Dielectric constant for water

In the atmosphere, most solutions available for reaction are aqueous. The dielectric constant for water at 25°C is e = 78.3. 

The dielectric constant (or relative permittivity) is usually expressed using the symbol c. The dielectric e is defined as the ratio of electric fields EJE for a vacuum and a substance placed between the plates of a capacitor. The dielectric constant of a vacuum is 1 and substances that can orient to greater or lesser extents in the applied field will have higher dielectric constants. The dielectric constant of heptane at 20°C is 1.9. Acetonitrile, CH3C=N , has a dielectric constant at 20°C of 37.5. The dielectric constant for water is near 80. 

Even if one assumes that the water near a surface has the same structure as it does in bulk, the oscillations of the short-range interactions between surfaces could be explained by a nonlocal dielectric constant for water.24 This model assumes that the dielectric displacement field (D = epE 4- P) at a position r not only depends on the local electric field [D(r) = r(r)E(r), but also depends on the electric field in the whole space D(r) = jr(r,r )E(r )dr. In this model, the oscillations of the interactions are due to charge overscreening25 and are analogous to the charge density waves in plasmas.24... 

Because of the experimental difficulties discussed above, there is limited opportunity for direct comparison between the different scales. There is a satisfactory qualitative trend in all the scales (including dielectric constant) for water, methanol, ethanol, propan-2-01, and f-butyl alcohol. However, it has been noted that a graph of Z versus Ej for pure alcohols and water is markedly curved (Fig. 16) and that 2,2,3,3-tetrafluoropropan-l-ol does not fit the curve (Dimroth et al., 1963). The results for both Y and Z scales suggest that the ET parameter for water is surprisingly low. Consequently,... 

There have been four recent evaluations of the dielectric constant for water. The earliest is that of Helgeson and Kirkham... 

The electrostatic bond strength depends strongly on the value assumed for the dielectric constant, and therefore on the separation of the charges and the proximity of any water molecules. One reason why the theoretical prediction of protein three-dimensional structure from the amino acid sequence is so difficult is because of the prevalence of water in vivo and the large value of the dielectric constant for water (80) as a result the force between charges varies dramatically depending on the number of water molecules interposed between the charges. 

Table 8.1 lists dielectric constants for water and a range of common organic solvents. The absolute permittivity... 

Capacitance Methods. Capacitance methods have been used to measure solids concentration in slurry pipelines (79). This method requires the dielectric constant of the solids and the carrying fluid to be significantly different. Sand-water slurry is a good example to use the capacitance method. In this case, the dielectric constant for water is 80, whereas that of the sand particles is 5. The method relies on the variation of the dielectric constant of the mixture, Em, with the solids concentration, C. For homogeneous slurries of spherical particles at low solids concentration, Maxwell s correlation can be used to predict the dielectric constant of the mixture. However, several investigators assumed that the relationship of the dielectric constant of the mixture and solids concentration was linear, as follows ... 

Table 9.1 lists dielectric constants for water and a range of common organic solvents. The absolute permittivity of a solvent is found using equation 9.3, but it is usual to discuss solvent properties in terms of the relative values. 

Qualitatively, the argument for using a low dielectric constant seems reasonable. The low dielectric constant comes into question, however, on closer scrutiny of experimental results on model proteins for which control of composition is possible, and it makes difficult what must be subjective judgments of a correct dielectric constant for water-filled clefts and crevices of various dimensions found in many protein-based machines. 

Table 1. Plasmon energies, real and imaginary parts of the dielectric constant for Au, and the dielectric constant for water.

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