Electric double layer counter-ions

The surface potential results in a concentration of the counter-ions and a depletion of the co-ions in the aqueous phase close to the charged surface, which results in the so-called electrical double layer. The ion concentrations in the aqueous phase at a given distance x from the charged surface are given by ... 

Ionic compounds such as halides, carboxylates or polyoxoanions, dissolved in (generally aqueous) solution can generate electrostatic stabilization. The adsorption of these compounds and their related counter ions on the metallic surface will generate an electrical double-layer around the particles (Fig. 1). The result is a coulombic repulsion between the particles. If the electric potential associated with the double layer is high enough, then the electrostatic repulsion will prevent particle aggregation [27,30]. 

Simple electrolyte ions like Cl, Na+, SO , Mg2+ and Ca2+ destabilize the iron(Hl) oxide colloids by compressing the electric double layer, i.e., by balancing the surface charge of the hematite with "counter ions" in the diffuse part of the double... 

Most particles acquire a surface electric charge when in contact with a polar medium. Ions of opposite charge (counter-ions) in the medium are attracted towards the surface and ions of like charge (co-ions) are repelled, and this process, together with the mixing tendency due to thermal motion, results in the creation of an electrical double-layer which comprises the charged surface and a neutralising excess of counter-ions over co-ions distributed in... 

Retention of proteins in ion exchange chromatography is mainly caused by electrostatic effects. Because both the protein and the surface have an electrical double layer associated to it, there is an increase in entropy when the two surfaces approach each other. This is due to a release of counter ions from the two double layers when they overlap. The model that is discussed here is based on a solution of the linearized Poisson-Boltzmann for two oppositely charged planar surfaces. We also show the result from a model where the protein is considered as a sphere and the... 

Figure 7.4. Schematic model of the Electrical Double Layer (EDL) at the metal oxide-aqueous solution interface showing elements of the Gouy-Chapman-Stern-Grahame model, including specifically adsorbed cations and non-specifically adsorbed solvated anions. The zero-plane is defined by the location of surface sites, which may be protonated or deprotonated. The inner Helmholtz plane, or [i-planc, is defined by the centers of specifically adsorbed anions and cations. The outer Helmholtz plane, d-plane, or Stern plane corresponds to the beginning of the diffuse layer of counter-ions and co-ions. Cation size has been exaggerated. Estimates of the dielectric constant of water, e, are indicated for the first and second water layers nearest the interface and for bulk water (modified after [6]).

Electrical Double Layer repulsion affected by counter ions such as salts in solution availability of hydrophobic and hydrophilic groups. 

While the linear adsorption isotherms of Figure 4 are illustrative only, they are not inconsistent with reality. The simplest theory of the electrical double layer, the Gouy-Chapman approximation, predicts that if the pH is not far from the isoelectric point, the charge represented by counter ions in the diffuse double layer is related to the surface potential as follows (4, 52, 86) ... 

The extent of the accumulation of such intermediates depends on their rates of the formation and those of the ensuing decomposition (or dissolution) reactions. If the latter are not high, the total density of such surface intermediates becomes so high that an appreciable surface potential At) is created by the electric double layer formed by the charge unbalance of these intermediates, as well as by the approach of counter ions and the hydration around these intermediates. The experimentally obtained difference between Ug(dark) and Ug (ill.) can be attributed to this AiJj. 

Figure 1. Potential variation through the electrical double layer for a higher concentration of potential-determining ion (a), a lower concentration of potential-determining ion (b), and in the presence of a specifically adsorbed counter ion with a potential-determining ion below the point-of-zero charge (c). Note that the potential in all three instances could be identical. (Reproduced, with permission, from Ref. 1. Copyright 1970, International Union of Pure and Applied Chemists.)...

Ion exchange involves an electric double layer situation in which two kinds of counter-ions are present, and can be represented by the equation... 

Ion diffusion, such as when a day partide, having internal crystal structure that carries an opposite charge due to isomorphic substitution, is placed in water and its counter-ions diffuse out to form an electric double layer. 

Figure 4.2 Illustration of the diffuse electric double layer showing the distributions of counter- and co-ions. Courtesy L.A. Ravina, Zeta-Meter, Inc., Staunton, Va.

Both association and aggregation thus seem to be favored in 0.1 - 0.2 M NaCl. At NaCl concentrations below 0.1 M dissociation into subunits may occur, but aggregation is suppressed due to the electric double layer and repulsion forces in the absence of mobile counter ions. 

All non-stoichiometric models adopt the electrical double layer concept and disagree with the stoichiometric hypothesis of an electroneutral stationary phase they emphasize the higher adsorbophilicity of the IPR compared to that of its counter ion a surface excess of IPR ions generates a primary charged layer and a charged interface. Like-sign co-ions are repelled from the surface while IPR counter ions are attracted by the charged surface. 

Conversely, according to the description of the electrical double layer based on the Stern-Gouy-Chapman (S-G-C) version of the theory [24], counter ions cannot get closer to the surface than a certain distance (plane of closest approach of counter ions). Chemically adsorbed ions are located at the inner Helmholtz plane (IHP), while non-chemically adsorbed ions are located in the outer Helmholtz plane (OHP) at a distance x from the surface. The potential difference between this plane and the bulk solution is 1 ohp- In this version of the theory, Pqhp replaces P in all equations. Two regions are discernible in the double layer the compact area between the charged surface and the OHP in which the potential decays linearly and the diffuse layer in which the potential decay is almost exponential due to screening effects. 

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