Electro-osmosis measurement

The value of q0 was determined from electro-osmosis measurements. Equations (64) and (65) were found to satisfy only at 0.01 N solution. On the other hand there was reasonable agreement with equation (69)... 

Recent surface force measurements revealed a similar trend (20). Comparing steam-treated to flame-treated silica sheets using site-dissociation/site-binding model, a decrease in silanol surface sites and apparent decrease in average pKa was observed upon heat treatment. Furthermore, a repulsive force other than double-layer and van der Waals forces was observed 15 A from the surface. This repulsion was attributed to hydration of the surface and was found to be independent of surface treatment and electrolyte concentration. In Bums treatment, an arbitrary plane of shear was introduced to provide a best model fit (l 3). A value of 9 A from the surface for the plane of shear was determined from electro-osmosis measurements. 

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. 

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. 

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... 

In the Nemst-Planck equations used the activity coefficients were neglected a term accounting for the electro-osmosis, however, is present. Calculated and measured concentration profiles could be made to inter-correspond by adapting the term for water transport. The values indirectly determined by electro-osmotic flow were now found to agree with those measured directly. 

We can observe electro-osmosis directly with an optical microscope using liquids, which contain small, yet visible, particles as markers. Most measurements are made in capillaries. An electric field is tangentially applied and the quantity of liquid transported per unit time is measured (Fig. 5.13). Capillaries have typical diameters from 10 fim up to 1 mm. The diameter is thus much larger than the Debye length. Then the flow rate will change only close to a solid-liquid interface. Some Debye lengths away from the boundary, the flow rate is constant. Neglecting the thickness of the electric double layer, the liquid volume V transported per time is... 

Electrophoretic measurements by the microscope method are complicated by the simultaneous occurrence of electro-osmosis. The internal glass surfaces of the cell are usually charged, which causes an electro-osmotic flow of liquid near to the tube walls together with (since the cell is closed) a compensating return flow of liquid with maximum velocity at the centre of the tube. This results in a parabolic distribution of liquid speeds with depth, and the true electrophoretic velocity is only observed at locations in the tube where the electro-osmotic flow and return flow of the liquid cancel. For a cylindrical cell the stationary level is located at 0.146 of the internal diameter from... 

It may be necessary to correct the experimental data for effects such as electro-osmosis in the measuring capillary tube and electro-osmotic leak back through the plug. 

Measurement of Electro-osmosis as a Method for Electrokinetic Surface Analysis... 

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. 

In the following sections an account of the origin and measurement of electroosmosis is elicited, Furthermore, it is shown how to employ its measurement as a characterization technique. The discussion will focus on the measurement of electro-osmosis in cylindrical chambers and in a novel rectangular chamber whereby electro-osmosis can be measured at small sample plates. Examples of using the measurement of electro-osmosis as a surface characterization technique are discussed in terms of interpretation of the source of electro-osmosis according to classical electrokinetic theory. 

The basic experimental approach when using electro-osmosis to characterize surfaces is to measure fluid mobility at a surface when electro-osmosis is in-... 

Electro-osmosis at a sample plate wall can then be taken from a single particle mobility measurement. 

One way to increase the range of measurement may be in the use of light scattering (Doppler electrophoresis) to determine particle velocities. Such methods are used in contemporary commercially available analytical particle electrophoresis apparatuses. However, presently available equipment is not designed for ready exchange (replacement) of chamber surfaces for electro-osmosis studies. 

With electro-osmosis data, on the other hand, interpretation is not subject to the complexities of the electrophoretic measurement. Analysis of zeta potential is straightforward, and a wide range of pH can be employed. In this light it would be promising to characterize ceramic and mineral materials of a wide variety of compositions and forms, e.g., powders and processed plates. 

Though the model presented and used does not give a complete account of the interface and the origin of measured electro-osmotic fluid mobility, it was proven useful in interpretation of surface properties. The range of electrolyte concentration that can be used in the manual particle electrophoresis chamber developed in this work is limited, and this limits the model of the origin of electro-osmosis that can be tested, such as inclusion of a Stern layer. 

Electrophoresis The most familiar electrokinetic experiment consists of setting up an electric field, E, in a solution containing charged particles and determining their velocity. The particle velocity, V, is measured by direct microscopic observation at the stagnation point (i.e., zero velocity point for electro-osmosis at the radius 0.707i c) in a capillaiy as shown in Figure 9.19. The zeta potential is then computed... 

Electro-osmosis is the counterpart of electrophoresis in that now the materials to be studied are stationary, whereas the liquid moves at a given velocity, driven by an applied field. In streaming potential measurements an applied pressure difference is the driving force in that case a potential difference is measured. In practice, two ways are open working with capillaries or with plugs. 

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