Particle surface counter ions

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]. 

Specifically sorbable species that coagulate colloids at low concentrations may restabilize these dispersions at higher concentrations. When the destabilization agent and the colloid are of opposite charge, this restabilization is accompanied by a reversal of the charge of the colloidal particles. Purely coulombic attraction would not permit an attraction of counter ions in excess of the original surface charge of the colloid. 

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... 

Ion chromatography. The mobile phase in this type of chromatography is a buffered solution and the stationary phase consists of spherical particles of a polymer, micrometres in diameter. The surface of the particles is modified chemically in order to generate ionic sites. These phases allow the exchange of their mobile counter ion, with ions of the same charge present in the sample. This separation relies on the coefficient of ionic distribution. 

Clays are generally considered to be effective barriers for flow of water and solutes due to their low permeability and high ion adsorption capacity. However, as environmental criteria for the emission of contaminants and water from clay barriers become increasingly stringent, it is crucial to be aware of all relevant driving forces and fluxes and to take them into account in model assessments. In this respect the processes of chemical and electro-osmosis may not be neglected in clayey materials of hydraulic conductivity < 10-9 m/s [7], At these low conductivities the surface charge of the clay particles and the counter-ion accumulation in diffuse double layers enable explanation and quantification of osmotic processes and semi-permeability in clays [1],... 

A net surface charge can be acquired by the adsorption of ions on the surface of the particle. Ion adsorption may be positive or negative. Surfaces, which are already charged (e.g. by ionisation), usually show a tendency to adsorb counter-ions. It is possible that counter-ion adsorption causes a reversal of charge. 

Counter ion — A mobile ion that balances the charge of another charged entity in a solution. It is a charged particle, whose charge is opposite to that of another electrically charged entity (an atom, molecule, micelle, or surface) in question [i]. Counter ions can form electrostatically bound clouds in the proximity of ionic macromolecules and in many cases, determine their electric properties in solution [ii]. 

That is, region II cannot extend up to r = a. Therefore, the entire region of r cannot be covered only with regions I and II and thus one needs region III very near the particle surface between r = a and r = a, where counterions are condensed (counter-ion condensation). In region III, y r) is very high so that Eq. (6.99) becomes... 

Eq. (6.168). The dependence of the surface potential upon the particle charge Q is considerably suppressed since the counterions are accumulated within and near the polyelectrolyte layer (counter-ion condensation). 

Distance from particle surface — (a) Prior to addition of counter ions... 

The choice of topics dealt with in this text refiects essentially the interests and experience of the author. It encompassed the applications of XPS and Auger spectroscopies to the elements constituting the zeolite lattice, to counter-ions and to probe molecules. This has left aside the very large applications of surface sciences to materials supported or occluded in the zeolitic pore lattice. These materials include highly dispersed metallic particles, finely spread oxidic phases, entrapped organo-metallic complexes or metallic clusters. To some extent however the analysis of the supported phase is not specific of the... 

The measurement of this surface potential (T g or Pq) is impossible due to the hydrodynamic behavior of the system that generates a thin layer of attached liquid around the particles. However, there is a plane where the shear starts (shear plane), and at this plane the surface potential can be measured and the value is known as the zeta potential ( P ). Besides the indifferent counter- and co-ions in solution, there are also so-called potential determining ions (chemists caU them adsorbing ions). For most systems these are and OH ions that can adsorb directly on the particle surface and alter the -potential. There is a pH value for which the potential becomes zero and is called the isoelectric point (lEP), as shown in Figure 11.6. 

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