Particles sedimentation potential

The final and less commonly dealt-with member of the family of electrokinetic phenomena is the sedimentation potential. If charged particles are caused to move relative to the medium as a result, say, of a gravitational or centrifugal field, there again will be an induced potential E. The formula relating to f and other parameters is [72, 77]... 

The 2eta potential (Fig. 8) is essentially the potential that can be measured at the surface of shear that forms if the sohd was to be moved relative to the surrounding ionic medium. Techniques for the measurement of the 2eta potentials of particles of various si2es are collectively known as electrokinetic potential measurement methods and include microelectrophoresis, streaming potential, sedimentation potential, and electro osmosis (19). A numerical value for 2eta potential from microelectrophoresis can be obtained to a first approximation from equation 2, where Tf = viscosity of the liquid, e = dielectric constant of the medium within the electrical double layer, = electrophoretic velocity, and E = electric field. 

There are four related electrokinetic phenomena which are generally defined as follows electrophoresis—the movement of a charged surface (i.e., suspended particle) relative to astationaiy hquid induced by an applied ectrical field, sedimentation potential— the electric field which is crested when charged particles move relative to a stationary hquid, electroosmosis—the movement of a liquid relative to a stationaiy charged surface (i.e., capiUaty wall), and streaming potential—the electric field which is created when liquid is made to flow relative to a stationary charged surface. The effects summarized by Eq. (22-26) form the basis of these electrokinetic phenomena. 

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 movement of a charged particle with respect to an adjacent liquid phase is the basic principle underlying four electrokinetic phenomena electrophoresis, electroosmosis, sedimentation potential, and streaming potential. 

A further electrokinetic phenomenon is the inverse of the former according to the Le Chatelier-Brown principle if motion occurs under the influence of an electric field, then an electric field must be formed by motion (in the presence of an electrokinetic potential). During the motion of particles bearing an electrical double layer in an electrolyte solution (e.g. as a result of a gravitational or centrifugal field), a potential difference is formed between the top and the bottom of the solution, called the sedimentation potential. 

The streaming potential (Dorn effect) relates to a movement of liquid that generates electric potential, and electroosmosis occurs when a direct electric potential causes movement of the liquid. The sedimentation potential relates to sedimentation (directed movement) of charged particles that generates electric potential, and electrophoresis occurs when a direct electric potential causes a movement of charged particles. 

Sedimentation potential A potential is created when charged particles settle out of a suspension. This gives rise to sedimentation potential,... 

Sedimentation potential This is due to the electric field created by charged particles sedimenting in a liquid. This situation is the opposite of electrophoresis. 

The opposite effect to electrophoresis is the generation of a sedimentation potential. If a charged particle moves in the gravitational field or in a centrifuge, an electric potential arises — the sedimentation potential. While the particle moves, the ions in the electric double layer lag somewhat behind due to the liquid flow. A dipole moment is generated. The sum of all dipoles causes the sedimentation potential. 

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. 

Sedimentation potential - the electric field which is created when charged particles move relative to stationary liquid (i.e. the opposite of electrophoresis). 

With solid samples (e.g. suspended particles, sediments and soils), determination of the species distribution pattern usually involves a series of selective chemical extraction steps, but it is now recognised that many experimental parameters can influence the amount extracted by the reagents, and there are many potential sources of error. For example, during an extraction step, metal ions released from one phase can resorb on other exposed surfaces and, where coatings are being removed in the process, the values obtained can be influenced by the order in which reagents are used. 

Zeta potential was the first, experimentally available value characterizing edl. The potential of the solid particles in the electrolyte solutions may be determined on the basis of one of the four following phenomena microelectrophoresis, streaming potential, sedimentation potential and electroosmosis. The most popular of them and the best described theoretically and methodically is the electrophoresis. Other papers, concerning the electrophoretic mobility, stationary level determination and the theory of the charged particles transportation in the electric field are still published. 

Dorn potential — Potential difference established during sedimentation of charged particles (see - sedimentation potential). 

Sedimentation potential— (also called electrophoretic or Dorn potential) Potential difference established during sedimentation (caused, e.g., by gravitation or centrifugation) of small charged particles (suspended in solution dispersion of solid particles or emulsion of immiscible liquid droplets). 

In the limit a = b, the polyelectrolyte layer vanishes so that 4 c = 4 and (J)s = 0. In this case, Eq. (24.40) gives the following result for sedimentation potential for concentrated suspension of hard particles of radius a for low zeta potentials, as expected ... 

Here Vp has been replaced with the pressure difference between the two points is AP, K°, and K are, respectively, the usual conductivity and the complex conductivity of the electrolyte solution in the absence of the particles, (f> is the particle volume fraction, (j)c is the volume fraction of the particle core, Vc is the volume of the particle core, volume fraction of the polyelectrolyte segments, I4 is the total volume of the polyelectrolyte segments coating one particle, and po, are respectively, the mass density of the particle core and that of the electrolyte solution, and ps is the mass density of the polyelectrolyte segment, V is the suspension volume, and p(cai) is the dynamic electrophoretic mobility of the particles. Equation (26.4) is an Onsager relation between CVP and pirn), which takes a similar form for an Onsager relation between sedimentation potential and static electrophoretic mobility (Chapter 24). 

Figure 11. Schematic representation of the electrophoretic mobility (A) measurement showing the major components. In an applied electric field, emulsion droplets move according to their surface charge. These charges can electrostatically stabilize an emulsion system by preventing the droplets from coming into contact and coalescing. The motion of the droplets is visually observed, and the electrophoretic mobilities of a number of particles are measured to determine zeta potential. The sedimentation potential (B) is also illustrated.

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. 

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