Electrophoresis electroosmosis

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 composition of the surface-bound species must be considered they contribute to the stability of the dispersions of metal nanoparticles. In the case of electrostatically stabilized dispersions, the techniques to measure the interfacial electronic phenomena, including electrophoresis, electroosmosis, etc., are useful (54). In order to understand the composition (as well as structures) of the chemical species bound in the surface of metal particles, spectroscopic measurements used for common organic substances are used as well as the elemental analysis. 

Electrokinetic Phenomena. Electrokinetic motion occurs when the mobile part of the EDL is sheared away from the inner layer (charged surface). There are four types of electrokinetic measurements, electrophoresis, electroosmosis, streaming potential, and sedimentation potential, of which the first finds the most use in industrial practice. Good descriptions of practical experimental techniques in electrophoresis and their limitations can be found in references 18-20. 

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. 

This book will make little reference to phenomena relating to this double layer. Yet a number of applications in electrochemistry, such as electrophoresis, electroosmosis, supercapacitors, etc. are rooted in these same phenomena. 

A buffer containing 1 mM Mg804 and 1 mM CaCl2 strongly reduces electroosmotic flow in capillary electrophoresis. Electroosmosis is restored by adding 3 mM EDTA to the buffer. 8uggest an explanation. 

Electrokinetic phenomena refers to dynamic processes that occur when viscous or electrical forces are applied to a charged interface. The most common of these phenomena are electrophoresis, electroosmosis, streaming potential and sedimentation potential. 

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. 

The effect known either as electroosmosis or electroendosmosis is a complement to that of electrophoresis. In the latter case, when a field F is applied, the surface or particle is mobile and moves relative to the solvent, which is fixed (in laboratory coordinates). If, however, the surface is fixed, it is the mobile diffuse layer that moves under an applied field, carrying solution with it. If one has a tube of radius r whose walls possess a certain potential and charge density, then Eqs. V-35 and V-36 again apply, with v now being the velocity of the diffuse layer. For water at 25°C, a field of about 1500 V/cm is needed to produce a velocity of 1 cm/sec if f is 100 mV (see Problem V-14). 

The electroosmotic flow profile is very different from that for a phase moving under forced pressure. Figure 12.40 compares the flow profile for electroosmosis with that for hydrodynamic pressure. The uniform, flat profile for electroosmosis helps to minimize band broadening in capillary electrophoresis, thus improving separation efficiency. 

The physical separation of charge represented allows externally apphed electric field forces to act on the solution in the diffuse layer. There are two phenomena associated with the electric double layer that are relevant electrophoresis when a particle is moved by an electric field relative to the bulk and electroosmosis, sometimes called electroendosmosis, when bulk fluid migrates with respect to an immobilized charged surface. 

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. 

Process Concept The application of a direct elecdric field of appropriate polarity when filtering should cause a net charged-particle migration relative to the filter medium (electrophoresis). The same direct electric field can also be used to cause a net fluid flow relative to the pores in a fixed filter cake or filter medium (electroosmosis). The exploitation of one or both of these phenomena form the basis of conventional electrofiltration. 

The mechanism by which analytes are transported in a non-discriminate manner (i.e. via bulk flow) in an electrophoresis capillary is termed electroosmosis. Eigure 9.1 depicts the inside of a fused silica capillary and illustrates the source that supports electroosmotic flow. Adjacent to the negatively charged capillary wall are specifically adsorbed counterions, which make up the fairly immobile Stern layer. The excess ions just outside the Stern layer form the diffuse layer, which is mobile under the influence of an electric field. The substantial frictional forces between molecules in solution allow for the movement of the diffuse layer to pull the bulk... 

Four different electrokinetic processes are known. Two of them, electroosmosis and electrophoresis, were described in 1809 by Ferdinand Friedrich Renss, a professor at the University of Moscow. The schematic of a cell appropriate for realizing and studying electroosmosis is shown in Fig. 31.1a. An electrolyte solution in a U-shaped cell is divided in two parts by a porous diaphragm. Auxiliary electrodes are placed in each of the half-cells to set up an electric held in the solution. Under the inhuence of this held, the solution starts to how through the diaphragm in the direction of one of the electrodes. The how continues until a hydrostahc pressure differential (height of liquid column) has been built up between the two cell parts which is such as to compensate the electroosmotic force. 

In 1861, Georg Hermann Quincke described a phenomenon that is the converse of electroosmosis When an electrolyte solution is forced through a porous diaphragm by means of an external hydrostatic pressure P (Fig. 31.1ft), a potential difference called the streaming potential arises between indicator electrodes placed on different sides of the diaphragm. Exactly in the same sense, in 1880, Friedrich Ernst Dorn described a phenomenon that is the converse of electrophoresis During... 

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. 

Auxiliary electrodes are placed into the solution to set up the electric field that is needed to produce electrophoresis or electroosmosis. Under these conditions an electric current passes through the solution and the external circuit its value depends on the applied voltage and on solution conductivity. The lower this conductivity, the higher will be the electric field strength E (or ohmic voltage drop) in the solution that can be realized at a given value of current. 

Electrophoresis The physical situation of relative motions of a solution and another (insulating) phase during electrophoresis is exactly the same as in electroosmosis. Hence, the linear velocity of a cylindrical particle (which is the equivalent of a cylindrical pore) is given by the value following from Eq. (31.4). With particles of dilferent shape, this velocity can be written as... 

Electroosmosis is used to remove liquid (moisture) from different porous solids (e.g., in drying soil for building purposes, which improves the bond between the foundations and the soil). A combination of electrophoresis and electroosmosis is sometimes used to dry peat or clay. In this way, the water content of peat can be reduced from 90% to 55-60%. Unfortunately, the energy required for a further reduction of the water content is very high. 

Ishii, C. Y. and Boxer, S. G. (2006) Controlling two-dimensional tethered vesicle motion using an electric field interplay of electrophoresis and electroosmosis. Langmuir, 22, 2384—2391. 

Theory Cross-flow-electrofiltration can theoretically be treated as if it were cross-flow filtration with superimposed electrical effects. These electrical effects include electroosmosis in the filter medium and cake and electrophoresis of the particles in the slurry. The addition of the applied electric field can, nowever, result in some qualitative differences in permeate-flux-parameter dependences. 

Jorgenson, J. W. and Lukacs, K. D, High-resolution separations based on electrophoresis and electroosmosis, /. Chromatogr. 218, 209, (1981)... 

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. 

With regard to the movement of liquid versus particles under direct current, electrophoresis is the reverse of the effect of electroosmosis.33 If particles move through a liquid that is stationary, this is called electrophoresis conversely, if the liquid moves through particles that are stationary, that is called electroosmosis. 

The mechanisms of the necrosis of the cancer tissue by electrochemical treatment (ECT) are complex and not fully understood although the nature of several factors involved has been indicated. Nordenstrom pointed out the importance of electroosmosis, electrophoresis, electrode reactions, pH changes and the general drastic change in the microenvironment of the cancer tissue10,18 during ECT this and related work has been reviewed by Nilsson and coworkers.19... 

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