Dipole orientational polarization

The majority of interfacial polarization loss processes can be closely approximated by some modification of the Debye description of orientational dipole polarization in homogeneous media (13). The subject of interfacial polarization effects and the dielectric properties of many classes of heterogeneous systems have been reviewed by Van Beek (14). 

The dielectric constant of unsymmetrical molecules containing dipoles (polar molecules) will be dependent on the internal viscosity of the dielectric. If very hard frozen ethyl alcohol is used as the dielectric the dielectric constant is approximately 3 at the melting point, when the molecules are free to orient themselves, the dielectric constant is about 55. Further heating reduces the ratio by increasing the energy of molecular motions which tend to disorient the molecules but at room temperature the dielectric constant is still as high as 35. 

Polar molecules, like nonpolar molecules, are attracted to one another by dispersion forces. In addition, they experience dipole forces as illustrated in Figure 9.9, which shows the orientation of polar molecules, such as Id, in a crystal. Adjacent molecules line up so that the negative pole of one molecule (small Q atom) is as dose as possible to the positive pole (large I atom) of its neighbor. Under these conditions, there is an electrical attractive force, referred to as a dipole force, between adjacent polar molecules. 

The collected fluorescence 3F [from Eq. (7.39)] clearly depends on the orientation distribution of the dipoles and the incident polarization through the dependences on 0 and E. We will assume a special but common case here randomly oriented dipoles with a z-dependent concentration near the surface, excited by a p-polarized evanescent wave. 

Figure 7.7. The weightings wj / versus i/(z) for the special case of p-polarized incident light and randomly oriented dipoles. In this case, k -1 = w]) 1.

In the liquid state, polar molecules (dipoles) orient themselves so that oppositely charged ends of the molecules are near to one another. The attractions between these opposite charges are called dipole-dipole forces. Figure 4.13 shows the orientation of polar molecules due to these forces in a liquid. 

Material response is typically studied using either direct (constant) applied voltage (DC) or alternating applied voltage (AC). The AC response as a function of frequency is characteristic of a material. In the future, such electric spectra may be used as a product identification tool, much like IR spectroscopy. Factors such as current strength, duration of measurement, specimen shape, temperature, and applied pressure affect the electric responses of materials. The response may be delayed because of a number of factors including the interaction between polymer chains, the presence within the chain of specific molecular groupings, and effects related to interactions in the specific atoms themselves. A number of properties, such as relaxation time, power loss, dissipation factor, and power factor are measures of this lag. The movement of dipoles (related to the dipole polarization (P) within a polymer can be divided into two types an orientation polarization (P ) and a dislocation or induced polarization. 

The surface potential arises because the lipid molecule orients with polar part toward the aqueous phase. This effects a change in dipole at the surface. There would thus be a change in surface potential when a monolayer is present, as compared to a clean surface. The surface potential, AV, is... 

The movement of dipoles (related to the dipole polarization, P0) within a polymer can be divided into two types, an orientation polarization, P 0, and a dislocating or induced polarization. 

When light passes through a condensed phase consisting of oriented electrical dipoles (polarizer), or when it is reflected from a dipolar interface it becomes polarized (Fig. 9.2). It means that orthogonally randomly oriented E, M vectors are now... 

The relatively sluggish permanent dipole polarizes the relatively frisky electrons on the nonpolar molecule and induces a dipole of opposite orientation. The direction of the induced dipole is such as to create attraction. 

Measurements of dielectric properties have been used to monitor chemical reactions in organic materials for more than fifty years. In 1934, Kienle and Race 11 reported the use of dielectric measurements to study polyesterification reactions. Remarkably, many of the major issues that are the subject of this review were identified in that early paper the fact that ionic conductivity often dominates the observed dielectric properties the equivalence between the conductivity measured with both DC and AC methods the correlation between viscosity and conductivity early in cure the fact that conductivity does not show an abrupt change at gelation the possible contribution of orientable dipoles and sample heterogeneities to measured dielectric properties and the importance of electrode polarization at low frequencies. 

This polarization has two components the induced polarization P (due to movement of the centers of charge, or to the static electric dipole polarizability a of molecules) and the dipole polarization Pjt (due to the orientation of the permanent dipoles m in the applied electric field E) ... 

To investigate the dependence of the polarization on molecular quantities, it is convenient to assume the polarization P to be divided into two parts the induced polarization Pa, caused by translation effects, and the dipole polarization P, caused by the orientation of the permanent dipoles. Note that in ionic polarization the transport of charge carriers and their trapping can also create induced polarization. 

