INTERPRETATION OF POLARIZATION

Anodic reactions represent metal deterioration since a corroding metal losses electrons. This is a surface electrochemical phenomenon that may cause drastic effects on metallic structures. Therefore, the driving force for corrosion is the overpotential (17) defined by 

On the other hand, if current flows, irreversible effects occur at the electrode surface due to electrochemical polarization effects. In this case, the corrosion potential and corrosion current density are compared with the cathodic and anodic terms. Thus, 

Additionally, electrochemical polarization is a measure of the overpotential and represents a deviation of the electrochemical state of half-cell electrodes induced by an applied external potential. Therefore, the driving force for electrochemical polarization is the overpotential. 

Consequently, the electrochemic polarization is divided into two classifications, such as anodic polarization and cathodic polarization. The corresponding overpotentials are defined by eqs. (3.35a) and (3.35b). Recall that the overpotentials for the anodic and cathodic parts of a polarization curve represent deviation from equilibrium anodicaUy or cathodicaUy. For convenience. 

In general, the higher the current density the higher the overpotential and the faster the electrochemical rate of reaction. Furthermore, other aspects of the polarization must be considered as a beneficial phenomenon for electrometallurgical operation, such as electrowinning, electrorefining, electroplating, and 

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Polarization equations of the type (14.35) or (14.38) contain the mean values of true current density. However, the rate-determining step is more often concentrated at just a few segments of the electrode the true working area changes continuously and an exact determination of this area is practically impossible. This gives rise to difficulties in an interpretation of polarization data. 

M. Maroncelli, V. P. Kumar, and A. Papazyan, A simple interpretation of polar solvation... 

Fig. 1 represents schematically the usual physical interpretation of polar SD The solute undergoes vertical electroitic excitation and the dynamic fluorescence Stokes shift arises Ifom the reorganization of the solvent molecules. In the case... 

M. Maroncelli, V. P. Kumar and A. Papazyan, A simple interpretation of polar solvation dynamics, J. Phys. Chem., 97 (1993) 13-17 E. W. Castner, Jr. and M. Maroncelli, Solvent dynamics derived from optical Kerr effect, dielectric dispersion, and time-resolved Stokes shift measurements an empirical comparison, J. Mol. Liq., 77 (1998) 1-36. 

The values of transfer constants directly indicate the danger of unfounded estimates of radical reactivities with respect to various substrates. The ratio of transfer rates of polystyrene and poly(methyl methacrylate) radicals to various substrates assumes a range of values [38] which are dependent on the substrate properties. The former radical is more reactive towards mercaptans, CBr4 or CC14, and the latter towards hydrocarbon transfer agents and trialkylamines which assume donor character in the transition complex. The interpretation of polar effects in macroradical reactivities is not yet satisfactory. 

The above description serves to give an idea of the basic measurements that are involved in this technique and the nature of the information that can be obtained. It leaves out many complicating factors that, in practice, have to be considered in the interpretation - of polarized infrared spectra, and it is to be emphasized that considerable caution is necessary in the use of this technique. 

The investigations presented focus on interpretation of polarization of fluorescence measurements and use of these measurements to study the structure of a representative spectrum of linear synthetic polypeptides, a vinyl polymer, and an intramolecularly cross-linked synthetic polypeptide. The methodological studies investigate the validity of the transition temperature as a structural parameter, the interaction of the fluorescent dye and the polymer to which it is conjugated, and the influence of the dye-polymer interaction on the measurements of various molecular parameters. The structural studies focus on the structure of the random coil, the helix-coil transition, the a-helix to conformation transition in polylysine, and the stability of the spatial structure in intramolecularly cross-linked synthetic polypeptides. 

These studies begin to provide a systematic basis for the detailed interpretation of polarization of fluorescence measurements in structural terms as well as hydrodynamic data about a wide range of linear and cross-linked synthetic polypeptides. The further development of such studies will provide a firm basis for applying polarization of fluorescence techniques to the study of the structure of native proteins in solution. 

The investigations presented in this study focus on the interpretation of polarization of fluorescence measurements and the use of these measurements to study the structure of a representative spectrum of linear synthetic polypeptides, of polyvinylamine, and of a cross-linked synthetic polypeptide. 

EXAMPLE 3.4. Interpretation of Polarization Data in Terms of a Reaction Model... 

The in-depth interpretation of the polarization curves frequently faces difficulties related to the non-uniform distributions of current and potential on the sample surface. This nonuniformity originates from the intrinsic effect of the sliding that causes an heterogeneity of the electrochemical surface reactivity, combined with the ohmic drop in the electrolyte. A full exploitation of the polarization curves in terms of local behavior is possible only if one can model the current and potential distributions xmder sliding conditions. This brings back to the same approach as in the case of the interpretation of open circuit potential measurements. Note that the effect of non-uniform distributions on the interpretation of polarization curves was already investigated in the absence of any sliding (Law Newman, 1979 Ponthiaux et al., 1995 Tiedemann et al., 1973). 

The quantities and 8 used for the molecular interpretation of polarization are the contributions due to the alignment of the molecular permanent moments and of the induced moments resulting from the polarizability a of the molecules. 

Third, the interpretation of polarization measurements normally requires assumptions about the nature of the motion leading to emission depolarization. Isotropic motion is easy to interpret. In some instances one can get a very detailed description of anisotropic rotational diffusion by combining nmr, light scattering and fluorescence depolarization measurements. 

FIGURE 5. Macroscopic interpretation of polarized light vector interaction. ... 

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