Inner 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) ... 

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). 

IHP Inner Helmholtz plane, see Helmholtz Double Layer. 

In principle it should be extremely fruitful to investigate the effect of the nature and concentration of the supporting electrolyte. The effect of the potential has been investiga ted by very many authors, and the most interesting from our point of view is one of the first papers [74], the authors of which came to the conclusion that the transition state in the reduction of nitromethane is located at the inner Helmholtz plane (see [10,17]). However, it is not possible to analyze the data which ha ve been accumula ted until a theory which takes account of the potential has been form-... 

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. 

There is compelling evidence that reducing agent oxidation and metal ion reduction are, more often than not, interdependent reactions. Nonetheless, virtually all established mechanisms of the electroless deposition fail to take into account this reaction interdependence. An alternative explanation is that the potentials applied in the partial solution cell studies are different to those measured in the full electroless solution studies. Notwithstanding some differences in the actual potentials at the inner Helmholtz plane in the full solution relative to the partial solutions, it is hard to see how this could be a universal reason for the difference in rates of deposition measured in both types of solution. 

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... 

In the discussion of the different models for the structure of double layer developed up to this point, no specific interactions have been considered. However, specific adsorption is a common phenomena in electrochemistry. Since the interactions implied have to be very short range in nature, the chemisorbed species are strongly bound to the electrode surface with the locus of their centers being the inner Helmholtz plane (IHP, see Fig. 1.10), or compact part of the double layer. 

Using this model, one cannot forecast the adsorption of the background electrolyte ions because this model do not consider the reactions responsible for such a process. Zeta potential values, calculated on the basis of this model, are usually too high, nevertheless, because of its simplicity the model is applied very often. In a more complicated model of edl, the three plate model (see Fig. 3), besides the mentioned surface plate and the diffusion layer, in Stern layer there are some specifically adsorbed ions. The surface charge is formed by = SOHJ and = SO- groups, also by other groups formed by complexation or pair formation with background electrolyte ions = SOHj An- and = SO Ct+. It is assumed that both, cation (Ct+) and anion (A-), are located in the same distance from the surface of the oxide and form the inner Helmholtz plane (IHP). In this case, beside mentioned parameters for two layer model, the additional parameters should be added, i.e., surface complex formation constants (with cation pKct or anion pKAn) and compact and diffuse layer capacities. 

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

Since specific adsorption is an important phenomenon in electrochemistry, the solution/metal interface has nevertheless been studied in various ways. An ion is considered to be adsorbed specifically in the inner Helmholtz plane when it is partially dehydrated and in direct contact with the metal surface (see, e.g.. Ref. 15). On the other hand, an ion that is adsorbed further away from the electrode with its hydration shell essentially intact is considered to be adsorbed non-specifically in the outer Helmholtz plane . In the classical treatment of contact adsorption, the balance between the energy of hydration of the ion and the strength of the image interactions determines which ions are specifically adsorbed and which ones are not... 

Inner Helmholtz Plane (IHP) See Helmholtz Double Layer. 

Specifically adsorbed ions " are those which directly contact the electrode surface. As indicated in Figure 1, specifically adsorbed ions are considered to be desolvated, and they displace solvent molecules adjacent to the electrode surface. Iodide, which is weakly solvated by water, is a good example of an ion which tends to specifically adsorb on electrode surfaces. The nature of specific adsorption is a function of both electrostatic and chemical interactions between the electrode and the ion. Specific adsorption can significantly alter the interfacial potential profiles as well as the kinetics of interfacial reactions. The thin solution layer closest to the electrode surface which contains specifically adsorbed ions as well as solvent molecules is often called the inner layer or the Helmholtz layer. The inner Helmholtz plane (IHP) is considered to pass through the centers of specifically adsorbed ions (see Figure 1). 

See color insert.) Electric double-layer models at interface of electrode and electrolyte solution. (a) Diffuse layer or Gouy-Chapman model, (b) Helmholtz layer or model the d represents the double-layer thickness, (c) Stern-Grahame layer or model in which the IHP represents the inner Helmholtz plane and the OHP represents the outer Helmholtz plane. 

In 1946, D. Grahame (27) introduced a refinement of the Stern theory, in which he distinguish between hydrated ions and ions which are adsorbed by covalent bonds or van der Waals forces (or both). Grahame introduced the terms inner-Helmholtz plane for the locus of the electrical centres of the adsorbed ions and outer-Helmholtz plane for the locus of the electrical centres for the hydrated ions in contact with the charged surface. This is illustrated in Figure 1.8. A number of more sophisticated models on the same theme as Grahame s model have been suggested (see ref. (28),... 

If specific adsorption occurs, the adsorbed ions are assumed to be located on an average plane b within the Stern layer. This plane is called the inner Helmholtz plane (IHP) (see Section 7.3.2). The adsorbed ions develop a charge c7b and are subjected to a potential i/), . The surface potential may therefore be written as... 

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. 

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.)...

The fractional occupation by a single species i corresponds to that given by the Langmuir adsorption isotherm (see, e.g., Delahay71) for which is the standard free energy of adsorption. The standard free energy of adsorption is therefore a function of Aequilibrium potential difference between the inner and outer Helmholtz planes. 

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