computational technique which would correlate the ionization constant of a weak electrolyte (e.g., weak acid, ionic complexes) in water and the ionization constant of the same electrolyte in a mixed-aqueous solvent. Consideration of Equations 8, 22, and 28 suggested that plots of experimental pKa vs. some linear combination of the reciprocals of bulk dielectric constants of the two solvents might yield the desirable functions. However, an acceptable plot should have the following properties it should be continuous without any maximum or minimum the plot should include the pKa values of an acid for as many systems as possible and the plot should be preferably linear. The empirical equation that fits this plot would be the function sought. Furthermore, the function should be analogous to some theoretical model so that a physical interpretation of the ionization process is still possible. 

Through a series of parametrization, the most suitable linear combination was (1/c — 1/c") where c = c + (c — corg), = 2c — c0rg, = bulk dielectric constant of the mixed solvent, corg = bulk dielectric constant of the pure organic component, and c = bulk dielectric constant of water. (The dielectric constant data were obtained from literature these references are not being cited in order to save space. Some of the c values were obtained by these authors by interpolation. For the convenience of the reader we have compiled the values of bulk... 

In many centred molecules the interactions between the electro-active centres in a given molecule modify the spacing of the formal Standard Potentials of the successive processes by an amount that depends in the case of coulombic forces in part on the dielectric properties of the local surrounding electrolyte solution. This feature has been observed in Ae case of bimetallic complexes in mixed solvent systems of relatively low bulk dielectric constants, has been used to ascertain Ae impact of electrolyte concentration in particular ion-pairing on electrolyte dielectric behaviour. 

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. ... 

Pratt and co-workers have proposed a quasichemical theory [118-122] in which the solvent is partitioned into inner-shell and outer-shell domains with the outer shell treated by a continuum electrostatic method. The cluster-continuum model, mixed discrete-continuum models, and the quasichemical theory are essentially three different names for the same approach to the problem [123], The quasichemical theory, the cluster-continuum model, other mixed discrete-continuum approaches, and the use of geometry-dependent atomic surface tensions provide different ways to account for the fact that the solvent does not retain its bulk properties right up to the solute-solvent boundary. Experience has shown that deviations from bulk behavior are mainly localized in the first solvation shell. Although these first-solvation-shell effects are sometimes classified into cavitation energy, dispersion, hydrophobic effects, hydrogen bonding, repulsion, and so forth, they clearly must also include the fact that the local dielectric constant (to the extent that such a quantity may even be defined) of the solvent is different near the solute than in the bulk (or near a different kind of solute or near a different part of the same solute). Furthermore... 

The combination of experimental evidence and computational modeling show conclusively that stable, homogeneously blended (bulk-immiscible) mixed-polymer composites can be formed in a single microparticle of variable size. To our knowledge, this represents a new method for suppressing phase-separation in polymer-blend systems without compatibilizers that allows formation of polymer composite micro- and nanoparticles with tunable properties such as dielectric constant. Conditions of rapid solvent evaporation (e.g. small (<10 pm) droplets or high vapor pressure solvents) and low polymer mobility must be satisfied in order to form homogeneous particles. While this work was obviously focused on polymeric systems, it should be pointed out that the... 

Mixed solvents have been widely used in thermodynamic and kinetic studies on ionic interactions in solution with changing solvent properties of the reaction medium. Dielectric constants and viscosities of solvents are changed by altering the solvent composition of the mixtures. Theories proposed for the explanation of the variation of thermodynamic and kinetic data usually assume a homogeneous continuum medium with specific bulk properties, and the theories which are successfully applied to a neat medium usually fail when applied to mixed solvent systems. The disagreement of theoretical values with experimental ones has traditionally been put down to the inadequacy of the simple assumption of continuum of the solvents. The different solvent composition in the solvation shell from that in the bulk may be another more important factor for causing these discrepancies. 

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