Interfacial potential gradient

Theoretically this term reflects the electrostatic work in transporting ions through the interfacial potential gradient (e.g., Stumm et al., 1970 Stumm and Morgan, 1981 Morel, 1983). Since... 

It is possible to find a range in which the electrode potential is changed and no steady state net current flows. An electrode is called ideally polarized when no charge flows accross the interface, regardless of the interfacial potential gradient. In real systems, this situation is observed only in a restricted potential range, either because electronic aceptors or donors in the electrolyte (redox systems) are absent or, even in their presence, when the electrode kinetics are far too slow in that potential range. This represents a non-equilibrium situation since the electrochemical potential of electrons is different in both phases. 

FIGURE 5.6 Schematic representation of the interfacial potential gradients established in the vicinity of the carbon surface. Cases (a)-(c) depend on the relative contributions of proton and electron transfer. (From Duval, J., et al., Langmuir, 17, 7573, 2001.)... 

Migration is the movement of ions due to a potential gradient. In an electrochemical cell the external electric field at the electrode/solution interface due to the drop in electrical potential between the two phases exerts an electrostatic force on the charged species present in the interfacial region, thus inducing movement of ions to or from the electrode. The magnitude is proportional to the concentration of the ion, the electric field and the ionic mobility. 

A number of metals, such as copper, cobalt and h on, form a number of oxide layers during oxidation in air. Providing that interfacial thermodynamic equilibrium exists at the boundaries between the various oxide layers, the relative thicknesses of the oxides will depend on die relative diffusion coefficients of the mobile species as well as the oxygen potential gradients across each oxide layer. The flux of ions and electrons is given by Einstein s mobility equation for each diffusing species in each layer... 

The analysis of oxidation processes to which diffusion control and interfacial equilibrium applied has been analysed by Wagner (1933) who used the Einstein mobility equation as a starting point. To describe the oxidation for example of nickel to the monoxide NiO, consideration must be given to tire respective fluxes of cations, anions and positive holes. These fluxes must be balanced to preserve local electroneutrality tliroughout the growing oxide. The flux equation for each species includes a term due to a chemical potential gradient plus a term due to the elecuic potential gradient... 

It follows from Eqn. 4—13 that the electron level o u/av) in the electrode is a function of the chemical potential p.(M) of electrons in the electrode, the interfacial potential (the inner potential difference) between the electrode and the electrolyte solution, and the surface potential Xs/v of the electrolyte solution. It appears that the electron level cx (ii/a/v) in the electrode depends on the interfacial potential of the electrode and the chemical potential of electron in the electrode but does not depend upon the chemical potential of electron in the electrolyte solution. Equation 4-13 is valid when no electrostatic potential gradient exists in the electrolyte solution. In the presence of a potential gradient, an additional electrostatic energy has to be included in Eqn. 4-13. 

V. INTERFACIAL CHEMICAL POTENTIAL GRADIENTS AND THE PHOTOVOLTAGE-DETERMINING MECHANISM... 

Use of a reference electrode to measure the electrode—electrolyte potential difference also introduces a new reference—solution interface, but this is designed so that the potential gradient at the new interface is constant regardless of whatever electrode process occurs isothermally at the working electrode. Changes in electrode potential are thus proportional to changes in the interfacial potential difference... 

At the n-type interface, the electric field generated causes photogenerated conduction band electrons to move into the bulk of the semiconductor, to the back metal contact, and into the external circuit. The valence band holes access the semiconductor interface due to the influence of the interfacial electric field (Fig. 28.2). Thus, redox species can be oxidized by the excited n-type semiconductor. These materials act as photoanodes. On the other hand, the electric field in a p-type material is reversed in potential gradient therefore, excited electrons move to the semiconductor surface, while holes move through the semiconductor to the external circuit (Fig. 28.2). These materials are photocathodes. The presence of an electric field at the semiconductor-electrolyte interface is usually depicted by a bending of the band edges as shown in Figure 28.2. Elec-... 

The reductive dissolution of metal oxides such as Mn(III/IV) oxides by organic reductants occurs by the following sequential steps (Stone, 1986) (1) diffusion of reductant molecules to the oxide surface, (2) surface chemical reaction, and (3) diffusion of reaction products from the oxide surface. Steps (1) and (3), which are transport steps, are influenced by both the interfacial concentration gradient and the electrical potential gradient due to the net charge of the oxide surface. 

The interfacial potential differences which develop in electrode-solution systems are limited to only a few volts at most. This may not seem like very much until you consider that this potential difference spans a very small distance. In the case of an electrode immersed in a solution, this distance corresponds to the thin layer of water molecules and ions that attach themselves to the electrode surface— normally only a few atomic diameters. Thus a very small voltage can produce a very large potential gradient. For example, a potential difference of one volt across a typical 1CT8 cm interfacial boundary amounts to a potential gradient of 100 million volts per centimeter— a very significant value indeed ... 

Quantum phenomena arise from the presence of the interfacial potential. To appreciate its effect, consider a sequence of classical particles in order of decreasing size. They become increasingly susceptible to the influence of the interfacial potential in the same order. At the interface, the potential gradient is very sharp and any assumption of constant potential, on the scale of the smallest particle, becomes totally untenable. This is the precise condition that differentiates between classical and non-classical behaviour. 

Hence, at rest the neuron is in a state of temporary electrochemical equilibrium in which the resting membrane potential maintains a chemical gradient of ions. The chemical gradient is present due to the diffusion equilibrium potentials for each ion, but also as a result of the interfacial potentials and the electrogenic separation of charges caused by the Na -K" active transport system. Excitability of neurons depends on disequilibrium. 

The function of the enzymes of the mitochondrial respiratory chain is to transform the energy of redox reactions into an electrochemical proton gradient across the hydrophobic barrier of a coupling membrane. Isolated oligoenzyme complexes of the respiratory chain of mitochondria, cytochrome c oxidase, succinate ytochrome c reductase, and NADH CoQ reductase, are able to catalyze charge transfer in model systems, e.g., at a water/octane interface, which can be followed by a change in the interfacial potential at this interface [20-... 

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