Osmosis electroosmosis

In the same ED cell pair, the overall water transport through the electromembranes from the dilute stream to the concentrate one can be expressed by accounting for electroosmosis (i.e., the migration of water molecules associated with ions, this being proportional to j) and osmosis phenomena ... 

The experiments demonstrate the development of a streaming potential in consolidated bentonite clay when flushed by a NaCl-solution of either low or high concentration. The streaming potential measured in our experiments is at least 5 to 10 times larger than values reported for bentonite in the literature. Apparently this is caused by a very low electric conductivity of the bentonite samples studied. This low conductivity might be ascribed to overlapping diffuse double layers on the clay particles, caused by the high compaction and the presence of monovalent ions in the equilibrium solution. The bentonite, thus compacted, will be a very effective medium for active application of electroosmosis. Compared with electrically shorted conditions, chemical osmosis will be reduced when the clay is not short-circuited. 

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. 

Safe and effective delivery of peptides has also been successfully demonstrated in human studies using iontophoresis, a technique that uses mild electric current to facilitate transport of molecules across the skin. ° Iontophoresis works primarily by a combination of two forces, electro-repulsion of charged drug molecule away from the electrode and into the skin, and electroosmosis, a convective solvent flow in the direction of the counter-ion transport. In general, cationic proteins and peptides are delivered more efficiently than anionic molecules because electro-osmosis works in the same direction as electro-migration for cationic species. 

Electro-osmosis in open systems is generally considered not to contribute to peak broadening. In hydrody-namically closed systems with nonsuppressed electroosmosis, or in cases of axially different electro-osmotic regimes, however, a considerable contribution may result. The corresponding plate-height term is... 

Electroosmotic effects also influence current efficiency, not only in terms of coupling effects on the fluxes of various species but also in terms of their impact on steady-state membrane water levels and polymer structure. The effects of electroosmosis on membrane permselectivity have recently been treated through the classical Nernst-Planck flux equations, and water transport numbers in chlor-alkali cell environments have been reported by several workers.Even with classical approaches, the relationship between electroosmosis and permselectivity is seen to be quite complicated. Treatments which include molecular transport of water can also affect membrane permselectivity, as seen in Fig. 17. The different results for the two types of experiments here can be attributed largely to the effects of osmosis. A slight improvement in current efficiency results when osmosis occurs from anolyte to catholyte. Another frequently observed consequence of water transport is higher membrane conductance, " " which is an important factor in the overall energy efficiency of an operating cell. 

In electrokinetic processes, there are two major transport mechanisms electromigration and electro-osmosis. Generally, in an electrical field, electromigration causes cationic metals such as cadmium, zinc, lead, nickel, and copper to move from the anode toward the cathode in electro-osmosis, the direction of movement of the pore water is toward the cathode when the zeta potential of the soil surface is negative. This can result in an enhanced removal of metals because the direction of transport of the ions in both mechanisms is the same. However, the direction of electromigration for anionic pollutants is toward the anode and that for electroosmosis is from anode to cathode, as stated previously. The opposite direction of movement means that the removal rate of anionic pollutants could be reduced. 

The solution is alkalized in the cathode compartment and it is acidified in the anode compartment As a result, the entire flow incoming to the electrodialysis apparatus can be separated into desalinated and concentrated streams. Coions absorbed by the membrane reduce the efficiency of this process. The highest possible degree of concentration can be achieved if no incoming solution is supplied to the concentrated compartments. In this case, water would be delivered to the concentrate compartments by osmosis and electroosmosis and in the hydration shells of the transported ions. 

Here is the electric potential drop across the nonequilibrium EDL - the generalized -potential. For very large (for a cation-selective interface, is negative), this expression is to replace the classical equilibrium diffusion-electroosmotic slip formula (for an ideal membrane, equilibrium diffusion-osmosis is equivalent to electroosmosis) ... 

The possibility of nonlinear electro-osmotic flow driven by non-equilibrium space charge at large currents was predicted by S.S. Dukhin in the 1980s [6, 13]. He coined the term electroosmosis of the second kind, to distinguish it fi om electrokinetic phenomena of the first kind (whether linear or nonlinear), which involve quasi-equiUbrium double layers and non-zero bulk concentrations. In spite of appealing to very different physical mechanisms, the maximum velocity of second-kind electro-osmosis is argued to be the same as that described above for ICEO flow (Eq. (4)) ... 

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