Density-dependent dielectric constant, ionic

D2O and the tritium analogue T2O (p. 41). The high bp is notable (cf. H2S, etc.) as is the temperature of maximum density and its marked dependence on the isotopic composition of water. The high dielectric constant and measurable ionic dissociation equilibrium are also unusual and important properties. The ionic mobilities of [H30] and [OH] in water are abnormally high (350 X 10 " and 192 x 10 cms per V cm... 

For molten salts one sets so = 1. For electrolyte solutions solvent-averaged potential [37]. Then, in real fluids, eo in Eq. (11) depends on the ion density [167]. Usually, one sets so = s, where e is the dielectric constant of the solvent. A further assumption inherent in all primitive models is in = , where is the dielectric constant inside the ionic spheres. This deficit can be compensated by a cavity term that, for electrolyte solutions with e > in, is repulsive. At zero ion density this cavity term decays as r-4 [17, 168]. At... 

The dielectric constant of a material depends primarily on its polarizability and, hence, strongly depends on density. The three types of polarization that contribute to the dielectric constant are electronic, ionic, and orientational polarizations and are given by the Debye equation ... 

Fig. 13 A cartoon of a profile of a smooth electrochemical interface. The half-space z < 0 is occupied by the metal ionic skeleton that, within the jellium model, is described as a continuum of positive charge density (n+) and the dielectric constant due to bound electrons (ei,), the value of which lies typically between 1 and 2. The gap accounts for a finite distance of closest approach of solvent molecules to the skeleton the gap is determined by the balance offerees that attract the molecules to the metal and the Pauli repulsion of the closed shells of the molecules from the free electron cloud of the metal of density n(z). The regions a < z < a + d and z> a + d correspond, respectively, to the first layer of solvent molecules (which can be roughly characterized by charge-dependent effective dielectric constant) and the diffuse-layer part.

The distance-dependence of the surface potentials depends on a number of parameters such as the dielectric constant, the ionic strength, the net excess surface charge density, temperature etc. An indication of the order of magnitude and distances involved of this is shown in Figure 5.3. 

Many density-dependent properties of H2O, such as viscosity, polarity (dielectric constant s changes from 74 to 2), heat capacity at constant pressure (which is infinite at the critical point), ion product and solvent power can be tuned for specific requirements by setting the correct temperature and pressure, and they show significant changes near the critical point (Figure 25.2). Several studies have demonstrated that the transition from sub- to supercritical conditions also affects the elementary steps in reaction mechanisms, and radical intermediates are favoured over ionic species. Another consequence is that subcritical water shows potential for acid catalysis. Reactions can be run either under non-polar/aprotic or polar/pH controlled conditions (water can take part in these reactions). Consequently, non-polar compounds like aromatics become soluble whereas inorganic salts precipitate. Therefore, the properties of water as a solvent are tunable over much wider parameter ranges than for most other compounds. 

Dielectric constants, dielectric loss factors and the temperature dependence of the dielectric properties of ionic liquids intended to be used in batteries were determined by the above described self-designed microwave dielectrometric apparatus (Figure 3) at the frequency of 2.45 GHz and at different temperatures (30°C, 40 "C, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C and 120°C). The speed of the change in temperature dep>ends on the electrical field strength in the material ( ), the absorbed microwave power, (Pv) density (p), the specific heat capacity (Cp) and the dielectric loss factor e" and can be given by Equation 16 (Gollei, 2009). The electrical conductivity values (G) were calculated by Equation 12 and Equation 13. 

The dielectric constant, e, depends on temperature only to the extent that the density changes with temperature, eg, a sharp change at the melting temperature, Tm. Except for the influence of ionic conductivity at low frequencies or temperatures above I m. the dissipation factor and the loss index, e", are essentially constant for an ideal, nonpolar polymer, such as PTFE, with some minor exceptions due to branching and other perturbations in the molecular structure. 

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