Electro-osmosis

Electro-osmosis is another electrokinetic phenomenon-in which an electric field is applied across a charged porous membrane or a slit of two charged nonporous membranes (see figure IV - 31). Due to the applied potential difference an electric current will flow and water molecules will flow with the ions (electro-osmotic flow) generating a pressure difference. As can be derived from nonequilibrium thermodynamics (sec chapter V) the following equation can be obtained indicating that both phenomena, electro-osmose and streaming potential, are similar 

The phenomenon of electro-osmosis has already been mentioned in connection with electrochemical realkalization (Section 7.8). It is well known that when a porous medium like concrete contains a solution, then an electric current applied between an anode and a cathode will move the water from the anode to the cathode. This leads to drying of anodes for pipelines in soils. The basis of the phenomenon is that when a compound dissolves, water molecules attach themselves to it. This happens more for positively charged metal ions than for negatively charged ions. Therefore more water is carried by the positive ions towards the negative cathode. 

It is claimed to be the reason for the transport of the sodium or potassium carbonate from the anode to the cathode in realkalization treatments as discussed in the previous section. 

There is therefore proprietary technology for applying this methodology to concrete. The proprietary part is the use of pulses to reduce the build up of charged ions at the electrodes which would increase the electrical resistance. The system works by the installation of cathodes in areas where the water can be discharged and anodes in areas where water must be removed. 

The main problem that the author is aware of with such systems is that the anode is heavily stressed if there is water ingress or run down. Very high current densities can occur on the anodes. This can lead to them failing. It is therefore likely that anodes are in need of further development before the technology can be considered fully developed for external applications where random wetting events cannot be avoided. 

In addition, the pulsing process requires sophisticated electronics which must prove durable over many years. They are likely to be less reliable than transformer rectifier systems providing straight DC for impressed current cathodic protection. 

We shall only consider electro-osmosis, streaming potential and electrophoresis in any detail. 

A diaphragm through which liquid is forced may be regarded as comprising a series of capillaries around the internal surface of which there exists a double layer of separated charges (Fig. 7.8). 

Cylindrically symmetrical double layer around the surface of a capillary. 

Let it be assumed that, during the movement of liquid through such a capillary, the fall of velocity is confined to the double layer by frictional forces. The velocity gradient in the layer is then v 8, while the potential gradient down the length of the tube is / = F (V cm ). 

If the surface charge per unit area is o, then the electrical force per unit area = Va. The viscous force per unit area = r)(v/8), rj being the coefficient of viscosity of the liquid. If liquid flows through the capillaries at a constant rate the electrical force balances the viscous force, i.e.  

When a direct current passes through an ion exchange membrane immersed in electrolyte solution, counter-ions are transported through the membrane, accompanied by water molecules, which is the electro-osmotic water. Because the liquid in a membrane pore has the same charge as the counter-ion, the liquid moves in 

The volume flow through the membrane per unit time per unit area (rate of electro-osmosis) is 

The transport number of the solvent (water) t0 is defined as the number of 

The parabola method makes it possible to measure the potential of cell walls. Usually, the cell is made of quartz, and the parabola method thus offers the possibility of determining the lEP of one material that has already been extensively studied. The potentials of macroscopic specimens of other materials can also be determined from the mobility profile [273-275] by replacement of the original cell wall of a commercial electrophoretic cell by a flat specimen of the material of interest. For example, in [276], the lEP of a basal plane of mica found from the mobility profile was different from the lEP of a mica dispersion. Only a few types of electrophoretic devices (most of which are no longer available on the market) can be used to determine potentials by means of electro-osmosis. 

Electro-osmosis has been chiefly discussed as a phenomenon accompanying electrophoresis (Section 2.1.1), but it also occurs separately and can be used to determine the potential and the lEP. 

Electro-osmosis is recommended for macroscopic specimens, for fibers, and for large particles, which do not form stable dispersions. Opaque solutions do not pose a problem. A difference in the refractive index between solution and particles is not required. A relatively large amount of material is necessary to carry out a measurement. Electro-osmosis is not recommended for measurements at high ionic strengths ( 0.1 M). Electro-osmosis in a mixture of different materials produces an averaged result, without fhe possibility of separating the components. 

