Colloids electro-osmosis

For well-dispersed colloid systems, particle electrophoresis has been the classic method of characterization with respect to electrostatic interactions. However, outside the colloidal realm, i.e., in the rest of the known world, the measurement of other electrokinetic phenomena must be used to characterize surfaces in this respect. The term electrokinetic refers to a number of effects induced by externally applied forces at a charged interface. These effects include electrophoresis, streaming potential, and electro-osmosis. 

To characterize a surface electrokinetically involves the measurement of one of the above electrokinetic effects. With disperse colloidal systems it is practical to measure the particle electrophoretic mobility (induced particle velocity per unit applied electric field strength). However, for a nondisperse system one must measure either an induced streaming potential or an electro-osmosis fluid flow about the surface. 

Bouriat, P. et al., A convenient apparatus to determine the zeta potential of grains by electro-osmosis, J. Colloid Interf. Sci., 209, 445, 1999. 

Particle charge plays a major role on the stabilization of colloidal systems. Especially when nanoparticles are stabilized by an adsorption layer of polyelectrolytes, zeta potential measurements are very useful. The stabilization of the nanoparticles results from a combination of ionic and steric contributions. The zeta potential can be detected by means of electro-osmosis, electrophoresis, streaming potential, and sedimentation potential measmements. The potential drop across the mobile part of electric donble layer can be determined experimentally, whenever one phase is made... 

Cataphoresis. The movement of colloidal particles in an electric field this forms the basis of the purification of clays by the so-called electro-osmosis (q.v.) method. 

The above relationship shows that zeta potenticd is a little lower than the Stem potential and this is because it Is located further out from the surface of the macromolecule. The zeta potential of any colloidal solution can be calculated by electrokinetic measurements like electrophoresis, electro-osmosis and streaming potential. Though the methods are different, all lead to the same calculated value of the zeta potential for any particular system. All the methods involve the relative motion of the two surfaces in contact. 

In our previous discussion we studied in brief the movement of the colloidal particles under the influence of an electric field. If the particles are now forcibly stopped from moving, we will see that the dispersion medium now starts moving. This movement of fluid in an electric field is called electro-endosmosis or electro-osmosis. 

ACEO is a phenomenon of induced-charge electro-osmosis (ICEO), where flow is generated by the action of an electric field on its own induced diffuse charge near a polarizable surface. The main difference with other examples of ICEO, such as flows around metal colloids, is that ACEO involves electrode surfaces, which supply both the electric field and the induced screening charge, in different regions at different times. For this reason, ACEO is inherently time-dependent (as the name inplies) and tied to the dynamics of diffuse charge, as ions move to screen the electrodes. 

Levitan JA, Devasenathipathy S, Studer V, Ben Y, Thorsen T, Squires TM, Bazant MZ (2005) Experimental observation of induced-charge electro-osmosis around a metal wire in a microchannel. Colloid Surf A Physicochem Eng Asp 267 122-132... 

The effect of electro-osmosis differs for different soils its influence is felt at a particular soil humidity for western pre-Caucasian black earth at a humidity of over 25%, for soddy podzol above 30%. The most favorable case is that of black earth containing clay and colloid particles the transfer of moisture to the cathode depends on the motion of these. 

Under the influence of a fall of potential almost all colloids migrate either to the cathode or to the anode. The phenomenon of electrical migration of colloids is closely associated with that of electro-osmosis. This latter phenomenon may be defined as the passage of liquids through membranes under the influence of a fall of electrical potential. When the particles move through the solution the phenomenon is called cata-phoresis. When the particles remain stationary and the liquid moves, the term endosmosis is applied. The earliest observations on these phenomena were made by Picton and Linder, Coehn,f Lottermoser, and Wiedemann. ... 

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... 

Charged polymers and colloids exhibit a wide variety of elec-trokinetic behavior with terms such as electrophoresis, electro-osmosis, streaming potential, sedimentation potential, and others. These have been thoroughly reviewed [50]. 

In the article on electro-osmosis (q.v.) a similar formula, but with 4 in place of the factor 6, is derived. Since electrophoresis is the reverse of electro-osmosis, the same expression should apply in both cases to the potential at the surface of shear between the two phases. The explanation of the apparent discrepancy is that instead of applying Stokes s law to a small sphere, the derivation of the electro-osmotic effect is based on the model of a parallel plate capacitor, i.e. on a large solid surface whose radius of curvature is negligible (compared with the thickness of the diffuse double layer). Closer analysis of the problem by Henry and Booth has shown that 4 is the correct factor for large particles, independent of their size and shape, but that for most systems, e.g. stable colloidal solutions, the factor varies between 4 and 6, depending on the size of the particle and the thickness of its atmosphere. 

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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