Part of electrical double layer

Thus, the properties of diffuse part of electrical double layer determine the dependence of electrostatic component of disjoining pressure on the thickness of film [15], The screening of the charged surface with a layer of counterions results in a sharp decrease of the electrostatic component of disjoining pressure with a corresponding increase in film thickness. For the surface bearing low charge, when... 

The nature of stability of disperse systems with solid dispersed phase and liquid continuous phase against coagulation is determined by phase composition, particle size and particle concentration. The stability of hydrosols at low electrolyte concentrations is usually related to the electrostatic component of disjoining pressure (Chapter VII), arising from the overlapping diffuse parts of electrical double layers. 

The difference in mobility of free mobile ions in the diffusive part of electrical double layer and on the charged surface determines the electrokinetic phenomena, which are totally determined by properties of electrical double layer. 

According to the model proposed by Verwey and Niessen (1939), an electric double layer is formed at an ITIES, which consists of two ionic space charge regions. As a whole the electric double layer is electrically neutral, so for the excess charge density in the part of the double layer in the aqueous phase, and for the part in the organic phase,... 

For semiconductor electrodes and also for the interface between two immiscible electrolyte solutions (ITIES), the greatest part of the potential difference between the two phases is represented by the potentials of the diffuse electric layers in the two phases (see Eq. 4.5.18). The rate of the charge transfer across the compact part of the double layer then depends very little on the overall potential difference. The potential dependence of the charge transfer rate is connected with the change in concentration of the transferred species at the boundary resulting from the potentials in the diffuse layers (Eq. 4.3.5), which, of course, depend on the overall potential difference between the two phases. In the case of simple ion transfer across ITIES, the process is very rapid being, in fact, a sort of diffusion accompanied with a resolvation in the recipient phase. 

The electrokinetic potential (zeta potential, Q is the potential drop across the mobile part of the double layer (Fig. 3.2c) that is responsible for electrokinetic phenomena, for example, elecrophoresis (= motion of colloidal particles in an electric field). It is assumed that the liquid adhering to the solid (particle) surface and the mobile liquid are separated by a shear plane (slipping plane). The electrokinetic charge is the charge on the shear plane. 

In a qualitative way, colloids are stable when they are electrically charged (we will not consider here the stability of hydrophilic colloids - gelatine, starch, proteins, macromolecules, biocolloids - where stability may be enhanced by steric arrangements and the affinity of organic functional groups to water). In a physical model of colloid stability particle repulsion due to electrostatic interaction is counteracted by attraction due to van der Waal interaction. The repulsion energy depends on the surface potential and its decrease in the diffuse part of the double layer the decay of the potential with distance is a function of the ionic strength (Fig. 3.2c and Fig. 

The value of the electric permittivity of water in the inner part of the double layer is commonly accepted as equal to 6. A much higher capacity of the inner layer at the Ga/solution interface was explained by the weak interaction of gallium with water, leading to a high value of As shown... 

According to the capacitor model of the double layer, assuming constant thickness and electric permittivity, the dependence of AG° on <7m should be linear. " Deviations from linearity can be viewed as resulting from changes of X2 and/or e in the inner part of the double layer. A linear plot ofAG° vs. is observed for adsorption of ions and thiourea. ... 

Consider a system in which a potential difference AV, in general different from the equilibrium potential between the two phases A 0, is applied from an external source to the phase boundary between two immiscible electrolyte solutions. Then an electric current is passed, which in the simplest case corresponds to the transfer of a single kind of ion across the phase boundary. Assume that the Butler-Volmer equation for the rate of an electrode reaction (see p. 255 of [18]) can also be used for charge transfer across the phase boundary between two electrolytes (cf. [16, 19]). It is mostly assumed (in the framework of the Frumkin correction) that only the potential difference in the compact part of the double layer affects the actual charge transfer, so that it follows for the current density in our system that... 

The fundamental electrochemical event, that is, electron transfer, occurs at the electrode surface. Peculiarities of electrochemical reactions include an electrical field, which in a special way complicates the phenomena of adsorption and desorption at the surface. The first layer of the solution, which is in contact with the electrode, possesses a specific structure. It is important for charged particles that the orientation of medium molecules in the vicinity of the electrode produces a decrease in dielectric permeability in the compact part of the double layer (Damaskin and Kryshtalik 1984). 

When particles or large molecules make contact with water or an aqueous solution, the polarity of the solvent promotes the formation of an electrically charged interface. The accumulation of charge can result from at least three mechanisms (a) ionization of acid and/or base groups on the particle s surface (b) the adsorption of anions, cations, ampholytes, and/or protons and (c) dissolution of ion-pairs that are discrete subunits of the crystalline particle, such as calcium-oxalate and calcium-phosphate complexes that are building blocks of kidney stone and bone crystal, respectively. The electric charging of the surface also influences how other solutes, ions, and water molecules are attracted to that surface. These interactions and the random thermal motion of ionic and polar solvent molecules establishes a diffuse part of what is termed the electric double layer, with the surface being the other part of this double layer. 

