Interfadal potential difference

The interfadal potential difference consists of the sum of the potential drops ... 

Next, we consider the interface M/S of a nonpolarizable electrode where electron or ion transfer is in equilibrium between a solid metal M and an aqueous solution S. Here, the interfadal potential is determined by the charge transfer equilibrium. As shown in Fig. 4-9, the electron transfer equilibrium equates the Fermi level, Enn) (= P (M)), of electrons in the metal with the Fermi level, erredox) (= P s)), of redox electrons in hydrated redox particles in the solution this gives rise to the inner and the outer potential differences, and respectively, as shown in Eqn. 4-10 ... 

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 <. ... 

In order to drive an electric current density across the system depicted in Fig. 1, an electric potential difference must be applied between the electrode and the bulk solution. This potential difference can he expressed in terms of three contrihutions the potential drop in the DEL, the equdih-rium interfadal or Nemst potential drop, and the activation overpotential. For large enough kinetic currents, the latter contribution can be neglected [7, 25] and the potential difference required for the electrode process is 7 eq — Arp, where gq is the equihhrium electrode potential (i.e. the electric potential of the electrode relative to that at the adjacent electrolyte solution). The value of gq is given by the Nemst equation. For the electrode Reaction (23), the Nemst equation takes the form... 

Because charge transfer is involved, the presence of an electric field at the interface affects the energies of the various species differently as they approach the interfadal region. In other words, the activation energy barrier for the reaction depends on the potential difference across the interface. It is convenient to express the potential dependence of the rate constants in the following manner ... 

Dove and Elston, this interfadal layer can be described by a triple layer snrface com-plexation model (TLM) as shown in Fig. 4.31. The interface consists of three electrostatically charged regions, each with an associated electric potential and snrface charge these are termed the o, p, and d planes. Hydrogen ions are permitted to coordinate with the nnsatnrated sites of the interface at the innermost o layer. Sodinm is positioned at the P layer or the d layer. The surface silicon-oxygen complex may have a different chemical character depending on the adsorbed species, hi a sodium chloride solution the surface complexes can be represented as sSiOHaCl, sSiOHj, =SiOH, =SiO-Na, and SiO". The concentration of each species depends on pH and salt concentration, and the sum of the fractions of these surface species equals 1 ... 

As outlined in the Introduction, a couple of suggested pathways have been proposed for the first electron transfer step (a) dissociative chemisorption of O2 (rds) probably accompanied by e-transfer and followed by proton transfer (b) simultaneous proton and electron transfer to a weakly adsorbed O2 molecule. We have recently shown through CPMD [21,69] and DFT [75] results that both pathways may take place under different conditions of the interfadal structure i.e., proton transfer may be involved in the first reduction step depending on the relative location of the O2 molecule with respect to the surface and to the proton, on the degree of proton hydration, and on the surface charge which is dependent on the electrode potential. Moreover, it was shown that proton transfer may precede or follow the first electron transfer, but in most cases the final product of the first step is an adsorbed HOO. ... 

Various parameters must be considered when selecting a reactor for multiphase reactions, such as the number of phases involved, the differences in the physical properties of the participating phases, the post-reaction separation, the inherent reaction nature (stoichiometry of reactants, intrinsic reaction rate, isothermal/ adiabatic conditions, etc.), the residence time required and the mass and heat transfer characteristics of the reactor For a given reaction system, the first four aspects are usually controlled to only a limited extent, if at aH, while the remainder serve as design variables to optimize reactor performance. High rates of heat and mass transfer improve effective rates and selectivities and the elimination of transport resistances, in particular for the rapid catalytic reactions, enables the reaction to achieve its chemical potential in the optimal temperature and concentration window. Transport processes can be ameliorated by greater heat exchange or interfadal surface areas and short diffusion paths. These are easily attained in microstructured reactors. 

This first chapter to Volume 2 Interfadal Kinetics and Mass Transport introduces the following sections, with particular focus on the distinctive feature of electrode reactions, namely, the exponential current-potential relationship, which reflects the strong effect of the interfacial electric field on the kinetics of chemical reactions at electrode surfaces. We then analyze the consequence of this accelerating effect on the reaction kinetics upon the surface concentration of reactants and products and the role played by mass transport on the current-potential curves. The theory of electron-transfer reactions, migration, and diffusion processes and digital simulation of convective-diffusion are analyzed in the first four chapters. New experimental evidence of mechanistic aspects in electrode kinetics from different in-situ spectroscopies and structural studies are discussed in the second section. The last... 

An entirely different situation may arise when the interfadal tension y becomes small enough to exhibit an appreciable curvature dependence that, approximately at least, is accounted for by the Helfrich expression, Eq. (52). In the Winsor I case, for instance, where we have small oil droplets dressed by surfactant dispersed in water and where an excess oil phase is present (Fig. 5), the (spherical) droplet free energy function 4 tP y passes through a minimum for P = Peq. At this particular radius, the Laplace equation written in the form of Eq. (76) yields AP = 0, and thus the condition of equal chemical potential in the droplet and the excess phase is satisfied. Around the minimum (where the curvature of the free energy function amounts to 8717 ), equilibrium fluctuations in size and shape occur, which have important entropic implications [40]. 

The translationally-restricted grand potential for the central interfadal hemi-micelle as a function of its size i2r,i(g ), called stability curve, is presented in Figure 5.3a for surfactants with different tail lengths. 

Figure 13.5 The Interfadal tension as a function of potential, for different concentrations of pyridine on polycrystalline gold. Lines (a)-(f) correspond to concentrations of 0, 1 x 10" M 4 x 10 M ...

In recent years there has been significant interest in the development of materials from blends of natural and synthetic polymers such as PE or EVOH (ethylene-vinyl-alcohol). To maintain the compostabUity feature, different biodegradable blends have been developed. These blends can be processed into useful disposable end products with potential to alleviate disposal problems by degrading in selective environments. The mechanical properties of polymer blends depend greatly on the adhesion of the different phases. Poor interfacial adhesion leads to lower ultimate properties, whereas strong interfadal adhesion leads to good mechanical properties and reduced molecular mobility. 

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