External electric fields changing

Our discussion of elecfronic effects has concentrated so far on permanent features of the cliarge distribution. Electrostatic interactions also arise from changes in the charge distribution of a molecule or atom caused by an external field, a process called polarisation. The primary effect of the external electric field (which in our case will be caused by neighbouring molecules) is to induce a dipole in the molecule. The magnitude of the induced dipole moment ginj is proportional to the electric field E, with the constant of proportionahty being the polarisability a ... 

The behavior of a polar dielectric in an electric field is of the same kind. If the dielectric, is exposed to an external electric field of intensity X, and this field is reduced in intensify by an amount SX, the temperature of the dielectric will not remain constant, unless a certain amount of heat enters the substance from outside, to compensate for the cooling which would otherwise occur. Alternatively, when the field is increased in intensity by an amount SX, we have the converse effect. In ionic solutions these effects are vciy important in any process which involves a change in the intensity of the ionic fields to which the solvent is exposed—that is to say, in almost all ionic processes. When, for example, ions are removed from a dilute solution, the portion of the solvent which was adjacent to each ion becomes free and no longer subject to the intense electric field of the ion. In the solution there is, therefore, for each ion removed, a cooling effect of the kind mentioned above. If the tempera-... 

Polyelectrolytes such as the ion exchange plastics form an interesting group of materials because of their ability to interact with water solutions. They have been used in medical applications involving the removal of heavy metal ions from the human body. They can be used to interact with external electric fields and change their physical properties drastically as is illustrated by the fact that some electrically active liquid crystals are polyelectrolytes of low molecular weight. 

The conformational changes are stimulated by cathodic compaction of the structure, by anodic oxidation and swelling, or, in absence of any external electric field, by thermal energy.177,178,188 In any case, they are a combination of the polymer-solvent interactions. So A can be expressed as a sum of three terms... 

By external stimulus at the plate electrode, migration of the ions in the solvent is induced, which changes the spatial concentration and so the local course of reactions [68]. By this means, weak electrical fields change the propagation velocity of the reaction zone through the capillary strong electrical fields ( supercritical ) further affect the global feature of the reaction in the capillary. 

Moreover, for the observables depending on external electric field, its specific effect has to be investigated the electric field induces new terms in the nuclear Hamiltonian, due to the change of equilibrium geometry and the nuclear motion perturbation. Pandey and Santry (14) has brought to the fore this effect and calculated the correction which only concerns the parallel component. It is represented by the following expression ... 

Pulsed electric field is another alternative to conventional methods of extraction. PEF enhances mass transfer rates using an external electrical field, which results in an electric potential across the membranes of matrix cells that minimizes thermal degradation and changes textural properties. PEF has been considered as a nonthermal pretreatment stage used to increase the extraction efficiency, increasing also permeability throughout the cell membranes. 

As was pointed out in Chapters 2 and 3, a dipole layer exists at the surface of a metal, which gives rise to a concomitant surface dipole potential x- The magnitude of this potential changes in the presence of an external electric field. A field E directed away from the surface induces an excess charge density, o e0e E, where e is the dielectric constant of the medium outside the metal. The field E pushes the electrons into the metal, producing the required excess charge, and decreasing the dipole potential (see Fig. 3.4). This has consequences for the interfacial capacity. 

An external electric field induces a change in the dipole moment known as polarization. However, the induced dipole moment has a saturation behavior which makes the polarization deviate for the linear response to the external L-field. and the total polarization p of the chromophore in an external electric field can be written as... 

However, another study concluded that the changes of the hydrogen-bond stability may be important in biological processes. For these, the influence of local electric fields created by Li+, Na+, and Mg2+ ions on the properties and reactivity of hydrogen bonds in HF and HC1 dimer has been carried out by means of ab initio self-consistent field (SCF) method [33]. A few years later, the effect of intensity and vector direction of the external electric field on activation barriers of unimole-cular reactions were studied using the semiempirical MINDO/3 method [34]. However, both semiempirical and ab initio calculations were performed to study the multiplicity change for carbene-like systems in external electric fields of different configurations (carbene and silylene) and the factor that determines the multiplicity and hence the reactivity of carbene-like structures is the nonuniformity of the field [35]. 

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