Electrokinetic phenomena electrophoresis

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. 

There are four electrokinetic phenomena (electrophoresis, electroosmosis, streaming potential, and sedimentation potential), all of which involve both the theory of the electric double layer and that of liquid flow. Among them electrophoresis has the greatest practical applicability to the study of biomolecules and biocell surface porperties. In this section, the relation between electrophoretic mobility and its related electrokinetic potential C will be discussed. 

III. NONEQUiLIBRIUM ELECTROPHORETIC MEASUREMENTS A Electrokinetic Phenomena—Electrophoresis... 

Another effect which is important in a discussion of the conductivity of colloidal systems is the surface conduction, i.e. a conductivity contribution from the double-layers. This contribution is important when the electrolyte content is relatively low in the bulk phase. The surface conductance is also important when measurements of electrokinetic phenomena (electrophoresis, electro-osmosis, etc.) need to be evaluated. Recently, it... 

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. 

Electrophoresis is an electrokinetic phenomenon whereby charged compounds in an electric field move through a continuous medium and separate by prefer-... 

A fundamental electrokinetic phenomenon is the electroosmotic flow of a liquid electrolyte (solution of positive and negative ions) past a charged surface in response to a tangential electric field. Electrophoresis is the related phenomenon of motion of a colloidal particle or molecule in a background electric field, propelled by electroosmotic flow in the opposite direction. The basic physics is as follows ... 

Let us mention that dielectrophoresis has also found wide application in manipulation and sorting of particles and biological cells. Together with standard electrophoresis, it is perhaps the most often used electrokinetic phenomenon with practical applications in mind. Even particle separation can be achieved by using microelectrode arrays [55]. Based on the dielectrophoresis phenomenon, a new technique has recently become available for particle or cell separation, namely the dielectrophoresis/gravitational field-flow fractionation (DEP/G-FFF). In DEP/ G-FFF, the relative positions and velocities of unequal particles or cells are controlled by the dielectric properties of the colloid and the frequency of the applied field. The method has been applied to model polystyrene beads, but, most interestingly, to suspensions of different biological cells [56]. 

Electrophoresis is an electrokinetic phenomenon whereby charged compounds in an electric field move through a continuous medium and separate by preferentially obtaining different electrophoretic mobilities according to their charges and sizes. Cations move toward the negative electrode (cathode) and anions move toward the positive electrode (anode). 

The most familiar type of electrokinetic experiment consists of setting up a potential gradient in a solution containing charged particles and determining their rate of motion. If the particles are small molecular ions, the phenomenon is called ionic conductance, if they are larger units, such as protein molecules, or colloidal particles, it is called electrophoresis. 

If the electric field E is applied to a system of colloidal particles in a closed cuvette where no streaming of the liquid can occur, the particles will move with velocity v. This phenomenon is termed electrophoresis. The force acting on a spherical colloidal particle with radius r in the electric field E is 4jrerE02 (for simplicity, the potential in the diffuse electric layer is identified with the electrokinetic potential). The resistance of the medium is given by the Stokes equation (2.6.2) and equals 6jtr]r. At a steady state of motion these two forces are equal and, to a first approximation, the electrophoretic mobility v/E is... 

Section 4.6 may be considered the prototype of modem electrokinetics, because all relevant features were covered the coupling of hydrodynamic and electric fluxes and double layer polarization. However, the elaboration remained restricted to electrophoresis, which is the most familiar electrokinetlc phenomenon. Other types of electrokinetics, summarized in table 4.1. basically require the same theory, although there may be considerable differences in the elaboration (what is stationary what is moving boundiuy condition , etc.). With sec. 4.6 we consider the fundamentals sufficiently explained and illustrated and we shall therefore not repeat and apply this theory to other electrokinetlc phenomena. Instead, two important extensions will now be briefly reviewed inclusion of double layer overlap, as occurs in plugs, in the present section and measurement in alternating fields in the following. 

The term electrokinetic is applied to a group of effects in which either an electric potential brings about movement, or movement produces an electric potential. For example, if macromolecuies are suspended in a liquid, and a potential is applied, the particles often move towards one or other of the electrodes. This phenomenon is called electrophoresis. The inverse of it is when the particles undergo sedimentation, in which case a sedimentation potential is developed. The occurrence of these electrokinetic effects is due to the existence of potential differences between the solid and liquid phases. 

The external screen is a diffusion layer in which the ions are poorly bonded as the distance from the Stem layer is increasing, and have the tendency to migrate into the solution. Therefore the electric potential in this layer decreases exponentially with the distance from the boundary of Stem layer. On this boundary it is called electrokinetic potential or potential. It can be derived from the measurements of the colloidal particles velocity in the electric field of intensity E. This phenomenon is known as electrophoresis. C potential can be determined from the formula given by Smoluchowski ... 

Electrokinetic flow covers in principle the transport of liquids (electroosmosis) and samples (electrophoresis) in respraise to an electric field. Both motions are associated with the electric double layer that is formed spraitaneously at the solid-liquid interface in which there is a net charge density. Compared to the traditional pressure-driven flow, electrokinetic flow is more suited to miniaturization due to its nearly plug-like velocity profile and much lower flow resistance. However, Joule heating is a ubiquitous phenomenon in electrokinetic flow that will affect the transport of both liquids and samples via temperature-sensitive material properties. 

It may be appreciated that electrokinetic phenomena are determined by electric properties at the plane of shear rather than at the real surface. In the following sections of this chapter, the relation between the measured property and is further analyzed. This is done for electroosmosis, electrophoresis, streaming current, and streaming potential. The sedimentation potential will not be discussed any further, because in practice this phenomenon does not play an important role. The electrokinetic charge density may then be derived from using the theory for the diffuse electrical double layer. 

Since all materials are polarizable to some degree, the surface charge is generally not fixed. This leads to a broad class of nonlinear electrokinetic phenomena, where bulk electric fields interact with induced diffuse charge in solution to produce nonlinear electrophoretic motion, U =f(E). In electrolytes, such effects of induced-charge electrophoresis (ICEP) occur in addition to the purely electrostatic effect of dielectrophoresis (DEP) in low-frequency AC fields (< 100 kHz), where there is enough time for diffuse-charge relaxation around the particle within each period. ICEP is a complex phenomenon, which can lead not only to nonlinear mobility (in the field direction) but also to rotation and motion in arbitrary directions, even in uniform fields. 

The four possible types of electrokinetic phenomena are streaming (current) potential (electric potential generated by fluid movement relative to another phase), sedimentation potential or Dorn phenomenon or Dom effect (due to dispersed particles motion relative to the fluid caused by sedimentation) and electrophoresis and electro-osmosis (movement of two phases is caused by an external potential difference). 

In Equation 19.12, Cq = 8.854 x j-i qi -1 jg jjjg dielectric constant in vacuum, e is the relative dielectric permittivity of the solvent (e = 78.5 for water at room temperature 298 K), and are the electrokinetic zeta potential defined at the shear plane (see Figure 19.3), r is the dynamic viscosity of the solvent (q = 8.91 x 10 kgm" s for water at room temperature 298 K), and E is the externally applied electric field. The first equation in Equation 19.12 represents the fluid motion in a stationary channel under the action of an externally appUed electric field. The motion is called electro-osmosis and the velocity is v. The second equation in Equation 19.12 gives the velocity v, of charged suspended colloidal particle (or a dissolved molecule) driven by the same electric field. This phenomenon is called electrophoresis. The EDL thickness 1/k depends on the concentration of background electrolyte [18,19,25,26]. 

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