Polarization interfacial

The dielectric constant is a measure of the ease with which charged species in a material can be displaced to form dipoles. There are four primary mechanisms of polarization in glasses (13) electronic, atomic, orientational, and interfacial polarization. Electronic polarization arises from the displacement of electron clouds and is important at optical (ultraviolet) frequencies. At optical frequencies, the dielectric constant of a glass is related to the refractive index k =. Atomic polarization occurs at infrared frequencies and involves the displacement of positive and negative ions. 

At lower frequencies, orientational polarization may occur if the glass contains permanent ionic or molecular dipoles, such as H2O or an Si—OH group, that can rotate or oscillate in the presence of an appHed electric field. Another source of orientational polarization at even lower frequencies is the oscillatory movement of mobile ions such as Na". The higher the amount of alkaH oxide in the glass, the higher the dielectric constant. When the movement of mobile charge carriers is obstmcted by a barrier, the accumulation of carriers at the interface leads to interfacial polarization. Interfacial polarization can occur in phase-separated glasses if the phases have different dielectric constants. 

Dees et al. [66, 67] reported that the sulfur poisoning was due to a large increase in anode interfacial polarization resistance (Rp). They found that total Rp for an Ni-YSZ cermet anode/electrolyte/anode symmetrical cell in 97% H2/3% H2 increased from 0.27 to 0.45 fl/cm2 (an -67% increase) when 100 ppm H2S was introduced into the... 

Fruitful interplay between experiment and theory has led to an increasingly detailed understanding of equilibrium and dynamic solvation properties in bulk solution. However, applying these ideas to solvent-solute and surface-solute interactions at interfaces is not straightforward due to the inherent anisotropic, short-range forces found in these environments. Our research will examine how different solvents and substrates conspire to alter solution-phase surface chemistry from the bulk solution limit. In particular, we intend to determine systematically and quantitatively the origins of interfacial polarity at solid-liquid interfaces as well as identify how surface-induced polar ordering... 

One aspect of the research will examine equilibrium aspects of solvation at hydro-phobic and hydrophilic interfaces. In these experiments, solvent dependent shifts in chromophore absorption spectra will be used to infer interfacial polarity. Preliminary results from these studies are presented. The polarity of solid-liquid interfaces arises from a complicated balance of anisotropic, intermolecular forces. It is hoped that results from these studies can aid in developing a general, predictive understanding of dielectric properties in inhomogeneous environments. 

In particular, we intend to systematically and quantitatively determine the origins of interfacial polarity at solid-liquid interfaces as well as identify how surface induced polar ordering affects dynamic properties of interfacial environments. (From Walker, 2001)... 

The polarizations noted above are the major types found in homogeneous materials. Other types of polarization, called interfacial polarizations, are the result of heterogeneity. Ceramics, polymers with additives, and paper are considered to be electrically heterogeneous. 

We can further describe the polarization, P, according to the different types of dipoles that either already exist or are induced in the dielectric material. The polarization of a dielectric material may be caused by four major types of polarization electronic polarization, ionic (atomic) polarization, orientation polarization, and space-charge (interfacial) polarization. Each type of polarization is shown schematically in Figure 6.24 and will be described in succession. In these descriptions, it will be useful to introduce a new term called the polarizability, a, which is simply a measure of the ability of a material to undergo the specific type of polarization. 

The final type of polarization is space-charge polarization, sometimes called interfacial polarization, and results from the accumulation of charge at structural interfaces in heterogeneous materials. Such polarization occurs when one of the phases has a much higher resistivity than the other, and it is found in a variety of ceramic materials, especially at elevated temperatures. The space-charge polarization, P c, has a corresponding space-charge polarizability, The two are related via a relationship of the form for the other types of polarization. 

The terms polarizability constant and dielectric constant can be utilized interchangeably in the qualitative discussion of the magnitude of the dielectric constant. The k values obtained utilizing dc and low-frequency measurements are a summation of electronic E, atomic A, dipole P0, and interfacial /, polarizations. Only the contribution by electronic polarizations is evident at high frequencies. The variation of dielectric constant with frequency for a material having interfacial, dipole, atomic, and electronic polarization contributions is shown in Figure 6.1. 

Electromagnetic measurements of high conductivity soil-fluid mixtures at low frequencies are difficult to obtain due to electrode polarization. Caution must be used when interpreting data in the literature, as electrode effects may be viewed as being material behaviour. In addition, difficulties with data interpretation arise at kHz and MHz frequencies for clay-fluid mixtures due to the possible manifestation of both double layer polarization and interfacial polarization phenomena. 

Although the mechanisms of electroporation, electrofusion, and electroinsertion are not known, biophysical data suggest that the primary field pulse effect is interfacial polarization by ion accumulation at the membrane surfaces. The resulting transmembrane electric field causes rearrangements of the lipids such that pores are formed1718. Electropores anneal slowly (over a period of minutes) when the pulse is switched off. 

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