Structure of the electrical double layer

The double-layer extension depends on electrolyte concentration and valency of the counterions, 

The double-layer extension increases with a decrease in electrolyte concentration. 

Distinction between Specific and Nonspecific Adsorbed Ions 

For the specifically adsorbed ions the range of interaction is short that is, these ions must reside at the distance of closest approach, possibly within the hydration shell. For indifferent ions the situation is different, and these ions are subjected to either an attractive (for the counterions) or a repulsive (for the co-ions) potential (energy = ZFy/(x)/RT). The space charge density due to these ions is high close 

When charged colloidal particles in a dispersion approach each other such that the double layers begin to overlap (when the particle separation becomes less than twice the double layer extension), then repulsion will occur. The individual double layers can no longer develop unrestrictedly, as the limited space does not allow complete potential decay [5]. 

The data tabulated below for 1 1 electrolyte (e.g. KCl) show that the double layer extension increases with decreasing electrolyte concentration. 

The potential drops linearly in the Stern region, and then exponentially. Grahame distinguished two types of ions in the Stern plane, physically adsorbed counter ions (outer Helmholtz plane) and chemically adsorbed ions (that lose part of their hydration shell) (iimer Helmholtz plane). 

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Fig. 1. The structure of the electrical double layer where Q represents the solvent CD, specifically adsorbed anions 0, anions and (D, cations. The inner Helmholtz plane (IHP) is the center of specifically adsorbed ions. The outer Helmholtz plane (OHP) is the closest point of approach for solvated cations or molecules. O, the corresponding electric potential across the double layer, is also shown.

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. 

Describe and draw clearly the structure of the electrical double layer (with its... 

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]

Fig. 4.1 Structure of the electric double layer and electric potential distribution at (A) a metal-electrolyte solution interface, (B) a semiconductor-electrolyte solution interface and (C) an interface of two immiscible electrolyte solutions (ITIES) in the absence of specific adsorption. The region between the electrode and the outer Helmholtz plane (OHP, at the distance jc2 from the electrode) contains a layer of oriented solvent molecules while in the Verwey and Niessen model of ITIES (C) this layer is absent...

At present it is impossible to formulate an exact theory of the structure of the electrical double layer, even in the simple case where no specific adsorption occurs. This is partly because of the lack of experimental data (e.g. on the permittivity in electric fields of up to 109 V m"1) and partly because even the largest computers are incapable of carrying out such a task. The analysis of a system where an electrically charged metal in which the positions of the ions in the lattice are known (the situation is more complicated with liquid metals) is in contact with an electrolyte solution should include the effect of the electrical field on the permittivity of the solvent, its structure and electrolyte ion concentrations in the vicinity of the interface, and, at the same time, the effect of varying ion concentrations on the structure and the permittivity of the solvent. Because of the unsolved difficulties in the solution of this problem, simplifying models must be employed the electrical double layer is divided into three regions that interact only electrostatically, i.e. the electrode itself, the compact layer and the diffuse layer. 

If the electrolyte components can react chemically, it often occurs that, in the absence of current flow, they are in chemical equilibrium, while their formation or consumption during the electrode process results in a chemical reaction leading to renewal of equilibrium. Electroactive substances mostly enter the charge transfer reaction when they approach the electrode to a distance roughly equal to that of the outer Helmholtz plane (Section 5.3.1). It is, however, sometimes necessary that they first be adsorbed. Similarly, adsorption of the products of the electrode reaction affects the electrode reaction and often retards it. Sometimes, the electroinactive components of the solution are also adsorbed, leading to a change in the structure of the electrical double layer which makes the approach of the electroactive substances to the electrode easier or more difficult. Electroactive substances can also be formed through surface reactions of the adsorbed substances. Crystallization processes can also play a role in processes connected with the formation of the solid phase, e.g. in the cathodic deposition of metals. 

The inhibition of electrode processes as a result of the adsorption of electroinactive surfactants has been studied in detail at catalytically inactive mercury electrodes. In contrast to solid metal electrodes where knowledge of the structure of the electrical double layer is small, it is often possible to determine whether the effect of adsorption on the electrode process at mercury electrodes is solely due to electrostatics (a change in potential 02)... 

