Electrical triple layer

Fig 20.13 (a) Electrical double layer and (b) electrical triple layer (after Bockris and Reddy ). Note that Layer 2 in (b) is produced by adsorption of negative ions on the negatively charged... 

FIG. 3 Schematic representation of the ionic distribution and potential characteristic of the double layer model of electrical triple layer at metal oxide-electrolyte 1 1 interface. 

Fig. 5.38. A. (a) An electrical double layer and (b) an electrical triple layer. B. Potential distribution at the interface. OHP = Outer Helmholtz Plane, IHP = Inner HP, <1> = Galvani potential.

Reactions and selected intrinsic equilibrium constants describing ionization of SOH and adsorption of Na and CF on quartz and corundum are listed in Table I. The formulas of surface complexes indicate the assumed location of constituents of the complex in the electrical triple layer. The entire surface complex is located directly on the surface unless a dash is present in the formula. If a dash is present, everything to the left of the dash is located on the surface, and everything to the right of the dash is located in the OHP. 

An electric current can be made to flow in the device twice by using (Fig. 23) a triple-layer design consisting of a conducting polymer, a two-sided tape, and a conducting polymer. When one of the polymer acts as anode, the second acts as a cathode. 

To be useful in modeling electrolyte sorption, a theory needs to describe hydrolysis and the mineral surface, account for electrical charge there, and provide for mass balance on the sorbing sites. In addition, an internally consistent and sufficiently broad database of sorption reactions should accompany the theory. Of the approaches available, a class known as surface complexation models (e.g., Adamson, 1976 Stumm, 1992) reflect such an ideal most closely. This class includes the double layer model (also known as the diffuse layer model) and the triple layer model (e.g., Westall and Hohl, 1980 Sverjensky, 1993). 

Gouy-Chapman, Stern, and triple layer). Methods which have been used for determining thermodynamic constants from experimental data for surface hydrolysis reactions are examined critically. One method of linear extrapolation of the logarithm of the activity quotient to zero surface charge is shown to bias the values which are obtained for the intrinsic acidity constants of the diprotic surface groups. The advantages of a simple model based on monoprotic surface groups and a Stern model of the electric double layer are discussed. The model is physically plausible, and mathematically consistent with adsorption and surface potential data. 

Diprotic Surface Groups. Most of the recent research on surface hydrolysis reactions has been interpreted in terms of the diprotic surface hydrolysis model with either the triple layer model or the constant capacitance model of the electric double layer. The example presented here is cast in terms of the constant capacitance model, but the conclusions which are drawn apply for the triple layer model as well. 

Fig. 5-8. An interfadal double layer model (triple-layer model) SS = solid surface OHP = outer Helmholtz plane inner potential tt z excess charge <2h = distance from the solid surface to the closest approach of hydrated ions (Helmluritz layer thickness) C = electric capacity.

The main, currently used, surface complexation models (SCMs) are the constant capacitance, the diffuse double layer (DDL) or two layer, the triple layer, the four layer and the CD-MUSIC models. These models differ mainly in their descriptions of the electrical double layer at the oxide/solution interface and, in particular, in the locations of the various adsorbing species. As a result, the electrostatic equations which are used to relate surface potential to surface charge, i. e. the way the free energy of adsorption is divided into its chemical and electrostatic components, are different for each model. A further difference is the method by which the weakly bound (non specifically adsorbing see below) ions are treated. The CD-MUSIC model differs from all the others in that it attempts to take into account the nature and arrangement of the surface functional groups of the adsorbent. These models, which are fully described in a number of reviews (Westall and Hohl, 1980 Westall, 1986, 1987 James and Parks, 1982 Sparks, 1986 Schindler and Stumm, 1987 Davis and Kent, 1990 Hiemstra and Van Riemsdijk, 1996 Venema et al., 1996) are summarised here. 

Thus, according to these theories, all univalent (1 1) electrolytes should behave the same way. However, this is not what was observed experimentally. Solutions of different 1 1 electrolytes (e.g., NaCl, NaBr, Nal, KI) show species-specific behavior. In order to interpret this specific behavior, Grahame (5) proposed a new model of the interphase the triple-layer model. The basic idea in the interpretation of the ion-specific behavior is that anions, when attracted into the interphase, may become dehydrated and thus get closer to the electrode. Each anion undergoes this to a different extent. This difference in the degree of dehydration and the difference in the size of ions results in the specific behavior of the anions. Ions that are partially or fully dehydrated are in contact with the electrode. This contact adsorption of ions allows short-range forces (e.g., electric image forces) to act between the metal elec-... 

Hydrogenation of double bonds can also be performed at the mercury cathode using methanol/tetramethylammonium salts as SSE 301 The solvated electron, stabilized by (CH3)4N+ and transferred in the electrical double layer to the double bond, is assumed to be the reductant. As in CH3NH2/LiCl conjugated double bonds may be reduced, while isolated ones remain untouched. Terminal triple bonds are hydrogenated to double bonds. 

