Electrical interfacial layer

Problems in modelling the electrical interfacial layer in metal/oxide aqueous systems... 

Before discussing problems in modelling the electrical interfacial layer (EIL), one should answer two basic questions What is modelling and what is its purpose ... 

The above considerations indicate some different areas of research activity in the field of the electrical interfacial layer. The state of the art in this field is far from that which is common in solution chemistry. The problem is that the situation in the interfacial region is so complicated that one is forced to introduce substantial simplifications in the course of the modelling procedure . In addition, the situation is sometimes unknown, so that one should introduce several hypotheses in treating the interfacial equilibria. With respect to the solution chemistry, the experimental data are significantly less accurate and reproducible so that several different models cannot be separated and may coexist. The choice of model used in an interpretation would depend on the taste and ability of the author. In this field it is an achievement to understand the phenomena on a semiquantitative basis in some cases it is possible to recalculate the measurements, but data acquisition is left for the future. It would be desirable to standardise the interpretation and to produce tables with equilibrium parameters, e.g. for different oxides in order to predict their properties under different conditions (temperature, pH, electrolyte concentration, etc.). In fact, the poor reproducibility of experimental systems leads to scattering of results, even for such simple characteristics as the point of zero charge [1,2]. The apparent advantage of the described state of art lies in the fact that experimental data can be fitted... 

This chapter is devoted to a description and interrelation of present theoretical models. The aim is to clarify the some problems and to suggest possible solutions. From the experimental point of view, one may develop new methods on the basis of existing experimental techniques, but it would be of essential interest to refine and develop new techniques, i.e. interfacial spectroscopy. The modelling of equilibria in the electrical interfacial layer involves ... 

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. 

The history of development of ideas concerning the electrical interfacial layer (EIL) originates in the mercury electrode phenomena. This concept was later applied and adapted to the metal oxide aqueous interface. The fundamental difference lies in the fact that the potential of a metal electrode is determined by an applied source of electricity, while the surface of an oxide is charged due to interactions and accumulation of ionic species at the interface. Even the simple situation at a metal oxide aqueous interface requires a relatively complicated picture of the EIL. Several different assumptions are in use. Two... 

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. 

Kosmuiski, M., Electrical interfacial layer in nonaqueous solvents, in Interfacial Dynamics, Kallay, N., ed.. Marcel Dekker, New York, 1999, p. 273. 

M.Kosmulski, Electrical interfacial layer in nonaquesnus solvents, in N. Kallay, Ed., Interfacial Dynamics", Marcel Dekker, New York, 2000, page 273-313... 

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]. 

M. R. Philpott, J. N. Glosli. Molecular dynamics simulation of interfacial electrochemical processes electric double layer screening. In G. Jerkiewicz, M. P. Soriaga, K. Uosaki, A. Wieckowski, eds. Solid Liquid Electrochemical Interfaces, Vol. 656 of ACS Symposium Series. Washington ACS, 1997, Chap. 2, pp. 13-30. 

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. 

In the above we have assumed that no other forces than the electrical are acting at the surface of separation. In general, there will be the capillary forces as well, and we have to take account of the influence of the electrical double layer in considering the adsorption of an electrolyte. If w is the area of the surface, o the interfacial tension, e the charge per unit area, and E the difference of potential, we shall have ... 

The temperature dependence of the electrical double-layer parameters has been determined for real393,398 as well as quasi-perfect Ag planes.382,394 For quasi-perfect Ag electrodes, the value of 3 ffa0/9rhas been found to be higher for Ag(100) than for Ag(lll), and so it was concluded that Ag(lll) is more hydrophilic than Ag(100). For real surfaces,382,385,386 dEff=0/BT increases in the order (110) < (100) <(111). The same order of planes has been observed for Au 446-448 BEa /BT linearly increases as AX (interfacial parameter) decreases, i.e., as the hydrophilicity of Ag and Au electrodes decreases.15 32 393 397 398 446 48... 

Anodically polished and then cathodically reduced Cd + Pb alloys have been studied by impedance in aqueous electrolyte solutions (NaF, KF, NaC104, NaN02, NaN03).827 For an alloy with 2% Pb at cNap 0.03 M, Emfo = -0.88 V (SCE) and depends on cNaF, which has been explained by weak specific adsorption of F" anions. Surface activity increases in the sequence F" < CIO4 < N02. The Parsons-Zobel plot at E is linear, with /pz = 1.33 and CT° = 0.31 F m"2. Since the electrical double-layer parameters are closer to those for pc-Pb than for pc-Cd, it has been concluded that Pb is the surface-active component in Cd + Pb alloys827 (Pb has a lower interfacial tension in the liquid state). 

Previously, we have proposed that SFG intensity due to interfacial water at quartz/ water interfaces reflects the number of oriented water molecules within the electric double layer and, in turn, the double layer thickness based on the p H dependence of the SFG intensity [10] and a linear relation between the SFG intensity and (ionic strength) [12]. In the case of the Pt/electrolyte solution interface the drop in the potential profile in the vicinity ofelectrode become precipitous as the electrode becomes more highly charged. Thus, the ordered water layer in the vicinity of the electrode surface becomes thiimer as the electrode is more highly charged. Since the number of ordered water molecules becomes smaller, the SFG intensity should become weaker at potentials away from the pzc. This is contrary to the experimental result. 

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 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. 

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]

Double integration with respect to EA yields the surface excess rB+ however, the calculation requires that the value of this excess be known, along with the value of the first differential 3TB+/3EA for a definite potential. This value can be found, for example, by measuring the interfacial tension, especially at the potential of the electrocapillary maximum. The surface excess is often found for solutions of the alkali metals on the basis of the assumption that, at potentials sufficiently more negative than the zero-charge potential, the electrode double layer has a diffuse character without specific adsorption of any component of the electrolyte. The theory of diffuse electrical double layer is then used to determine TB+ and dTB+/3EA (see Section 4.3.1). 

Of the quantities connected with the electrical double layer, the interfacial tension y, the potential of the electrocapillary maximum Epzc, the differential capacity C of the double layer and the surface charge density q(m) can be measured directly. The latter quantity can be measured only in extremely pure solutions. The great majority of measurements has been carried out at mercury electrodes. 

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