Distribution potential, ITIES

According to Eq. (32.10), the distribution potential corresponding to the equilibrium partition of the electrolyte RX is independent of the electrolyte concentration. On the other hand, when more than two ions are involved in the partition equilibrium, there always exists a thermodynamic relationship between the potential difference and the concentrations of ions present. More specifically, let us consider an ITIES with a different electrolyte in each phase. 

Koryta et al. (1977) have shown that the distribution potential for such an ITIES fulfills the inequality cp s+ 9 ix- 9 )r+> provided that RX consists of... 

The interpretation of phenomenological electron-transfer kinetics in terms of fundamental models based on transition state theory [1,3-6,10] has been hindered by our primitive understanding of the interfacial structure and potential distribution across ITIES. The structure of ITIES was initially studied by electrochemical and thermodynamic analyses, and more recently by computer simulations and interfacial spectroscopy. Classical electrochemical analysis based on differential capacitance and surface tension measurements has been extensively discussed in the literature [11-18]. The picture that emerged from... 

This distribution potential, expounded by Randles, defines a nonpolarizable ITIES [18]. 

The maximum electrical potential in the compact layer A < - includes a dipolar potential which is shown schematically as a narrow region at the sharp interface. A dipolar layer can be located not only in the compact layer but can also occupy part of the diffuse layer. The amplitude and sign of isPpg can differ from the total interfadal potential. Figure 4 illustrates four possibilities for potential distribution at ITIES. Generally, the dipolar potential depends on the total interfadal potential A <. ... 

The Electrical Potential Distribution at the ITIES in the Presence of Zwitterionic Phospholipids... 

In both cases, the half-wave potential shifts by RT/ ziF)vaN per pH unit, and a typical example of such a behavior is given in Fig. 9 for the transfer of two acidic fi-diketones at the water-nitrobenzene interface. These results were unexpected, since a current wave is measured at a pH where the compound of interest is by a very large majority neutral, but they in fact represent the typical behavior of ionizable compounds at the ITIES and prove that the interfacial potential and the transfer of protons plays a key role for the distribution in biphasic systems. 

Fig. 4.1 Structure of the electric double layer and electric potential distribution at (A) a metal-electrolyte solution interface, (B) a semiconductor-electrolyte solution interface and (C) an interface of two immiscible electrolyte solutions (ITIES) in the absence of specific adsorption. The region between the electrode and the outer Helmholtz plane (OHP, at the distance jc2 from the electrode) contains a layer of oriented solvent molecules while in the Verwey and Niessen model of ITIES (C) this layer is absent...

Fig. 14.10 Folate metabolism and role of MTHFR. Genetically reduced MTHFR activity affects the distribution between folate species required for protein and DNA synthesis. Higher availabil ity of 5,10-methylenetetrahydrofolate (CH2THF) potentiates the TS inhibition by 5-FdUMP, the active metabolite of 5-FU. Hey, homocysteine Met, methionine CH3HF, 5-methyltetrahydrofolate TS, thymidylate synthase 5-FdUMP, fluorodeoxyuridine monophosphate.

Fig. 2.1. Distribution of the electric potential 0 in the vicinity of ITIES. The position of ITIES is indicated by xq, cf. (2.4.1).

A reference electrode which can directly be dipped in the organic phase is not available, except the AgPh4B/Ag electrode [44]. It is customary to use a nonpolarized liquid-liquid interface, i.e., a reference ITIES. The potential drop across this interface is primarily determined by an ionic species distributed commonly in both aqueous and organic phases. There are two points to be taken into account in using this reference ITIES the deviation from nernstian behavior and limited reversibility. 

First, when only one ionic species can be distributed between the phases, there is a contact equilibrium [23] at the interface and the potential difference is completely determined by the standard ion transfer potential of this potential-determining ion. In actual cases, however, this contact equilibrium is seldom achieved at the ITIES, since the transfer of counter ions of the potential-determining ion across the interface is not always negligible. If a distribution equilibrium is established t the reference ITIES, the degree of interference can be estimated using Eq. (3). However, the distribution equilibrium is not usually realized in an actual reference ITIES. Rather, the final equilibrium, if achieved, would nullify the potential difference between the two aqueous phases. Consider the following example ... 

Hydration of olefins to alcohols is equilibrium limited and hence CD is potentially suitable for such applications. The catalysts used for the process are acidic catalysts such as cation-exchange resins or zeolites. The hydration of isobutylene to produce tert-h ity alcohol via CD results in a higher conversion and there is no need to recycle the water. The hydration process is catalyzed by acidic ion-exchanged resins at 85°C and about 1200 kPa. The CD process configuration involves feeding the isobutylene below the catalyst zone and the water is fed above the catalyst zone. Flooding of the reaction zone is introduced in the process to improve the contact between the catalyst and the liquid and to ensure that the water is in constant contact with the catalyst sites. Flooding of the catalyst zone apparently improves the catalyst lifetime and performance because catalyst deactivation is caused by mass transfer and liquid distribution problems. Some recent publications on the hydration of isobutylene include a patent and a study of the kinetics of the hydration process and discussions on the merits of the application of CD for hydration. 

In the study of the interface with two immiscible electrolyte solutions (ITIES), considerable attention has been focused on the estimation of the Galvani potential difference at the water oil interface on the basis of a reasonable extra-thermodynamic assumptions. The discussion of these estimates is often made in terms of the ionic distribution coefficient, which is defined on the basis of equations (8.9.5) and (8.9.6). Generalizing this equation for the ot P interface at which ion i with charge z,- is transferred, one may write... 

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