Concentration polarization and inhibition

Coupled Effect of Concentration Polarization and Inhibition by Carbon Monoxide... 

In order to discuss on the separate effect of concentration polarization and inhibition by CO, the system shown in Figure 14.1 is considered. Referring to this sketch, three different zones can be identified ... 

According to their definitions, CPC and IC provide quantitative measures of the flux decrease due to concentration polarization and inhibition, respectively. However, it is sometimes useful to have an indication of the overall flux... 

For this aim, an overall permeance reduction coelScient , PRCP was introduced, to take into account the permeation reduction owing to both concentration polarization and inhibition at the same time. Its definition, reported in eqn (14.19), arises from the simple consideration that the lowest permeance is evaluated at bulk conditions when both polarization and inhibition affect the membrane system, whereas, on the other hand, the ideal membrane behavior and the maximum permeance can be found under pure hydrogen conditions ... 

In this section, the analysis is focussed on the direct effect that inhibition by CO and concentration polarization have on hydrogen permeance and transmembrane flux. To do that, the operating conditions reported in Table 14.4 are considered.As shown in the table, two mixtures are considered to separate the CO contribution to concentration polarization and inhibition. 

According to its definition, the coefficient PRC is able to deal with the possible situations in which a membrane system affected by concentration polarization and inhibition can operate (Table 14.5). 

Finally, in order to observe quantitatively both the influence of concentration polarization and inhibition by CO, the behavior of the overall permeation reduction coeificient is shown in Figure 14.7, which represents a map analogous to the ones presented for CPC and IC. 

In this chapter, an innovative approach dealing with the combined effect of concentration polarization and inhibition by CO in Pd-based membranes is discussed and analyzed by means of appropriate coefficients measuring the permeation reduction due to these phenomena. 

A remarkable result obtained from such an investigation consisted in the quantification of the mutual interactions between concentration polarization and inhibition, for which the non-additivity of the effects was shown in terms of their respective coefficients. In fact, it was mathematically demonstrated that the physical phenomenon according to which the concentration polarization in the presence of inhibition is less than in its absence due to a lower permeating flux. 

The reverse micelles stabilized by SDS retard the autoxidation of ethylbenzene [27]. It was proved that the SDS micelles catalyze hydroperoxide decomposition without the formation of free radicals. The introduction of cyclohexanol and cyclohexanone in the system decreases the rate of hydroperoxide decay (ethylbenzene, 363 K, [SDS] = 10 3mol L [cyclohexanol] =0.03 mol L-1, and [cyclohexanone] = 0.01 mol L 1 [27]). Such an effect proves that the decay of MePhCHOOH proceeds in the layer of polar molecules surrounding the micelle. The addition of alcohol or ketone lowers the hydroperoxide concentration in such a layer and, therefore, retards hydroperoxide decomposition. The surfactant AOT apparently creates such a layer around water moleculesthat is very thick and creates difficulties for the penetration of hydroperoxide molecules close to polar water. The phenomenology of micellar catalysis is close to that of heterogeneous catalysis and inhibition (see Chapters 10 and 20). 

It should be understood that even for micro surface features the potential is uniform and the ohmic resistance through the bath to peaks and valleys is about the same. Also, electrode potential against SCE will be uniform. What is different is that over micro patterns the boundary of the diffusion layer does not quite follow the pattern contour (Fig. 12.3). Rather, it thus lies farther from depth or vias than from bump peaks. The effective thickness, 8N, of the diffusion layer shows greater variations. This variation of 8N over a micro profile therefore produces a variation in the amount of concentration polarization locally. Since the potential is virtually uniform, differences in the local rate of metal deposition result if it is controlled by the diffusion rate of either the depositing ions or the inhibiting addition (leveling) agents. 

In most cases, drugs are converted to metabolites, which, in the more polar and water-soluble forms, are readily excreted. Often, the concentration of a metabolite far exceeds the concentration of the drug. For example, orally administered propranolol is rapidly converted to 4-hydroxypropranolol, which has a concentration that is severalfold higher than that of propranolol. Sometimes the metabolites are able to inhibit the further metabolism of the parent drug. For example, phenytoin becomes metabolized to hydroxyphenytoin. When administered in higher-than-recommended individual doses, hydroxyphenytoin inhibits the hydroxylase system that metabolizes phenytoin, increasing its concentration in free form and its potential to produce toxicity. 

In an environment with a constant redox condition (e.g., permanently aerated and/or constant pH), a condition not uncommon in industrial and environmental situations, corr could shift in the positive direction for a number of reasons. Incongruent dissolution of an alloy could lead to surface ennoblement. Alternatively, as corrosion progresses, the formation of a corrosion product deposit could polarize (i.e., increase the overpotential, i), for) the anodic reaction as illustrated in the Evans diagram of Fig. 4. Polarization in this manner may be due to the introduction of anodic concentration polarization in the deposit as the rate of transport of dissolved metal species away from the corroding surface becomes steadily inhibited by the thickening of the surface deposit i.e., the anodic half-reaction becomes transport controlled. 

The effect of ultrasound on the process of tellurium anodic dissolution in alkaline solutions was studied by the method of plotting polarization and galvanostatic curves [148]. Tests were made in NaOH solutions (concentrations of 0—20 g/L), subjected to the action of ultrasound at a frequency 17.5 kHz and using Te electrodeposited under ultrasound. The anodic polarization curves plotted without ultrasound and in its presence shifted with increased NaOH concentration towards negative values as a result of the increasing rate of Te anodic dissolution. The presence of ultrasound inhibited the process of Te anodic dissolution, probably due to the desorption of OFT anions from the anode surface. This sonoelectrodeposited Te thus showed greater corrosion resistance in alkaline solution than that deposited... 

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