The variation of the electric potential in the electric double layer with the distance from the charged surface is depicted in Figure 6.2. The potential at the surface ( /o) linearly decreases in the Stem layer to the value of the zeta potential (0- This is the electric potential at the plane of shear between the Stern layer (and that part of the double layer occupied by the molecules of solvent associated with the adsorbed ions) and the diffuse part of the double layer. The zeta potential decays exponentially from to zero with the distance from the plane of shear between the Stern layer and the diffuse part of the double layer. The location of the plane of shear a small distance further out from the surface than the Stem plane renders the zeta potential marginally smaller in magnitude than the potential at the Stem plane ( /5). However, in order to simplify the mathematical models describing the electric double layer, it is customary to assume the identity of (ti/j) and The bulk experimental evidence indicates that errors introduced through this approximation are usually small. 

It is convenient to think of the diffuse part of the double layer as an ionic atmosphere surrounding the particle. Any movement of the particle affects the particle s ionic atmosphere, which can be thought of as being dragged along through bulk motion and diffusional motion of the ions. The resulting electrical contribution to the resistance to particle motion manifests itself as an additional viscous effect, known as the electroviscous effect. Further,... 

Figure 26-20 (a) Electric double layer created by negatively charged silica surface and nearby cations. (fc>) Predominance of cations in diffuse part of the double layer produces net electroosmotic flow toward the cathode when an external field is applied. 

In an electric field, cations are attracted to the cathode and anions are attracted to the anode (Figure 26-20b). Excess cations in the diffuse part of the double layer impart net momentum toward the cathode. This pumping action, called electroosmosis (or electroen-dosmosis), is driven by cations within — lOnm of the walls and creates uniform pluglike electroosmotic flow of the entire solution toward the cathode (Figure 26-21a). This process is in sharp contrast with hydrodynamic flow, which is driven by a pressure difference. In hydro-... 

Cations striking a cathode liberate electrons. A series of dynodes multiplies the number of electrons by 105 before they reach the anode, electroosmosis Bulk flow of fluid in a capillary tube induced by an electric field. Mobile ions in the diffuse part of the double layer at the wall of the capillary serve as the pump. Also called electroendosmosis. electroosmotic flow Uniform, pluglike flow of fluid in a capillary tube under the influence of an electric field. The greater the charge on the wall of the capillary, the greater the number of counterions in the double layer and the stronger the electroosmotic flow. 

The electric field which actually affects the charge transfer kinetics is that between the electrode and the plane of closest approach of the solvated electroactive species ( outer Helmholtz plane ), as shown in Fig. 2.2. While the potential drop across this region generally corresponds to the major component of the polarization voltage, a further potential fall occurs in the diffuse double layer which extends from the outer Hemlholtz plane into the bulk of the solution. In addition, when ions are specifically absorbed at the electrode surface (Fig. 2.2c), the potential distribution in the inner part of the double layer is no longer a simple function of the polarization voltage. Under these circumstances, serious deviations from Tafel-like behaviour are common. 

The electric double layer can be regarded as consisting of two regions an inner region which may include adsorbed ions, and a diffuse region in which ions are distributed according to the influence of electrical forces and random thermal motion. The diffuse part of the double layer will be considered first. 

Consider the motion of liquid in the diffuse part of the double layer relative to that of a non-conducting flat surface when an electric field... 

The distribution of ions in the diffuse part of the double layer gives rise to a conductivity in this region which is in excess of that in the bulk electrolyte medium. Surface conductance will affect the distribution of electric field near to the surface of a charged particle and so influence its electrokinetic behaviour. The effect of surface conductance on electrophoretic behaviour can be neglected when ka is small, since the applied electric field is hardly affected by the particle in any case. When tea is not small, calculated zeta potentials may be significantly low, on account of surface conductance. 

The ions in the mobile part of the double layer show a net movement in a direction opposite to that of the particle under the influence of the applied electric field. This creates a local movement of liquid which opposes the motion of the particle, and is known as electrophoretic retardation. It is allowed for in the Henry equation. 

Electrokinetic measurements at 25°C on silver iodide in 10 3 mol dm-3 aqueous potassium nitrate give d /d(pAg) = -35 mV at the zero point of charge. Assuming no specific adsorption of K+ or NO3 ions and no potential drop within the solid, estimate the capacity of the inner part of the electric double layer. Taking the thickness of the inner part of the double layer to be 0.4 nm, what value for the dielectric constant near to the interface does this imply Comment on the result. 

This statement requires explanation in terms of the electrical double layer at the surface of all cells, including at the sarcolemma of myofibers and the plasma membrane of neurons. The inner part of this double layer (Stmt layer) can be regarded as a condenser with its complement of ions largely giving it a certain numerical value for permittivity (dielectric constant). This is charged when the membranes of muscle and nerve are at rest (resting potential). 

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