Electrophoresis, electrokinetic effects

If a liquid moves tangential to a charged surface, then so-called electrokinetic phenomena arise [101]. Electrokinetic phenomena can be divided into four categories Electrophoresis, electro-osmosis, streaming potential, and sedimentation potential [102], In all these phenomena the zeta potential plays a crucial role. The classic theory of electrokinetic effects was proposed by Smoluchowski2 [103],... 

Figure 13. Basic electrokinetic effects. (According to Atkins et al. [242].) (a) Electroosmotic flow (EOF), (b) electrophoresis (EP), (c) dielectrophoresis (DEP).

Electroosmosis is one of several electrokinetic effects that deal with phenomena associated with the relative motion of a charged solid and a solution. A related effect is the streaming potential that arises between two electrodes placed as in Figure 9.8.1 when a solution streams down the tube (essentially the inverse of the electroosmotic effect). Another is electrophoresis, where charged particles in a solution move in an electric field. These effects have been studied for a long time (37, 38). Electrophoresis is widely used for separations of proteins and DNA (gel electrophoresis) and many other substances (capillary electrophoresis). 

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 electrokinetics are a class of several different interfacial effects that become important in micron and submicron dimensions. The most important and widespread categories of the electrokinetic effects are the electroosmosis and the electrophoresis. When the ionized liquids are in contact with stationary charged surfaces, counterions accumulate near the surface and buUd a layer that is called the electric double layer (EDL). The presence of an external electric field moves this layer and consequently generates the bulk flow field in the channels. This effect is named as electroosmosis and the generated flow is electroosmotic or electrokinetic flow. The external electric field also moves charged species and macromolecules in the micro- and nanochaimels which is usually referred to electrophoresis or electrophoretic effect. 

In the presence of an external electric field, the nanoparticle mainly moves due to electrophoresis and electroosmosis (the electrokinetic effects). Here, it should mention that the Brownian motion is one of the main signatures of the nanoparticle motion. However, it was shown previously that the effect of the Brownian force on the nanoparticle is negligible compared with the electrokinetic effects (the electrophoretic and the electroosmotic forces) [5]. In the presence of the external electric field, the nanoparticles are mainly manipulated by the electrokinetic effects, and the Brownian force has negligible effect on the nanoparticle motion. 

The classic experimental approach when measuring electrokinetic phenomena is to measure either the velocity of the respective phases, in the case of electrophoresis and electroosmosis, or the strength of the induced electric field under relative phase motion, in the case of streaming potential and sedimentation potential. A number of experimental set-ups can be used to measure electrokinetic phenomena. However, the appropriate experimental set-up for a given system is usually determined by the size, shape and state of the substrate of interest. The following provides a discussion of some common experimental regimes for measurement of electrokinetic effects. 

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 Dorn effect is one of the electrokinetic effects (q.v.), and is the converse of electrophoresis (q.v.). Charged particles falling through a stationary liquid set up a potential difference in the liquid column, opposing the motion of the particles. 

Electro-osmosis is one of the electrokinetic effects (q.v.). If a potential difference is applied between the ends of a capillary tube containing electrolyte, or across a plug of finely divided material (which can be regarded as a bundle of capillaries), a movement of the liquid is observed. This is the reverse effect of electrophoresis (q.v.), where particles move through a liquid which is stationary. The effect can be studied in an apparatus such as that sketched in figure E.IO. A plug of the finely divided material is in the centre of the tube, which is completely filled with liquid, and a potential of, say, 200 V is applied between the two calomel electrodes. An air-bubble trapped in the capillary measures the rate of movement of the liquid. 

This is not immediately applicable to electrophoresis, however. The solid particle with its fixed double layer (net charge Q) is moving relative to a solution in which the diffuse double layer is distributed (see electrical double layer). The latter is equivalent to a charge -Q spread out on a concentric sphere of radius where this is the thickness of the ionic atmosphere. The presence of this atmosphere reduces the mobility, and the potential at the surface of the particle, by the factor 1/(1+ Kr) so that in place of equation (E.16) the zeta-potential (see electrokinetic effects) is given by... 

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. 

Klampfl, C.W., Solvent effects in microemulsion electrokinetic chromatography. Electrophoresis, 24, 1537, 2003. 

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. 

Micellar electrokinetic capillary chromatography (MECC), in contrast to capillary electrophoresis (CE) and capillary zone electrophoresis (CZE), is useful for the separation of neutral and partially charged species [266,267]. In MECC, a surfactant, usually sodium dodecyl sulfate (SDS), is added to the buffer solution above its critical micellar concentration to form micelles. Although SDS is certainly the most popular anionic surfactant in MECC, other surfactants such as bile salts have proved to be very effective in separating nonpolar analytes that could not be resolved using SDS [268]. 

The Effect of SDS Micelle on the Rate of a Reaction, J. Chem. Ed. 1992, 69, 1024 C. P. Palmer, Demonstrating Chemical and Analytical Concepts Using Electrophoresis and Micellar Electrokinetic Chromatography, J. Chem. Ed 1999, 76, 1542. 

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