Diffuse electric double layer motion

In 1910, Georges Gouy (1854-1926) and independently, in 1913, David L. Chapman (1869-1958) introduced the notion of a diffuse electrical double layer at the surface of electrodes resulting from a thermal motion of ions and their electrostatic interactions with the surface. 

Surface charge influences the distribution of nearby ions in a polar medium ions of opposite charge (counter-ions) are attracted to the surface while those of like charge (co-ions) are repelled. Together with mixing caused by thermal motion, a diffuse electric double layer is formed. 

If the external electric field is parallel to the charged surface, then the electric force is acting on the ions in the diffusion electric double layer. This force is directed along the surface and causes migration of ions along the wall and consequently, motion of the solution as a whole. 

Electrical neutrality is established near the surfaces of the particles and a charge with the opposite sign, equivalent to the surface charge, gathers like a cloud in the form of ions around the particle surface (refer to Figure 54). In this structure, the layer attached to the particle surface is called the Stem layer, and outside that, the layer that is present as a result of equilibrium distribution through the balance between electrostatic attraction and diffusion force from thermal motion, is called the diffuse electric double layer. If an external electric field is applied to this kind of dispersion system, the particle and diffuse electric double layer are drawn electrically to an electrode of the opposite sign, and relative motion occurs at the boundary of a certain slide plane . The potential of this slide plane is called the zeta potential, and it is used as the scale of surface potential [6, 7, 8]. 

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. 

In the absence of specific adsorption of anions, the GCSG model regards the electrical double layer as two plate capacitors in series that correspond respectively, to two regions of the electrolyte adjacent to the electrode, (a) An inner compact layer of solvent molecules (one or two layers) and immobile ions attracted by Coulombic forces (Helmholtz inner plane in Fig. 2). Specific adsorption of anions at the electrode surface may occur in this region by electronic orbital coupling with the metal, (b) An outer diffuse region of coulombically attracted ions in thermal motion that complete the countercharge of the electrode. 

In the years 1910-1917 Gouy2 and Chapman3 went a step further. They took into account a thermal motion of the ions. Thermal fluctuations tend to drive the counterions away form the surface. They lead to the formation of a diffuse layer, which is more extended than a molecular layer. For the simple case of a planar, negatively charged plane this is illustrated in Fig. 4.1. Gouy and Chapman applied their theory on the electric double layer to planar surfaces [54-56], Later, Debye and Hiickel calculated the potential and ion distribution around spherical surfaces [57],... 

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. 

The particles suspended and surfaces immersed in a liquid are usually charged by the adsorption of the ions from solution. The charge on the surface of the particle or any other surface immersed in liquid is balanced by an equal but oppositely charged layer in the adjacent liquid, resulting in a so-called electric double layer discussed earlier in Chapters 4 and 5. In a liquid with ions and molecules under constant thermal motion, one expects a diffused zone of charges in the solution and a compacted layer on the solid surface. Total charge density in the two zones must be equal and opposite in sign. When the liquid or the particle is in motion (with respect to each other) the compacted layer on the... 

FIGURE 5.1 Electric double layer in the vicinity of an adsorption layer of ionic surfactant, (a) The diffuse layer contains free ions involved in Brownian motion, whereas the Stem layer consists of adsorbed (bonnd) counterions, (b) Near the charged snrface there is an accnmnlation of counterions and a depletion of coions. 

Figure 3.4 The electrical double layer (a) according to the Helmholtz model, (b) the diffuse double layer resulting from thermal motion. Q positive charge, 9 negative charge.

The sorption mechanism includes diffusion motion, facilitating the penetration of molecules and ions to the active surface of colloids, their release into the medium and mutual exchanges. The diffusion is manifested during the motion of ions in the electric double layer of colloids as well as during the motion of ions and molecules to the surface of plant and microbial cells. 

A further complication arises when attention is focussed on the electron density distribution within the semiconductor solid. This, in contrast to the metal case, now is able to vary from a low to a high concentration level as electrons in a conduction band or as holes in a valence band. The electric field on the solid side of the electrical double layer now has spatial extent - a diffuse double layer character exists within the solid. The conventional electric field effects previously associated with ion motion and ion distributions in the electrolyte have a counterpart within the solid phase. 

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