Most of the new molecular-level results concern the structure and dynamics of water at interfaces. We begin this review with a brief summary of this area. Several recent review articles and books can be consulted for additional information. " We then examine in some detail the new insight gained from molecular dynamic simulations of the structure of the electric double layer and the general behavior of ions at the water/metal interface. We conclude by examining recent developments in the modeling of electron transfer reactions. 

Although our knowledge of the structure of the electric double layer is based on experimental data collected at finite electrolyte concentrations, understanding the structure of the electric double layer at the microscopic level must begin with knowledge of the structure of a single solvated ion at the interface. This information has been obtained in recent years from molecular dynamics computer simulations. 

This series covers recent advances in electrocatalysis and electrochemistry and depicts prospects for their contribution to 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 structure 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. 

Fig. 2.2 Structure of the electric double layer under different conditions of electrode polarization (a) metal positively charged, anions present at the inner Helmholtz plane (chemically interacting with metal) and in the diffuse double layer beyond the outer Helmholtz plane (b) metal negatively charged, inner Helmholtz plane empty, cations in diffuse layer (c) metal positively charged, strong adsorption of anions in inner Helmholtz plane, balancing cations in the diffuse layer...

Next comes a layer of nonspecifically adsorbed counterions with their hydration shell. Still, the permittivity is significantly reduced because the water molecules are not free to rotate. This layer specifies the outer Helmholtz plane. Finally there is the diffuse layer. A detailed discussion of the structure of the electric double layer at a metal surface is included in Ref. [65],... 

Figure 7.2 Schematic representation of the structure of the electric double layer according to Stern s theory... 

This chapter has been written with the intention of introducing the reader to some analytical methods which can be employed to describe the structure of the electrical double layer adjacent to a charged surface and double layer interactions between two charged surfaces across an electrolyte. Since the... 

The structure of the electric double layers has been described by different models. All models consider the charges on solid phase to be compact, forming... 

The number of surface sites, surface area, and structure of the electric double layer, and its surface charge and potential have to be known in order to use these programs. The law of mass action, electroneutrality, and mass balances have to be taken into consideration. 

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.. 

R. Parsons, The structure of the electrical double layer and its influence on the rates of electrode reactions, in Advances in Electrochemistry and Electrochemical Engineering, Vol. 1, P. Delahay, editor, Wiley-lnterscience, New York, 1961, pp. 1-64. 

L. Blum, Structure of the Electric Double Layer, Adv. Chent Phys. 78 (1990) 171. (Review of various advanced theoretical models.)... 

M.A.V. Devanathan, B.V.K.S.R.A. Tllak, The Structure of the Electrical Double Layer at the Metal-Soludon Interface, Chent Revs. 65 (1965) 635. (Review with over 300 references to older literature emphasis on mercury.)... 

Structure of the Electrical Double Layer on HTSC Oxides... 

In 1940, Frumkin explored the relationships among the double-layer structure on mercury electrodes, the capacitance measured by use of a Wheatstone bridge, and the surface tension, following the theoretical underpinnings of the Lippmann equation. Grahame ° expanded this treatment of the mercury electrode, providing a fundamental understanding of the structure of the electrical double layer. Dolin and Ershler applied the concept of an equivalent circuit to electrochemical kinetics for which the circuit elements were independent of frequency. Randles developed an equivalent circuit for an ideally polarized mercury electrode that accounted for the kinetics of adsorption reactions. ... 

Furusawa, K., Tomotsu, N. Direct Observation Studies for the Structure of the Electrical Double Layer of Concentrated Monodisperse Latices. J. Colloid Interface Sci., 1983, 92, p. 504. 

Activation-limited growth tends to favor compact columnar or equiaxed deposits, while mass transport-limited growth favors formation of loose dendritic deposits. The deposit morphology is modified by using additives. Additives act as grain refiners and levelers because of their effects on electrode kinetics and the structure of the electrical double layer at the cathode surface. Additives that reduce primarily the nucleation overpotential can be considered to be grain-refining additives because of increased secondary nucleation events. 

Tadros, T.F. and Lyklema, J., The operative mechanism of glass electrodes and the structure of the electrical double layer on glass, 7. Electroanal. Chem., 22, 9, 1969. 

Janusz, W.. The structure of the electrical double layer at the Lichrospher-type adsorbent/aqueous electrolyte solution interface, Adsorption Sci. Technol.. 14, 151, 1996. 

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