Electrokinetic phenomenon

Electrokinetic properties are associated with phases in contact with each other and are of particular significance for colloidal systems, although by no means restricted to these. Imposition of e.m.f. s across such interfaces causes 

Motion caused by imposed e,m.f e.m.f produced by movement of phases 

Electraosmosis - liquid caused to move through a static diaphragm Streaming potential - potential produced by hquid being forced through a diaphragm 

Electrokinetic phenomena designate the transport phenomena involving electrolytes near charged interfaces. 

Obviously the boundary conditions have a considerable influence on the observed transport processes. 

If the electrolyte flow moves perpendicularly to the interface, it will be stopped by it, except if some particular event such as an electrochemical reaction occurs, giving a source (or a well) of solute. Moreover, in this situation, the solvent, which is in most liquids uncompressible, will have to escape (or to arrive) in another direction and then to move in a direction parallel to the interface. 

The most common situation concerning the motion of electrolytes near interfaces is then that concerning a motion parallel to the interface. This will involve not only the solute flow, but also the solvent flow. 

Since most interfaces in contact with a liquid bear a superficial charge, the combination between hydrodynamic and electrostatic conditions will be the key of the observed processes, which are known as eletrokinetic phenomena. 

The potential at the shear plane is termed the electrokinetic or (zeta) potential and represents the actual value determined in the procedures discussed in the next section. It is generally assumed in tests of double-layer theory that the potential and ips are the same, since any error introduced will be small under ordinary circumstances. More significant errors may be introduced at high potentials, high electrolyte concentrations, or in the presence of adsorbed bulky nonionic species that force the shear plane further away from the surface, reducing the potential relative to ips- 

Considering all the assumptions and approximations involved in the derivation of the Gouy-Chapman model of the double layer, it should be obvious that a real situation is likely to be much more complex. Nevertheless, results obtained based on that model have served (and continue to serve) well in furthering our understanding of electrical phenomena in colloidal systems. Further refinements of double layer theory have succeeded in explaining a number of bothersome observations in specific situations, especially very high surface potentials. However, the complications involved in their application, and the benefits derived, do not generally warrant such effort in most practical situations. 

An important consequence of the existence of electrical charges at interfaces, whether they are colloids, porous materials, or some other system, is that they will exhibit certain phenomena under the influence of an applied electric field related to movement of some part of their electrical double layer. Those phenomena (illustrated schematically in Fig. 5.7) are collectively defined as electrokinetic phenomena and include four main classes  

Electroosmosis. The movement of a liquid relative to a stationary charged interface under the influence of an electric field. The fixed surface will typically be a capillary tube or porous plug. 

Electrophoresis. The movement of a charged interface (usually colloidal particles or macromolecules) plus its electrical double layer relative to a stationary liquid, under the influence of an applied field. Electrophoresis is, of course, the complement of electroosmosis. 

Electrokinetic is the general description applied to four phenomena which arise when attempts are made to shear off the mobile part of the electric double layer from a charged surface. 

Electrophoresis - the movement of a charged surface plus attached material (i.e. dissolved or suspended material) relative to stationary liquid by an applied electric field. 

Streaming potential - the electric field which is created when liquid is made to flow along a stationary charged surface (i.e. the opposite of electro-osmosis). 

Driving Force Mobile Phase Stationary Phase Measured Property Electrokinetic Phenomenon 

Electric field Liquid Porous plug capillary Motion of the liquid Electroosmosis 

Electric field Dispersed Liquid Motion of the Electrophoresis 

Pressure gradient particles Liquid Porous plug capillary particles Potential difference Streaming potential  

Gravitational or Dispersed Liquid electric current Potential difference streaming current Sedimentation 

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] 

Marlow and Rowell discuss the deviation from Eq. V-47 when electrostatic and hydrodynamic interactions between the particles must be considered [78]. In a suspension of glass spheres, beyond a volume fraction of 0.018, these interparticle forces cause nonlinearities in Eq. V-47, diminishing the induced potential 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]. 

The response of an electrified solid-liquid interface to shearing stress applied tp or induced in a contiguous liquid phase is termed an electro-kinetic phenomenon. The fundamental physical assumptions on which the molecular interpretation of electrokmetic phenomena is based are that an 

The other dependent variable of interest is the liquid velocity, v (x), defined to be a solution of a linearized form of the Navier-Stokes equation  

The net electric current /, which under steady-state conditions, is produced by the convection of charged species in the mobile region of the liquid phase, can be expressed mathematically with the equation 

The first step in Eq. 3.32 is the result of substituting Eq. 3.26 into Eq.3. 30 the second step is an integration by parts the third step makes use of Eqs. 3.27 and 3.29a and invokes the assumption that the mobile liquid phase is a homogeneous dielectric medium the fourth step is another integration by parts the fifth step is the result of Eqs. 3.27 and 3.28 along with the assumption that the mobile liquid phase has a uniform viscosity coefficient the sixth step involves the identity 

To the extent that the liquid phase retains bulk dielectric characteristics outside the region enclosed by the plane of shear, Eqs. 3.31 and 3.32 lead 

For electrolyte solutions, the term particle will be applied to particles of sufficiently large size, which makes it possible to describe them as particles that form a separate phase. For example, this term may refer to colloidal particles of various nature and origin, particles of emulsions of water in oil (w/o) or oil in water (o/w) type, etc. These particles, as a rule, carry a negative charge. 

At the boundary between the phases, one of which is an electrolyte solution, a region of charged solution is formed - an electric double layer whose presence causes peculiar kinds of electrohydrodynamic effects that manifest themselves in the motion of particles in the electrolyte solution and are called electrokinetic phenomena. All electrokinetic phenomena have common mechanism and arise from the relative motion of phases [3, 23]. 

If external electric field is applied to a solution containing dispersed particles, the particles will start moving. Such motion is called electrophoresis. An effect opposite to electrophoresis is also known the emergence of an electric field due to particles motion under non-electrical forces. For example, the so-called sedimentation potential arises in the case of particle sedimentation in a gravitational field. When particles move in a flow, there appears the electric potential of the flow. Generally, for any relative motion of phases, a corresponding potential drop arises in the mixture. 

EDLs can be present at the gas-liquid interfaces between bubbles in foams. 

In this case, since the interfaces on each side of the thin film are equivalent, any interfacial charge will be equally carried on each side of the film. If a foam film is stabilized by ionic surfactants, then their presence at the interfaces will induce a repulsive force opposing the thinning process. The magnitude of the force will depend on the charge density and the film thickness. 

Electrokinetic motion occurs when the mobile part of the EDL is sheared away from the inner layer (charged surface). There are several types of electrokinetic measurements  

Electrophoresis, in which an electric field is applied causing dispersed species, with their charged surfaces plus some attached fluid, to move relative to 

Electrokinetic sonic amplitude ESA, also termed electrosonic amplitude), an electro-acoustical method involving detection of the sound wave generated when dispersed species are made to move (oscillate) by an imposed alternating electric field (the principal features of the technique are shown in 

However, it must be realised that the potential is not a constant but dependent on the ionic environment. It is dependent on two parameters, the surface charge of the membrane and of the ionic strength. The surface charge may be strongly dependent on pH. The ionic strength both depends on the concentration and on the valence of the ions involved. 

An increase of the ionic strength results in a decrease of the double thickness (kt ) and of the potential (see figure IV - 32). 

From these experiment the iso-clectric point (lEP) of both membranes, i.c the point without effective charge, can clearly be ob.served. The rejection behaviour towards ionic species may well be predicted as a inunction of the pH if the surface charge is known (see chapter V). 

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

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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