There is a range of equations used describing the experimental data for the interactions of a substance as liquid and solid phases. They extend from simple empirical equations (sorption isotherms) to complicated mechanistic models based on surface complexation for the determination of electric potentials, e.g. constant-capacitance, diffuse-double layer and triple-layer model. 

On this basis, three models will be discussed, which enable a calculation of the electrical potential, namely the constant-capacitance, the diffuse-double-layer, and the triple-layer model. 

Fig. 14 Idealized distribution of the electrical potential in the vicinity of hydrated oxide surfaces after the (a) diffuse-layer model (b) the constant-capacitance model (c) triple-layer model (after Drever 1997).

The TLM (Davis and Leckie, 1978) is the most complex model described in Figure 4. It is an example of an SCM. These models describe sorption within a framework similar to that used to describe reactions between metals and ligands in solutions (Kentef fll., 1988 Davis and Kent, 1990 Stumm, 1992). Reactions involving surface sites and solution species are postulated based on experimental data and theoretical principles. Mass balance, charge balance, and mass action laws are used to predict sorption as a function of solution chemistry. Different SCMs incorporate different assumptions about the nature of the solid - solution interface. These include the number of distinct surface planes where cations and anions can attach (double layer versus triple layer) and the relations between surface charge, electrical capacitance, and activity coefficients of surface species. 

Dove and Elston, this interfadal layer can be described by a triple layer snrface com-plexation model (TLM) as shown in Fig. 4.31. The interface consists of three electrostatically charged regions, each with an associated electric potential and snrface charge these are termed the o, p, and d planes. Hydrogen ions are permitted to coordinate with the nnsatnrated sites of the interface at the innermost o layer. Sodinm is positioned at the P layer or the d layer. The surface silicon-oxygen complex may have a different chemical character depending on the adsorbed species, hi a sodium chloride solution the surface complexes can be represented as sSiOHaCl, sSiOHj, =SiOH, =SiO-Na, and SiO". The concentration of each species depends on pH and salt concentration, and the sum of the fractions of these surface species equals 1 ... 

Several SCM s have been described in the literature. The more commonly used models include the Constant Capacitance Model (Schindler and Stumm, 1987), the Diffuse Double Layer Model (Stumm et al., 1970) and the Triple Layer Model (Davis et al., 1978 Yates et al, 1974). All are based on electric double layer theory but differ in their geometric description of the oxide-water interface and the treatment of the electrostatic interactions. 

The models describing hydrolysis and adsorption on oxide surfaces are called surface complexation models in literature. They differ in the assumptions concerning the structure of the double electrical layer, i.e. in the definition of planes situation, where adsorbed ions are located and equations asociating the surface potential with surface charge (t/> = f(5)). The most important models are presented in the papers by Westall and Hohl [102]. Tbe most commonly used is the triple layer model proposed by Davis et al. [103-105] from conceptualization of the electrical double layer discussed by Yates et al. [106] and by Chan et al. [107]. Reviews and representative applications of this model have been given by Davis and Leckie [108] and by Morel et al. [109]. We will base our consideration on this model. 

Sonnefeld, J., Lobbus, M., and Vogelsberger, W.. Determination of electric double layer parameters for spherical silica particles under application of the triple layer model using surface charge density data and results of electrokinetic sonic amplitude measurements, Colloids Surf. A, 195, 215, 2001. 

In triple layer approximations the location of each adsorbate with respect to the surface must be specified. Protons and all ions assumed bound as inner-sphere complexes (specifically or chemically adsorbed species) are assumed to lose part of their hydration sheaths, bonding directly to sites in the surface itself. Adsorbates assumed to remain hydrated, forming outer-sphere surface complexes, are assigned to the OHP. In the intrinsic equilibrium constants for adsorption reactions, K ", the activities of ions transferred from solution to the surface are corrected for the electrical potential they experience, % or (27). 

The specifics of surface complexation is associated with the participation of the surface and minerals electrostatic field whose potential depends on the structure of the dual electric layer. Due to this, there are several different models of surface complexation. Most commonly used are the constant capacitance model, dual diffuse-layer model and triple layer model. 

The triple layer model is applicable for solutions with a wide range of ionic strength. To use it, it is necessary to know the concentrations of active centres acidity constants and and electric capacitances and of the mineral surface, and also equilibrium constants of all specific and nonspecific complexation reactions. 

G. A. Parks, Characterization of aqueous colloids by their electrical doublelayer and intrinsic surface chemical properties, Surface Colloid Sci. 12 119 (1982). A computer-based algorithm for the triple layer model is described by J. Westall, op. cit. ... 

Usually, artificial muscle based on electrostrictive, piezoelectric, electrostatic, or ferroelectric materials have been manufactured as a film of the dry polymer, both sides coated with a thin metallic film required to apply the electric field. Electrokinetic artificial muscles [5,6] are constituted by films of polymeric gel (polymer, solvent, and salt) and two electrodes, located as close as possible to the material or coating both on sides, which are required to apply the electric field that drives the electroosmotic process. Any of the actuators described in this paragraph has a triple layer structure metal-electroactive polymer-metal (Figure 16.2). 

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