Outer Helmholtz plane . See

What, therefore, is the potential difference to be used Is it MzfraP< ), the potential difference from the metal to the contact adsorption plane, or IHP (inner Helmholtz plane, see Fig. 6.88), or is it MzfOHP<[>, the potential difference from the metal to the OHP (outer Helmholtz plane, see Fig. 6.88), or MzfSpotential difference from the bulk of the metal to the bulk of the electrolytic solution In respect to P, does one consider it to multiply the whole potential difference across the interface or only a fraction of this potential difference Similarly, what concentrations of electron acceptors and donors must be fed into the basic equation Bulk values or the values at the OHP or the values at the contact-adsorbed species (Fig. 6.88) ... 

OHP Outer Helmholtz plane. See Helmholtz Double Layer. 

When the electrode acts as a source or sink for electrons with the reactant sitting in the outer Helmholtz plane (see Section 1.4.4), reaction rate is not usually dependent on the nature of the electrode material. For some reactions to proceed, however, the reactant or an intermediate must be adsorbed on the electrode surface. Then the nature of the electrode material often has a significant effect on the reaction rate by virtue of its ability or lack of ability to provide the necessary sites. We shall first discuss briefly the phenomenon of adsorption and then derive an example of kinetic expressions when adsorption is taken into account. 

IHP) (the Helmholtz condenser formula is used in connection with it), located at the surface of the layer of Stem adsorbed ions, and an outer Helmholtz plane (OHP), located on the plane of centers of the next layer of ions marking the beginning of the diffuse layer. These planes, marked IHP and OHP in Fig. V-3 are merely planes of average electrical property the actual local potentials, if they could be measured, must vary wildly between locations where there is an adsorbed ion and places where only water resides on the surface. For liquid surfaces, discussed in Section V-7C, the interface will not be smooth due to thermal waves (Section IV-3). Sweeney and co-workers applied gradient theory (see Chapter III) to model the electric double layer and interfacial tension of a hydrocarbon-aqueous electrolyte interface [27]. 

All factors influencing the potentials of the inner or outer Helmholtz plane will also influence the zeta potential. For instance, when, owing to the adsorption of surface-active anions, a positively charged metal surface will, at constant value of electrode potential, be converted to a negatively charged surface (see Fig. 10.3, curve 2), the zeta potential will also become negative. The zeta potential is zero around the point of zero charge, where an ionic edl is absent. 

As a result of the above considerations, the Helmholtz model of the interface now shows two planes of interest (see Figure 2.8). The inner Helmholtz plane (IHP) has the solvent molecules and specifically adsorbed ions (usually anions) the outer Helmholtz plane (OHP), the solvated ions, both cations and anions. It can be seen from Figure 2.8 that the dielectric in the capacitor space now comprises two sorts of water that specifically adsorbed at the electrode surface and that lying between the two Helmholtz planes. Continuing the analogy with capacitance, these two forms of water act as the dielectric in two capacitors connected in series. 

That volume within which the ions having charge opposite to that on the electrode have a concentration higher than those in the bulk of the solution (in the absence of specific adsorption). Under the conditions typically employed in electrochemical measurements, i.e. high ionic strength, this would correspond simply to a volume bounded by the outer Helmholtz plane, a few angstroms (see section on electrocapillarity). 

The electrified interface is generally referred to as the electric double layer (EDL). This name originates from the simple parallel plate capacitor model of the interface attributed to Helmholtz.1,9 In this model, the charge on the surface of the electrode is balanced by a plane of charge (in the form of nonspecifically adsorbed ions) equal in magnitude, but opposite in sign, in the solution. These ions have only a coulombic interaction with the electrode surface, and the plane they form is called the outer Helmholtz plane (OHP). Helmholtz s model assumes a linear variation of potential from the electrode to the OHP. The bulk solution begins immediately beyond the OHP and is constant in potential (see Fig. 1). 

It is probable that ions—the physisorbed ones—do react from this outer plane. However, there is a closer plane—the inner Helmholtz plane (see Hg, 6.88)—and this is occupied principally by ions that chemisorb on the electrode (and are not separated from the electrode by a water layer). 

