Dielectric constant of water

T. Simonson. Accurate calculation of the dielectric constant of water from simulations of a microscopic droplet in vacuum. Chem. Phys. Lett, 250 450-454, 1996. 

The variation of the dielectric constant of the HCl + H2O mixtures is not appreciably different from that of pure water (78.30) at 25°C until the hydrogen chloride concentration teaches a minimum of 0.2%. It increases slightly over the dielectric constant of water as the concentration increases. 

Solvent. The solvent properties of water and steam are a consequence of the dielectric constant. At 25°C, the dielectric constant of water is 78.4, which enables ready dissolution of salts. As the temperature increases, the dielectric constant decreases. At the critical point, the dielectric constant is only 2, which is similar to the dielectric constants of many organic compounds at 25°C. The solubiUty of many salts declines at high temperatures. As a consequence, steam is a poor solvent for salts. However, at the critical point and above, water is a good solvent for organic molecules. 

Along the saturation line and the critical isobar (22.1 MPa (3205 psi)), the dielectric constant of water declines with temperature (see Fig. 10). In the last 24°C below the critical point, the dielectric constant drops precipitously from 14.49 to 4.77 in the next 5°C, it further declines to 2.53 and by 400°C it has declined to 1.86. In the region of the critical point, the dielectric constant of water becomes similar to the dielectric constants of typical organic solvents (Table 6). The solubiHty of organic materials increases markedly in the region near the critical point, and the solubiHty of salts tends to decline as the temperature increases toward the critical temperature. 

The formation of acids from heteroatoms creates a corrosion problem. At the working temperatures, stainless steels are easily corroded by the acids. Even platinum and gold are not immune to corrosion. One solution is to add sodium hydroxide to the reactant mixture to neutralize the acids as they form. However, because the dielectric constant of water is low at the temperatures and pressure in use, the salts formed have low solubiHty at the supercritical temperatures and tend to precipitate and plug reaction tubes. Most hydrothermal processing is oxidation, and has been called supercritical water oxidation. 

It is the hydrogen bond that determines in the main the magnitude and nature of the mutual interactions of water molecules and that is consequently responsible for the striking physical properties of this uniquely important substance. In this section we shall discuss the melting point, boiling point, and dielectric constant of water and related substances other properties of water are treated later (Sec. 12-4). 

Water in its supercritical state has fascinating properties as a reaction medium and behaves very differently from water under standard conditions [771]. The density of SC-H2O as well as its viscosity, dielectric constant and the solubility of various materials can be changed continuously between gas-like and liquid-like values by varying the pressure over a range of a few bars. At ordinary temperatures this is not possible. For instance, the dielectric constant of water at the critical temperature has a value similar to that of toluene. Under these conditions, apolar compounds such as alkanes may be completely miscible with sc-H2O which behaves almost like a non-aqueous fluid. 

The high dielectric constant of water normally militates against the formation of ion-pairs for simple salts because a high dielectric constant reduces the strength of the electrostatic forces. The phenomenon is more readily observed in solvents of low dielectric constant for a typical mono-monovalent salt, ion-pair formation takes place only when the dielectric constant is less than 41 (Fuoss Kraus, 1933). 

Dielectric constant Also called permittivity. The dielectric constant of a substance is the ratio of the attractive force between two opposite charges measured in a vacuum to that force measured in the substance. The high dielectric constant of water makes it a good solvent for ionic compounds. 

Water is the most common solvent used to dissolve ionic compounds. Principally, the reasons for dissolution of ionic crystals in water are two. Not stated in any order of sequence of importance, the first one maybe mentioned as the weakening of the electrostatic forces of attraction in an ionic crystal known, and the effect may be alternatively be expressed as the consequence of the presence of highly polar water molecules. The high dielectric constant of water implies that the attractive forces between the cations and anions in an ionic salt come down by a factor of 80 when water happens to be the leaching medium. The second responsible factor is the tendency of the ionic crystals to hydrate. 

TABLE 5.2 Dielectric Constants of Water-Lipid Interfaces (Expanded from Ref. 453) ... 

Neumann M (1985) The dielectric constant of water. Computer simulations with the MCY potential. J Chem Phys 82(12) 5663-5672... 

According to the Kirkwood theory of polar dielectrics, simple relations (23) between molecular dipole moment vectors and the mean-square total dipole moment of water clusters can be used to compute the static dielectric constant of water. As the normalized mean-square total dipole moment increases towards unity, theory predicts decreases in the static dielectric constant. Since MD results indicate that the mean-square total dipole moment of interfacial water is greater than that for bulk water (48), the static dielectric... 

Fig. 9. Logarithm plots of the dielectric constant of water (7), methanol (1), and various aqueous-organic mixtures (2-6, Methanol-water at 80 20, 70 30,60 40, 50 50, and 40 60 v/v, respectively, and 8, ethylene glycol-water at 50 50 v/v) vs temperature. As the concentradon of the organic component increases, the dielectric constant decreases, but this effect is reversed by lowering the temperature.

For glycine, it is known that the dielectric constants of water increases rapidly and linearly with the concentration of the amino acid, reaching a value of about 135 at a concentration of 2.5 mol liter" at 25 C. D is given by D = 78.54 + 22.58C, where 78.54 is the measured value of D for pure water, 22.58 is the numerical value of the dielectric increment, and C is the concentration in mol/liter. This great increase of D reflects the extremely large moment of glycine as a dipolar ion, and the linearity of the relationship represents the proportionality between D and polarizability that is characteristic of strongly polar media. 

Fundamental constants and the densities of water used in calculating these constants were obtained fiom Ref. 10. Experimental values of the dielectric constant of water were obtained fiom Ref. 11. 

In Fig. 4.24, we plot the experimental free energies W(l, 1) as a function of the proton-proton distance Rjfu. We also plot the theoretical curve for the Coulombic interaction between the two protons, as modified by the macroscopic dielectric constant of water D = 78.54. It is clear that the values of W(l, 1) for the larger molecules follow closely the theoretical curve with a fixed value of D. Large... 

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