Electrical behavior, electrolytes

Having covered the chemical behavior of electrolytes, the text is now directed to their electrical behavior. The importance of the chemical and the electrical behaviors of electrolytes in galvanics and electrolytics hardly needs any elaboration. The term galvanics, used here, implies the generation of electrical energy directly from a spontaneous chemical reaction. 

The elucidation of the electrical behavior of electrolytes owes much to Arrhenius, who was the originator of the theory of electrolytic dissociation, generally, known as the ionic theory. 

Although simple impedance measurement can tell the existence of an anodic film, electrochemical impedance spectroscopy (EIS) can obtain more information about the electrochemical processes. In general, the anode/electrolyte interface consists of an anodic film (under mass transport limited conditions) and a diffuse mobile layer (anion concentrated), as illustrated in Fig. 10.13a. The anodic film can be a salt film or a cation (e.g., Cu ) concentrated layer. The two layers double layer) behave like a capacitor under AC electric field. The diffuse mobile layer can move toward or away from anode depending on the characteristics of the anode potential. The electrical behavior of the anode/electrolyte interface structure can be characterized by an equivalent circuit as shown in Fig. 10.13. Impedance of the circuit may be expressed as... 

Electric behaviors of colloidal particles in a salt-free medium containing counterions only are quite different from those in electrolyte solutions, as shown in Chapter 6. In this chapter, we consider the electrostatic interaction between two ion-penetrable membranes (i.e., porous plates) in a salt-free medium [1]. 

A biomembrane surface-biological fluid interface may be regarded as a solid-liquid interface exhibiting electrical behavior similar to that at an electrode-electrolyte solution interface (Pilla, 1974). Similarities between electrode interfaces and biomembranes in contact with aqueous solutions have recently been noted (Berry et al., 1985 Bowden et al., 1985). 

Pradhan, D.K., R. Choudhary, and B. Samantaray, Studies of structural, thermal and electrical behavior of polymer nanocomposite electrolytes. Express Polymer Letters, 2008. 2 630-638. 

The problem when trying to make an electrical model of the physical or chemical processes in tissue is often that it is not possible to mimic the electrical behavior with ordinary lumped, physically realisable components such as resistors (R), capacitors (C), inductors, semiconductor components, and batteries. Let us mention three examples 1) The constant phase element (CPE), not realizable with a finite number of ideal resistors and capacitors. 2) The double layer in the electrolyte in contact with a metal surface. Such a layer has capacitive properties, but perhaps with a capacitance that is voltage or frequency dependent. 3) Diffusion-controlled processes (see Section 2.4). Distributed components such as a CPE can be considered composed of an infinite number of lumped components, even if the mathematical expression for a CPE is simple. 

In the simplest case, the equivalent circuit comprised of a capacitance C and a resistance Ri connected in parallel can describe the electrical behavior of the electrodesolution interface (Chapter 3). When a current flows, an ohmic resistance Rq must be added in series to take into account the ohmic drop in the electrolyte between the reference electrode and working electrode. Equations (5.141) and (5.142) express the impedance of the equivalent circuit presented in Figure 5.26. In these equations Zq represents the impedance of the double layer. 

Apart from the intrinsic physical interest in the role of potential-dependent free-electron distribution in determining the electrical behavior of metal/electrolyte interfaces, work on this problem has been stimulated by the experimental observation that the compact component of the metal/electrolyte interfacial capacitance (Grahame, 1947) is metal-dependent, e.g., for Hg relative to (liquid) Ga (Frumkin and Damaskin, 1974) in the supposed absence of specific chemisorption of solvent dipoles or solute ions. In the liquid state, of course, the specific... 

The Dehye-Hbckel theory of electrolytes based on the electric field surrounding each ion forms the basis for modern concepts of electrolyte behavior (16,17). The two components of the theory are the relaxation and the electrophoretic effect. Each ion has an ion atmosphere of equal opposite charge surrounding it. During movement the ion may not be exacdy in the center of its ion atmosphere, thereby producing a retarding electrical force on the ion. 

A finite time is required to reestabUsh the ion atmosphere at any new location. Thus the ion atmosphere produces a drag on the ions in motion and restricts their freedom of movement. This is termed a relaxation effect. When a negative ion moves under the influence of an electric field, it travels against the flow of positive ions and solvent moving in the opposite direction. This is termed an electrophoretic effect. The Debye-Huckel theory combines both effects to calculate the behavior of electrolytes. The theory predicts the behavior of dilute (<0.05 molal) solutions but does not portray accurately the behavior of concentrated solutions found in practical batteries. 

The behavior of ionic liquids as electrolytes is strongly influenced by the transport properties of their ionic constituents. These transport properties relate to the rate of ion movement and to the manner in which the ions move (as individual ions, ion-pairs, or ion aggregates). Conductivity, for example, depends on the number and mobility of charge carriers. If an ionic liquid is dominated by highly mobile but neutral ion-pairs it will have a small number of available charge carriers and thus a low conductivity. The two quantities often used to evaluate the transport properties of electrolytes are the ion-diffusion coefficients and the ion-transport numbers. The diffusion coefficient is a measure of the rate of movement of an ion in a solution, and the transport number is a measure of the fraction of charge carried by that ion in the presence of an electric field. 

Polymers bearing electric charges, i.e., poly electrolytes, exhibit a very different behavior. See Chapter XIV. 

A detailed analysis of this behavior, as well as its analogy to the mercury-KF solution system, can be found in several papers [1-3,8,14]. The ions of both electrolytes, existing in the system of Scheme 13, are practically present only in one of the phases, respectively. This allows them to function as supporting electrolytes in both solvents. Hence, the above system is necessary to study electrical double layer structure, zero-charge potentials and the kinetics of ion and electron reactions at interface between immiscible electrolyte solutions. 

Electric interactions are screened by electrolytes. Hence, by adding electrolytes, the adsorption behavior is made to resemble that of uncharged macromolecules (see Fig. 4.17). 

The measurement of properties such as the resistivity or dielectric constant of PS requires some kind of contact with the PS layer. Evaporation of a metal onto the PS film-covered silicon sample produces a metal/PS/Si sandwich, which behaves like an MIS structure with an imperfect insulator. Such sandwich structures usually exhibit a rectifying behavior, which has to be taken into account when determining the resistivity [Si3, Bel4]. This can be circumvented by four-terminal measurements of free-standing PS films, but for such contacts the applied electric field has to be limited to rather small values to avoid undesirable heating effects. An electrolytic contact can also be used to probe PS films, but the interpretation of the results is more complicated, because it is difficult to distinguish between ionic and electronic contributions to the measured conductivity. The electrolyte in the porous matrix may short-circuit the silicon filaments, and wetting of PS in-... 

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