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Related phenomena are electro-osmosis, where a liquid flows past a surface under the influence of an electric field and the reverse effect, the streaming potential due to the flow of a liquid past a charged surface. 

Electro osmosis often accompanies electrophoresis. It is the transport of Hquid past a surface or through a porous soHd, which is electricaHy charged but immovable, toward the electrode with the same charge as that of the surface. Electrophoresis reverts to electroosmotic flow when the charged particles are made immovable if the electroosmotic flow is forcibly prevented, pressure builds up and is caHed electroosmotic pressure. 

Electrophoresis and electro osmosis can be used to enhance conventional cake filtration. Electrodes of suitable polarity are placed on either side of the filter medium so that the incoming particles move toward the upstream electrode, away from the medium. As most particles carry negative charge, the electrode upstream of the medium is usuaHy positive. The electric field can cause the suspended particles to form a more open cake or, in the extreme, to prevent cake formation altogether by keeping aH particles away from the medium. 

The 2eta potential (Fig. 8) is essentially the potential that can be measured at the surface of shear that forms if the sohd was to be moved relative to the surrounding ionic medium. Techniques for the measurement of the 2eta potentials of particles of various si2es are collectively known as electrokinetic potential measurement methods and include microelectrophoresis, streaming potential, sedimentation potential, and electro osmosis (19). A numerical value for 2eta potential from microelectrophoresis can be obtained to a first approximation from equation 2, where Tf = viscosity of the liquid, e = dielectric constant of the medium within the electrical double layer, = electrophoretic velocity, and E = electric field. 

An innovative technology called the "lasagna" process is based on the electrokinetic phenomenon called electro osmosis. The lasagna process was created to treat difficult wastes in low permeabiUty, sdt- and clay-laden soils (40). The lasagna process is so named because it consists of a number of layered subsurface electrodes and treatment zones. These layers can be constmcted either horizontally where contaminants are forced to more upward or in vertical position where lateral contaminant movement is desired. 

Electrically assisted transdermal dmg deflvery, ie, electrotransport or iontophoresis, involves the three key transport processes of passive diffusion, electromigration, and electro osmosis. In passive diffusion, which plays a relatively small role in the transport of ionic compounds, the permeation rate of a compound is deterrnined by its diffusion coefficient and the concentration gradient. Electromigration is the transport of electrically charged ions in an electrical field, that is, the movement of anions and cations toward the anode and cathode, respectively. Electro osmosis is the volume flow of solvent through an electrically charged membrane or tissue in the presence of an appHed electrical field. As the solvent moves, it carries dissolved solutes. 

Once a matrix of particles is formed, whether filter cake, thickened underflow, or soil, applying a current to the fluid causes a movement of ions in the water and, with the ions, water of hydration. The phenomenon is called electro osmosis. The pressure generated on the fluid is given by (127) ... 

Electroosmotic flow in a capillary also makes it possible to analyze both cations and anions in the same sample. The only requirement is that the electroosmotic flow downstream is of a greater magnitude than electrophoresis of the oppositely charged ions upstream. Electro osmosis is the preferred method of generating flow in the capillary, because the variation in the flow profile occurs within a fraction of Kr from the wall (49). When electro osmosis is used for sample injection, differing amounts of analyte can be found between the sample in the capillary and the uninjected sample, because of different electrophoretic mobilities of analytes (50). Two other methods of generating flow are with gravity or with a pump. 

A. Electro-osmosis in polar (nonionic) and ionic solutions 786... 

A. Electro-osmosis In Polar (Nonlonic) and Ionic Solutions... 

Electro-osmosis has been defined in the literature in many indirect ways, but the simplest definition comes from the Oxford English Dictionary, which defines it as the effect of an external electric held on a system undergoing osmosis or reverse osmosis. Electro-osmosis is not a well-understood phenomenon, and this especially apphes to polar non-ionic solutions. Recent hterature and many standard text and reference books present a rather confused picture, and some imply directly or indirectly that it cannot take place in uniform electric fields [31-35]. This assumption is perhaps based on the fact that the interaction of an external electric held on a polar molecule can produce only a net torque, but no net force. This therefore appears to be an ideal problem for molecular simulation to address, and we will describe here how molecular simulation has helped to understand this phenomenon [26]. Electro-osmosis has many important applications in both the hfe and physical sciences, including processes as diverse as water desahnation, soil purification, and drug delivery. 