In considering the mechanism of interaction of microwave energy and materials, a simplified model of a capacitor with the material between charged plates can illustrate the more important aspects of heating (4). The ability of the material to maintain the charge separation (that is, resist current flow) is closely related to the inverse of the dielectric constant (c ). When materials are subjected to the electric field between the plates, those with permanent dipoles (polar molecules) will orient... 

Orientational polarization results from the fact that randomly oriented dipoles tend to be aligned parallel to the applied field, in which position they have minimum electric energy. This tendency, however, is usually counteracted by rotational diffusion, i.e. thermal movements of dipole axes which tend to restore a random distribution. Ordinarily we deal with small electric field strengths satisfying the condition... 

Experimentally, the polarity of molecules is measured indirectly by measuring the dielectric constant, which is the ratio of the capacitance of a cell filled with the substance to be measured to the capacitance of the same cell with a vacuum between the electrodes. Orientation of polar molecules in the electric field partially cancels the effect of the field and results in a larger dielectric constant. Measurements at different temperatures allow calculation of the dipole moment for the molecule, defined as... 

Figure 2.43. Reaction field of a polar solute, a) Dipole field of the isolated molecule, b) orientation of polar solvent molecules parallel to the dipole field, and c) reaction field from the solvent molecules in the cavity if the solute molecule has been removed after freezing all electronic and nuclear coordinates (adapted from Liptay, l%9).

An electrode interface has a layered structure in which a nonunifomi electric field (potential slope) is generated by polarization of the electrode. An extremely strong electric field of around 10 Vcm in the innermost layer, the so-called electron transfer layer, which is very thin, 10 A or less, might cause a variety of polar effects. Since not only the electron transfer step but also adsorption and some of the chemical steps involved in an electrolytic reaction take place in the electron transfer layer, the electrochemical reaction should be strongly influenced by polar factors. The orientation of polar adsorbed species, such as ions and dipoles, is electrostatically influenced, and consequently the stereochemistry of their reactions is also controlled by this kind of electrostatic factor. 

There must be forces between the molecules of a non-polar compound, since even such compounds can solidify. Such attractions are called van der Waals forces. The existence of these forces is accounted for by quantum mechanics. We can roughly visualize them arising in the following way. The average distribution of charge about, say, a methane molecule is symmetrical, so that there is no net dipole moment. However, the electrons move about, so that at any instant of time the distribution will probably be distorted, and a small dipole will exist. This momentary dipole will affect the electron distribution in a second methane molecule nearby. The negative end of the dipole tends to repel electrons, and the positive end tends to attract electrons the dipole thus induces an oppositely oriented dipole in the neighboring molecule ... 

Polarization can be factorized into two main contributions induced polarization Pa, due to translation effects, and dipole polarization due to orientation of permanent dipoles. Moreover, induced polarization can be viewed as being due to the contribution of electronic polarization P and atomic polarization P. ... 

Fig. 10.3. Piezoelectric ceramic with groups of oriented dipoles (a group is indicated with one arrow) (a) Prior to orientation in an electric field, the dipoles point in random directions (b) while a strong electric field is applied the dipoles align (c) a polarized piezoelectric ceramic ready to use.

A proper solvated electron is a particle localized in the potential well of a polar medium, the well being created by the interaction of electron charge with the permanent and induced dipole moments of the nearest as well as remote neighbours. This notion of the nature of a solvated electron, based on the idea that the Landau-Pekar theory initially advanced for solid bodies can be applied also to liquid systems, was advanced in 1948 since then considerable efforts have been made to develop it and verify it experimentally. In most liquid systems, localization of an electron is followed by the formation of a cavity where most of the density of the solvated electrons is concentrated. The cavity is surrounded by the orientated dipoles of the solvent. Usually, the radius of this cavity equals about 3-3.5 A which conforms to a solvated-electron molar volume of 70-100 cm . This is the reason why solutions with large concentrations of solvated electrons have a lower density. 

Conway et al examined this question in terms of the BDM (Bockris-Devanathan-Muller) two-state model of solvent dipole orientation. The dipole polarization, expressed as a surface potential contribution, g ip, is given by... 

Physical Measurements. Molecular weights were determined in "x 0.07 M benzene solutions at 37°C by vapor pressure osmometry. Dipole moments were measured in dilute benzene solution at 25. 00 + 0.05°C using a previously described method (12). Values of the atomic polarization, P2 for the admh and acac ligands were estimated from the total polarization, TP2, of trans-Pd(admh)2 (13) and Co-(acac)g. Assuming that Co(acac)3 and trans- Pd(admh) have Dg and symmetry, respectively, the orientation molar polarizations,... 

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