Electro-osmosis conductivity

Combination of equations (7.30) and (7.32) gives a general relationship between electro-osmosis and streaming potential, Saxen s equation, which, by a more general derivation, can be shown to apply independent of plug structure and surface conductance ... 

Clays are generally considered to be effective barriers for flow of water and solutes due to their low permeability and high ion adsorption capacity. However, as environmental criteria for the emission of contaminants and water from clay barriers become increasingly stringent, it is crucial to be aware of all relevant driving forces and fluxes and to take them into account in model assessments. In this respect the processes of chemical and electro-osmosis may not be neglected in clayey materials of hydraulic conductivity < 10-9 m/s [7], At these low conductivities the surface charge of the clay particles and the counter-ion accumulation in diffuse double layers enable explanation and quantification of osmotic processes and semi-permeability in clays [1],... 

Iv) Ion transport, which in the bulk proceeds by conduction, is considerably modified by the presence of the particle. When, as will be assumed here, the particle is kept stationary, the electric force exerted on the countercharge sets this charge into motion. By viscous traction liquid is entrained. This phenomenon is called electro-osmosis-, it will be treated in sec. 4.3b. 

With plugs and capillaries a number of electrokinetlc (streaming potentials, electro-osmosis, streaming currents) and related phenomena (conductivity, permeability) can be measured, all of these requiring a different mode of operation. 

Failure of electroklnetlc consistency if K° is ignored has been observed by several researchers. In table 4.2 this is illustrated for negative polystyrene particles. The, mostly minor, spread in f from conductivity is again caused by uncertainties in the models applied to account for particle Interaction. The difference between electrophoresis or electro-osmosis on the one hand and conductivity on the other is obvious moreover f(ef) does not systematically decrease with c. Zukoski and Savllle ) found a similar difference for other latlces. In a series of papers from Savllle s group it was concluded that low... 

Table 4.2. Electrokinetic potentials of negative polystyrene latex particles obtained from electro-osmosis, electrophoresis and conductivity. In the conversion of mobilities surface conduction behind the slip plane was ignored. (Data from A.G. van der Put, PhD. thesis. Agricultural University Wagenlngen. NL (1980) as elaborated by O Brlen, J. CoOoid Interface Set 110 (1986) 477.)...

Finally, electrodialysis may be mentioned, a process widely used to de-salt aqueous solutions. An electric field is applied across a stack of alternating cation-exchange and anion-exchange membranes. Ions in the electrolyte solutions between these membranes are transported till they meet a membrane of the same sign, so that electrolyte-rich and electrol)rte-freed solutions are created. The process involves conduction and electro-osmosis. Obviously, irreversible thermod mamics appears very suitable to describe the various flows... 

Figure 16. Experimental system for measuring cadmium or zinc movement throngh the barrier, conducted in parallel, with and without electric field (Ruggeri, 2005). Arrows show the direction of the hydraulic flux, and electroosmotic and ion migratory flow. The dark area is the cadmium-contaminated soil. EO, electro-osmosis EM, electro-migration (ion migration).

Electroosmotic blistering. Electro-osmosis involves water transport though a membrane or a capillary system under the influence of a potential gradient. It is relatively unimportant because with increasing salt concentration the conductivity increases, decreasing the water transport by this mechanism. 

The most challenging task, to suggest chemical modifications of membranes, which minimize electro-osmosis and/or methanol diffusion, maximize proton conductance and membrane stability at a reduced price is only indirectly affected by theory and simulation. Most conclusions, once made, are rather obvious (such as higher channel connectivity, homogeneity of swelling, higher sidechain density, etc). Others are less trivial, such as the... 

Kinetic theory, non-equilibrium statistical mechanics and non-equilibrium molecular dynamics (NEMD) have proved to be useful in estimating both straight and cross-coefficients such as thermal conductivity, viscosity and electrical conductivity. In a typical case, cross-coefficient in case of electro-osmosis has also been estimated by NEMD. Experimental data on thermo-electric power has been analysed in terms of free electron gas theory and non-equilibrium thermodynamic theory [9]. It is found that phenomenological coefficients are temperature dependent. Free electron gas theory has been used for estimating the coefficients in homogeneous conductors and thermo-couples. 

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