The Stern surface is drawn through the ions that are assumed to be adsorbed on the charged wall. (This surface is also known as the inner Helmholtz plane [IHP], and the surface running parallel to the IHP, through the surface of shear (see Chapter 12) shown in Figure 11.9, is called the outer Helmholtz plane [OHP]. Notice that the diffuse part of the ionic cloud beyond the OHP is the diffuse double layer, which is also known as the Gouy-Chapman... 

Ey2 reversible half-wave potential, k, vxl> experimental standard rate constant, a transfer coefficient, (outer-Helmholtz plane, L ron standard rate constant after correction for the double-layer effect (see 4)), lcex rate constant for the homogeneous self-exchange electron-transfer (see 4) in Chapter 9) obtained with a HMDE in DMF-0.5 M BU4NCIO4 at 22 2°C, except the last two obtained with a DME in DMF-0.1 M Bu4NI at 30°C. 

The size of the particles that is calculated from these experiments corresponds to particle dimensions plus the double layer thickness, in this case defined by the shear plane inside which the adsorbed species are rigidly held, and outside of which there is free movement. The shear plane can therefore be associated roughly with the outer Helmholtz plane, an approximation often made. The value of the electrostatic potential at the shear plane with respect to the value in bulk solution is called the electrokinetic or zeta potential, 33 (see Section 3.3). 

The static - double-layer effect has been accounted for by assuming an equilibrium ionic distribution up to the positions located close to the interface in phases w and o, respectively, presumably at the corresponding outer Helmholtz plane (-> Frumkin correction) [iii], see also -> Verwey-Niessen model. Significance of the Frumkin correction was discussed critically to show that it applies only at equilibrium, that is, in the absence of faradaic current [vi]. Instead, the dynamic Levich correction should be used if the system is not at equilibrium [vi, vii]. Theoretical description of the ion transfer has remained a matter of continuing discussion. It has not been clear whether ion transfer across ITIES is better described as an activated (Butler-Volmer) process [viii], as a mass transport (Nernst-Planck) phenomenon [ix, x], or as a combination of both [xi]. Evidence has been also provided that the Frumkin correction overestimates the effect of electric double layer [xii]. Molecular dynamics (MD) computer simulations highlighted the dynamic role of the water protrusions (fingers) and friction effects [xiii, xiv], which has been further studied theoretically [xv,xvi]. 

Assume that specific adsorption takes place at the outer Helmholtz plane. This means that yf is identified with yf. See flg. 3.21. For this case equation (3.6.31) reduces to... 

Helmholtz planes see inner Helmholtz plane and outer Helmholtz plane Helmholtz-Smoluchowskl equation see electrophoretic mobility Henderson equation,... 

Tafel analysis of voltammetric curves (see Chapter 3) allows us to attribute the electrocatalytic process at -eO.92 V to the formation of relatively strong vanadium-glucose adducts, whereas the electrocatalytic process at -1-1.15 V involves the oxidation of glucose molecules located at the outer Helmholtz plane. The catalytic pathway can be described on assuming that the electrochemical process is initiated by the formation of vanadium-glucose surface-confined complexes ... 

FIGURE 5.67 Schematic presentation of the structure of the EDL. The surface charge is created by ionized surface groups and/or by ions tightly adsorbed in the Stem layer. The plane of closest approach of the ions from the diffuse part of the EDL is called the outer Helmholtz plane (OHP). The electric potential in the OHP plane is referred to as the surface potential, /j, in the text. The shear plane, x = x, separates the hydrodynam-ically immobile liquid that moves together with the surface, x x, which has nonzero relative velocity with respect to the surface. Note that the ions in the immobile part of the EDL can move with respect to the surface under an applied electric field, which gives rise to the anomalous surface conductivity (see Section 5.8.8). 

Fig. 12. Cyclic voltammogram and model of the electrical double layer at a silver electrode surface. Arrows indicate the direct-ions of molecular dipoles in the water (smallest circles) and pyridine (largest circles, Py) molecules, the arrow head being the positive end. The cations (solvated) could he Na+ or K+, the anions (unsolvated) Cl or SOJ-. IHP and OHP designate the inner and outer Helmholtz planes, respectively, and PZC is the potential of zero charge (see text for further explanations). (Reproduced with permission from ref. 14.)...

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