Electrical interfacial layer structure

The effect of the phospholipids on the rate of ion transfer has been controversial over the last years. While the early studies found a retardation effect [6-8], more recent ones reported that the rate of ion transfer is either not retarded [9,10] or even enhanced due to the presence of the monolayer [11 14]. Furthermore, the theoretical efforts to explain this effect were unsatisfactory. The retardation observed in the early studies was explained in terms of the blocking of the interfacial area by the phospholipids, and therefore was related to the size of the transferring ion and the state of the monolayer [8,15]. The enhancement observed in the following years was attributed to electrical double layer effects, but a Frumkin-type correction to the Butler Volmer (BV) equation was found unsuitable to explain the observations [11,16]. Recently, Manzanares et al. showed that the enhancement can be described by an electrical double layer correction provided that an accurate picture of the electrical double layer structure is used [17]. This theoretical approach will be the subject of Section III.C. 

In the area of interfacial charging at the solid/liquid interface of metal oxide aqueous suspensions, the "surface complexation or site binding concept is commonly used [3-20]. This concept is characterised by consideration of specific ionic reactions with surface groups, rather than assuming simple binding of ions to the surface or their accumulation at the interface (adsorption). In the past decade several different models were introduced on the basis of the surface complexation model (SCM) they differ in the assumed structure of the electrical interfacial layer (EIL) and in the proposed mechanisms and stoichiometries of surface reactions leading to surface charge. 

It could be concluded that different assumed structures of electrical interfacial layer were not distinguished by the applied procedure. It is clear that another data are necessary for that purpose. One way would be to introduce simultaneous electrokinetic measurements and assume relationship between electrokinetic potential and the potential at the onset of diffuse layer. 

Oscillations may exert a strong effect on adsorption processes in the frictional contact. Adsorption of particles on the electrode with a certain potential is known [23] to occur at a finite speed. Under low oscillation frequencies the adsorption manages to follow the potential and participate in the variation of the interfacial layer structure. At high frequencies the adsorption mechanism does not work, giving place to electrostatic charging of the layer as a condenser, i.e. the generation of the double electric layer (DEL). A mechanical model of the interfacial DEL has been elaborated by Shepenkov [24]. It follows from the model that, if a periodic mechanical force acts on the double layer from the side of the liquid or electrode, the electrode potential will vary periodically with the same excitation frequency. 

L. Blum. Structure of the electric double layer. In I. Prigogine, S. A. Rice, eds. Advances in Chemical Physics, Vol. 78, New York Wiley, 1990, pp. 171-222. L. Blum. The electric double layer—a comprehensive approach. In C. A. Croxton, ed. Fluid Interfacial Phenomena. New York Wiley, 1986, pp. 391-436. 

This series covers recent advances in electrocatalysis and electrochemistry and depicts prospects for their contribution into the present and future of the industrial world. It illustrates the transition of electrochemical sciences from a solid chapter of physical electrochemistry (covering mainly electron transfer reactions, concepts of electrode potentials and stmcture of the electrical double layer) to the field in which electrochemical reactivity is shown as a unique chapter of heterogeneous catalysis, is supported by high-level theory, connects to other areas of science, and includes focus on electrode surface structure, reaction environment, and interfacial spectroscopy. 

The description of the ion transfer process is closely related to the structure of the electrical double layer at the ITIES [50]. The most widely used approach is the combination of the BV equation and the modified Verwey-Niessen (MVN) model. In the MVN model, the electrical double layer at the ITIES is composed of two diffuse layers and one ion-free or inner layer (Fig. 8). The positions delimiting the inner layer are denoted by X2 and X2, and represent the positions of closest approach of the transferring ion to the ITIES from the organic and aqueous side, respectively. The total Galvani potential drop across the interfacial region, AgCp = cj) — [Pg.545]

The central issue which has to be addressed in any comprehensive study of electrode-surface phenomena is the determination of an unambiguous correlation between interfacial composition, interfacial structure, and interfacial reactivity. This principal concern is of course identical to the goal of fundamental studies in heterogeneous catalysis at gas-solid interfaces. However, electrochemical systems are far more complicated since a full treatment of the electrode-solution interface must incorporate not only the compact (inner) layer but also the boundary (outer) layer of the electrical double-layer. The effect of the outer layer on electrode reactions has been neglected in most surface electrochemical studies but in certain situations, such as in conducting polymers and... 

Electrocapillary is the study of the interfacial tension as a function of the electrode potential. Such a study can shed useful light on the structure and properties of the electrical double layer. The influence of the electrode-solution potential difference on the surface tension (y) is particularly pronounced at nonrigid electrodes (such as the dropping mercury one, discussed in Section 4.5). A plot of the surface tension versus the potential (like the ones shown in Fig. 1.13) is called an electrocapillary curve. 

Electrical Double Layer. In order to model the structure of the electrical double layer (EDL) of oxide colloids, it is necessary to formulate 1) the reactions which result in the formation of surface charge (cTq), and 2) the potential and charge relationships in the interfacial region. It has been generally assumed that surface charge (O ), defined experimentally by the net uptake of protons by the surface, results from simple ionization of oxide surface sites (5, 11 12, 13), i.e.. 

It is clear from the above discussion that three aspects of the electrical double layer must be considered in order to understand experimental observations and double layer phenomena. The first of these is the role of the metal and its influence on double layer properties. The second aspect concerns the inner layer or region immediately next to the metal. In the simplest case, this region is occupied only by solvent molecules. If adsorption is present, then some of these molecules are replaced by ions or solute molecules. In many cases the inner layer plays a dominant role in determining interfacial capacity. Thus, considerable effort has been expended to develop models for solvent structure in this region and adsorption. 

In aqueous media, the surfactant molecules are oriented, in all these structures, with their polar heads predominantly toward the aqueous phase and their hydro-phobic groups away from it. In vesicles, there will also be an aqueous phase in the interior of the structure. In ionic micelles, the aqueous solution-micelle interfacial region contains the ionic head groups, the Stern layer of the electrical double layer with the bound counterions, and water. The remaining counterions are contained in the Gouy-Chapman portion of the double layer that extends further into the aqueous phase. For POE nonionics the structure is essentially the same, except that the outer region contains no counterions, but includes coils of hydrated POE chains. 

---