Colloids sedimentation potential

The electrokinetic processes can actually be observed only when one of the phases is highly disperse (i.e., with electrolyte in the fine capillaries of a porous solid in the cases of electroosmosis and streaming potentials), with finely divided particles in the cases of electrophoresis and sedimentation potentials (we are concerned here with degrees of dispersion where the particles retain the properties of an individual phase, not of particles molecularly dispersed, such as individual molecules or ions). These processes are of great importance in particular for colloidal systems. 

The electric field induced by sedimentation of colloidal particles under gravity is termed the sedimentation potential. The potential difference is sensed by two identical electrodes placed at different heights. No commercial or home-made apparatus for measurements of potential and IEP based on sedimentation potential have been reported in the recent literature. 

The sedimentation potential method has been used only to a limited extent to determine the surface charge characteristics of blood cells. In this method, the colloidal particles (e.g., blood cells) are allowed to fall through a vertical column and the potential difference between two electrodes, vertically separated, is measured. An apparatus, used for measurements on erythrocytes, is shown in Figure 19. The zeta potential across the colloidal particle-solution interface is calculated from the sedimentation potential (E) according to the equation ... 

Particle charge plays a major role on the stabilization of colloidal systems. Especially when nanoparticles are stabilized by an adsorption layer of polyelectrolytes, zeta potential measurements are very useful. The stabilization of the nanoparticles results from a combination of ionic and steric contributions. The zeta potential can be detected by means of electro-osmosis, electrophoresis, streaming potential, and sedimentation potential measmements. The potential drop across the mobile part of electric donble layer can be determined experimentally, whenever one phase is made... 

Levine, S., Neale, G., and Epstein, N., The prediction of electrokinetic phenomena within multiparticle systems, n. Sedimentation potential, J. Colloid Interface Set, 57, 424, 1976. 

Ohshima, H., Sedimentation potential in a concentrated suspension of spherical colloidal particles, J. Colloid Interface Sci., 208, 295, 1998. 

Ozaki, M., Ando, T., and Mizuno, K., A new method for the measurement of the sedimentation potential rotating cobunn methods. Colloids Surf A, Physicochem. Eng. Aspects, 159, 478-480, 1999. 

Of these four, the phenomenon of greatest practical interest is electrophoresis. Over the years, several relatively easy techniques for the study and application of electrophoresis have been developed and today these are important tools in many areas of science and technology, including colloid science, polymer science, biology, and medicine. Of lesser practical importance, and less intensely studied, are electroosmosis and streaming potential. Sedimentation potential has received relatively little attention because of experimental difficulties. 

Charged polymers and colloids exhibit a wide variety of elec-trokinetic behavior with terms such as electrophoresis, electro-osmosis, streaming potential, sedimentation potential, and others. These have been thoroughly reviewed [50]. 

The most popular and straightforward way to determine zeta potential is to apply an electric field to a colloidal suspension. In the case of neutral particles nothing happens, while particles carrying surface charges will have an oriented motion dependent on the direction of the electric field. Several phenomena (collectively known as electrokinetic effects) are observed i.e., electrophoresis, electroosmosis, streaming potential, and sedimentation potential. In this chapter we will discuss the first two effects. 

Contaminated bed sediments exist at numerous locations in the United States and around the world. These result mainly from past indiscriminate pollution of our aquatic environments and consist of freshwater and marine bodies including streams, lakes, wetlands, and estuaries. The bed sediments contain many hydrophobic organic compounds and metal ions that in the course of time act as sources of pollutants of the overlying aqueous phase. There are a number of transport pathways by which pollutants are transferred to the aqueous phase from contaminated sediments. One of the lesser known, but potentially important, modes of transport of pollutants from bed sediments is by diffusion and advection of contaminants associated with colloidal-size dissolved macromolecules in pore water. These colloids are measured in the aqueous phase as dissolved organic compounds (DOCs). (These are defined operationally as particles with a diameter smaller than 0.45 micrometer.)... 

The treatment of sedimentation and diffusion is a little more complicated when the particles under consideration are charged. The smaller counter-ions (see Chapter 7) tend to sediment at a slower rate and lag behind the sedimenting colloidal particles. A potential is thus set up which tends to restore the original condition of overall electrical neutrality by accelerating the motion of the counter-ions and retarding the motion of the colloidal particles. 

In addition to the soluble chemical species and possible solid phase species described in the previous sections no discussion on speciation can be complete without the consideration of surface species. These include the inorganic and organic ions adsorbed on the surface of particles. Natural systems such as soils, sediments and waters abound with colloids such as the hydrous oxides of iron, aluminium, manganese and silicon which have the potential to form surface complexes with the various cationic and anionic dissolved species (Evans, 1989). 

The situation is more complicated when the particles are charged because the smaller counter-ions sediment at a slower rate than do the larger colloidal particles. This creates an electrical potential that tends to speed up the counter-ions and slow down (drag) the colloidal particles. At high enough electrolyte concentrations the electric potentials are quickly dissipated and this effect vanishes. 

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