Electrokinetics phenomena

A number of electrokinetic phenomena have in common the feature that relative motion between a charged surface and the bulk solution is involved. Essen-... 

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]... 

E. Interrelationships in Electrokinetic Phenomena In electroosmosis, the volumetric flow and current are related through... 

The presence of surface conductance behind the slip plane alters the relationships between the various electrokinetic phenomena [83, 84] further complications arise in solvent mixtures [85]. Surface conductance can have a profound effect on the streaming current and electrophoretic mobility of polymer latices [86, 87]. In order to obtain an accurate interpretation of the electrostatic properties of a suspension, one must perform more than one type of electrokinetic experiment. One novel approach is to measure electrophoretic mobility and dielectric spectroscopy in a single instrument [88]. 

In particular, in polar solvents, the surface of a colloidal particle tends to be charged. As will be discussed in section C2.6.4.2, this has a large influence on particle interactions. A few key concepts are introduced here. For more details, see [32] (eh 13), [33] (eh 7), [36] (eh 4) and [34] (eh 12). The presence of these surface charges gives rise to a number of electrokinetic phenomena, in particular electrophoresis. 

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. 

It is very difficult and scarce to find literature to study the electrokinetic phenomena of proteins or macromolecules in solution therefore limit us to the basic concepts of electrokinetic changes observed, they are conformational change because of the presence of salts and the zeta potential change in pH. 

Electric double layers are formed in heterogeneous electrochemical systems at interfaces between the electrolyte solution and other condncting or nonconducting phases this implies that charges of opposite sign accumnlate at the surfaces of the adjacent phases. When an electric held is present in the solntion phase which acts along snch an interface, forces arise that produce (when this is possible) a relative motion of the phases in opposite directions. The associated phenomena historically came to be known as electrokinetic phenomena or electrokinetic processes. These terms are not very fortunate, since a similar term, electrochemical kinetics, commonly has a different meaning (see Part 11). 

In 1873, Gabriel Lippmann (1845-1921 Nobel prize, 1908) performed extensive experiments of the electrocapiUary behavior of mercury and established his equation describing the potential dependence of the surface tension of mercury in solutions. In 1853, H. Helmholtz, analyzing electrokinetic phenomena, introduced the notion of a capacitor-like electric double layer on the surface of electrodes. These publications... 

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. 

The adsorption of ions at insulator surfaces or ionization of surface groups can lead to the formation of an electrical double layer with the diffuse layer present in solution. The ions contained in the diffuse layer are mobile while the layer of adsorbed ions is immobile. The presence of this mobile space charge is the source of the electrokinetic phenomena.t Electrokinetic phenomena are typical for insulator systems or for a poorly conductive electrolyte containing a suspension or an emulsion, but they can also occur at metal-electrolyte solution interfaces. 

Of the four electrokinetic phenomena, two (electroosmotic flow and the streaming potential) fall into the region of membrane phenomena and will thus be considered in Chapter 6. This section will deal with the electrophoresis and sedimentation potentials. 

The relationship of electrokinetic phenomena and the movement of petroleum constituents is not of high importance however, it can be important for the transport of some solutes related to a remedial technology such as electroosmosis remediation. 

The electrokinetic potential (zeta potential, Q is the potential drop across the mobile part of the double layer (Fig. 3.2c) that is responsible for electrokinetic phenomena, for example, elecrophoresis (= motion of colloidal particles in an electric field). It is assumed that the liquid adhering to the solid (particle) surface and the mobile liquid are separated by a shear plane (slipping plane). The electrokinetic charge is the charge on the shear plane. 

Electrostatic vs. Chemical Interactions in Surface Phenomena. There are three phenomena to which these surface equilibrium models are applied regularly (i) adsorption reactions, (ii) electrokinetic phenomena (e.g., colloid stability, electrophoretic mobility), and (iii) chemical reactions at surfaces (precipitation, dissolution, heterogeneous catalysis). 

In the following text, let us consider what happens if the charged particle or surface is under dynamic motion of some kind. Further, there are different systems under which electrokinetic phenomena are investigated. These systems are... 

The word electrokinetic implies the joint effects of motion and electrical phenomena. We are interested in the electrokinetic phenomena that originate the motion of a liqnid within a capillary tube and the migration of charged species within the liquid that surrounds them. In the first case, the electrokinetic phenomenon is called electroosmosis whereas the motion of charged species within the solution where they are dissolved is called electrophoresis. This section provides a brief illns-tration of the basic principles of these electrokinetic phenomena, based on text books on physical chemistry [7-9] and specialized articles and books [10-12] to which a reader interested to stndy in deep the mentioned theoretical aspects should refer to. 

Section 6.2.1 has briefly examined the electrokinetic phenomena of electroosmosis, which refers to the motion of a liquid relative to a surface under the action of an electric field. This section examines the motion of ions in an applied electric field relative to the solution that surrounds them. 

John L. Anderson (Co-Chair) is a University Professor of Chemical Engineering and is affihated with the Center for Complex Fluids Engineering at Carnegie Mellon University. He is also the dean of the College of Engineering. He received his B.S. from the University of Delaware and his Ph.D. from the University of Illinois. His research interests are membranes, colloidal science, electrophoresis and other electrokinetic phenomena, polymers at interfaces, and biomedical engineering. He is a former co-chair of the BCST and is a member of the National Academy of Engineering. 

Malek, R. A. I. Roy, D. M. 1985. Electrokinetic phenomena and surface characteristics of fly ash particles. Materials Research Society Symposium Proceedings, 86, 41-50. 

To give an atomistic interpretation of electrokinetic phenomena, one must consider questions such as What happens when one of the phases moves relative to the other For example, what happens when the electrolyte is made to flow past an electrode at rest ... 

The charge in the diffuse layer can be considered equivalent to the Gouy charge density qd placed at a distance K-1 from the OHP. This gives rise to a parallel-plate condenser model. The potential at one plate—deep in the solution side—is taken at zero, while the potential at the other plate—which coincides with the OHP—is [f0. This latter potential is often referred to in the study of electrokinetic phenomena as the zeta ( ) potential. Thus,... 

But v/X is the electro-osmotic velocity of the fluid per unit of electric field, i.e., the electro-osmotic mobility. It is interesting to note that both the electro-osmotic mobility v/X = a2 and the streaming-current coefficient j/AP = a3 have beat proved to be equal to each other and to ltfZ/4nr. This only means that the Onsager reciprocity relation has been shown to be consistent with a simple model of some electrokinetic phenomena. 

Electrokinetic phenomena depend on the relative motion of the phases constituting the double layer. In the treatment of electro-osmotic mobility, the electrolyte was considered to move within a stationary capillary—a moving cylinder of liquids within a static cylinder of solid. But the arguments only need relative motion the arguments would be equally valid if one considered a moving cylindrical solid within a stationary liquid. 

Electrokinetic phenomena such as electroosmosis, streaming potential, and viscoelectric effects (Chapter 12)... 

The first group, consisting of Sections 2.2-2.4, covers sedimentation. After some preliminaries, we discuss Stokes s law, a hydrodynamic equation that will appear again when we discuss electrokinetic phenomena in Chapter 12 and the kinetics of coagulation in Chapter 13. Stokes s law is a key relationship in understanding the rate of sedimentation and is used in the derivation of the sedimentation equation for spherical particles. Following this, the equation for the sedimentation coefficient, a... 

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