The simulations to investigate electro-osmosis were carried out using the molecular dynamics method of Murad and Powles [22] described earher. For nonionic polar fluids the solvent molecule was modeled as a rigid homo-nuclear diatomic with charges q and —q on the two active LJ sites. The solute molecules were modeled as spherical LJ particles [26], as were the molecules that constituted the single molecular layer membrane. The effect of uniform external fields with directions either perpendicular to the membrane or along the diagonal direction (i.e. Ex = Ey = E ) was monitored. The simulation system is shown in Fig. 2. The density profiles, mean squared displacement, and movement of the solvent molecules across the membrane were examined, with and without an external held, to establish whether electro-osmosis can take place in polar systems. The results clearly estab-hshed that electro-osmosis can indeed take place in such solutions. 

The molecular simulations also showed that electro-osmosis is also observed in aqueous electrolyte solutions, as long as the external electric field is reversed periodically to prevent the ions from accumulating near the membrane. An example of this is shown in Fig. 10, which shows the effect of an electric field on a 4.67 mole percent aqueous LiCl solution at 25°C. It is quite clear that the mobility of the solvent molecules increases as a result of... 

F. Paritosh, S. Murad. Molecular simulation of osmosis and reverse osmosis in aqueous electrolyte solutions. AIChE J 42 2984, 1996 S. Murad, K. Oder, J. Lin. Molecular simulation of osmosis, reverse osmosis, and electro-osmosis in aqueous and methanolic electrolyte solutions. Mol Phys 95 401, 1998 J. G. Powles, S. Murad. The molecular simulation of semi-permeable membranes—osmosis, reverse osmosis and electro-osmosis. J Mol Liq 75 225, 1998. 

S. Murad, R. Madhusudan, J. G. Powles. A molecular simulation to investigate the possibility of electro-osmosis in non-ionic solutions with uniform electric fields. Mol Phys 90 671, 1997 R. Madhususan, J. Lin, S. Murad. Molecular simulations of electro-osmosis in fluid mixtures using semi-permeable membranes. Eluid Phase Equil 150 91, 1998. 

Electro-osmosis passage of a liquid through a porous medium (such as a soil) under the influence of a potential difference. 

Usually, this phenomenon limits the lifetime of a battery because the storage capacity falls below a reasonable lower limit. One reason for this zinc migration was identified by McBreen [35] an inhomogeneous current distribution makes the zinc move away from high current density areas. Another mechanism seems to be active as well an electrolyte convection induced by electro-osmosis through the separator [36],... 

Electro-osmosis generated flows are interesting for micro-electronics, biomedical diagnostic techniques, and a number of other applications. Important results related to heat transfer in such flows were obtained recently by Maynes and Webb... 

Kim, M. J., Beskok, A., and Kihm, K. D., "Electro-Osmosis-Driven Micro Channel Flows A Comparative Study of Microscopic Particle Image Velocimetry Measurements and Numerical Simulations, Exp. Fluids, Vol. 33, No. 1, 2002, pp. 170-180. 

Pretorius, V., Hopkins, B.J., and Schieke, J.D., Electro-osmosis, a new concept for high-speed liquid chromatography, /. Chromatogr. 99 23-30 (1974). 

Factors Affecting Ionic Migration. Effect of Temperature. pH and Ionic Strength. Electro-osmosis. Supporting Medium. Detection of Separated Components. Applications of Traditional Zone Electrophoresis. High-performance Capillary Electrophoresis. Capillary Electrochromatography. Applications of Capillary El ectrochromatography. ... 

During the migration of cations and anions towards their respective electrodes, each ion tends to carry solvated water along with it. As cations are usually more solvated than anions, a net flow of water towards the cathode occurs during the separation process. This effect, known as electro-osmosis, results in a movement of neutral species which would normally be expected to remain at the point of application of the sample. If required, a correction can be applied to the distances migrated by ionic species by measuring them... 

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