Polarization Phenomena

Microwave effects result from material-wave interactions and, because of the dipolar polarization phenomenon, the greater the polarity of a molecule (such as the solvent) the more pronounced the microwave effect when the rise in temperature [43] is considered. In terms of reactivity and kinetics the specific effect has therefore to be considered according to the reaction mechanism and, particularly, with regard to how the... 

Sidebar 3.6 describes some molecular aspects of the electrical polarization phenomenon. 

Many of the systems used for electrochemical studies of ion transfer processes taking place at the ITIES are systems of a single polarized interface. In these kinds of systems, the polarization phenomenon is only effective at the sample solution/... 

In these kinds of systems, the polarization phenomenon is effective at the two interfaces involved (see also Sect. 2.3.2). Specifically, in membrane systems comprising two ITIES, this behavior is achieved when the membrane contains a hydrophobic supporting electrolyte and the sample aqueous solution (the inner one) contains hydrophilic supporting electrolytes, and there is no common ion between any of the adjacent phases. In this case, the potential drop cannot be controlled individually and the processes taking place at both interfaces are linked to each other by virtue of the same electrical current intensity. 

To achieve a high membrane rejection towards the substrate it is important that the pore size of the membrane is smaller than the size of the molecules to be retained. Nevertheless, other factors influence the separation properties of a membrane, such as the shape and flexibility of the substrate and its acid-base properties, as well as the concentration-polarization phenomenon and the membrane fouling. 

This finding depends on different factors and, as previously described, was partially solved by enhancing the turbulent flow on the membrane surface. In this way, the deposition of the substrate and the catalyst was limited, avoiding the concentration polarization phenomenon that also affects the water flux across the membrane. This aspect is currently under study and some preliminary results obtained in a study on the Gemfibrozil degradation in the described PMR are reported in Table 15.4. 

Scaled membranes exhibit lower productivity and lower salt rejection. This lower salt rejection is a function of the concentration polarization phenomenon (see Chapter 3.4). When membranes are scaled, the surface of the membrane has a higher concentration of solutes than in the bulk solution. Since the membrane rejects when the membrane "sees," the passage of salts will be higher, even though the absolute or true rejection stays constant. 

Another general class of solids that has been prepared as thin films using PLD is ferroelectric materials. A potentially useful characteristic of ferroelectric materials is that they can be polarized by an electric field and retain this polarization when the field is removed. In ferroelectric thin films it may be possible to exploit this polarization phenomenon to make sensors, displays, and memory devices. A number of techniques have been used to prepare ferroelectric thin films. However, it has been difficult to control the stoichiometry (and correspondingly the properties) of these materials using thermal and sputtering techniques. In part, the difficulty in maintaining correct stoichiometry is due to the volatility of a component in the material (e.g. Pb in PbTiOs). 

Concentration polarization Convective transport and retention of solutes by the membrane results in an accumulation of species at wall. Local concentrations, C , are higher than in the bulk, Cb, and a back-diffusion from near the wall into the bulk liquid phase takes place. This is the so-called "concentration polarization" phenomenon (Fig. 12.1). A simple mass balance leads to the classical equation ... 

Polar molecules display the property that they can be oriented along an electric field dipolar polarization phenomenon). In the absence of this phenomenon, dipoles are orientated at random and molecules submitted to Brownian movement only. In the presence of a continuous electric current, aU the dipoles are lined up together in the same direction. If submitted to an alternating current, the electric field is inversed at each alternance with a subsequent tendancy for dipoles to move together to follow the field. Such a characteristic induces stirring and friction of molecules which dissipates as internal homogeneous heating (Scheme 21). 

Geophysics. Utilization of dynamic proton polarization phenomenon in nitroxyl radical solutions facilitated the improvement of spin-precision magnetometers by synchronizing the processes of nuclear polarization and precision measurement. Deuterated radical solutions serve as sensors in such instruments. Instruments of this type have already been produced in France. They find application in geophysical research, geological explorations and in searches for metallic objects under water. 

Sudden polarization phenomenon was investigated in ethylene using nonempiri-cal molecular electronic structural theory [38]. The studies predicted that D2d symmetry of the ground state ethylene undergoes twisting in the excited state to attain a pyramidalized shape. 

Discrepancies between theory and practice are explained by assuming that the very complex polarization phenomenon is characterized by a range of relaxation times rather than by a single one.

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To illustrate the concentration polarization phenomenon, we consider an infinitely long electrodialysis cell pair having parallel channels in which the flow is fully developed and laminar. The qualitative behavior of the development of the salt concentration and potential distributions along the channels of a dialysate and concentrate cell pair are shown schematically in Fig. 6.2.3 for the case where the inlet salt concentrations are the same in both channels (Probstein 1972). 

Other researchers used CFD calculation with complex 3D models to study reactive and separating systems with Pd-based membranes " to take into account the concentration polarization phenomenon. However, their attention was strongly directed towards a specific process and, thus, it is very difficult to achieve and extrapolate general considerations about it. 

A further polarization phenomenon is depicted in the bottom part of Figure 2.8, namely that of circular polarization. A circular-polarized wave can be thought of as the sum of two plane-polarized waves of equal ampUtude and at right angles to each other, which differ in phase by 90°. 

The concept of concentration polarization was given earlier and a simple model based on the him theory was developed to describe the phenomenon mathematically. In this chapter an attempt is made to develop a differential equation of the solute material balance, the solution of which will rigorously establish a concentration profile of the solute in the vicinity of the solution-membrane boundary. It is hoped that this approach will furnish a deeper understanding of the concentration polarization phenomenon. 

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