Electroosmosis and streaming potential

The electrokinetic processes can actually be observed only when one of the phases is highly disperse (i.e., with electrolyte in the fine capillaries of a porous solid in the cases of electroosmosis and streaming potentials), with finely divided particles in the cases of electrophoresis and sedimentation potentials (we are concerned here with degrees of dispersion where the particles retain the properties of an individual phase, not of particles molecularly dispersed, such as individual molecules or ions). These processes are of great importance in particular for colloidal systems. 

It has already been noted that there is a close similarity between electroosmosis and streaming potential. Therefore we consider this additional electrokinetic phenomenon next. 

Two conditions must be met to justify comparisons between f values determined by different electrokinetic measurements (a) the effects of relaxation and surface conductivity must be either negligible or taken into account and (b) the surface of shear must divide comparable double layers in all cases being compared. This second limitation is really no problem when electroosmosis and streaming potential are compared since, in principle, the same capillary can be used for both experiments. However, obtaining a capillary and a migrating particle wiih identical surfaces may not be as readily accomplished. One means by which particles and capillaries may be compared is to coat both with a layer of adsorbed protein. It is an experimental fact that this procedure levels off differences between substrates The surface characteristics of each are totally determined by the adsorbed protein. This technique also permits the use of microelectrophoresis for proteins since adsorbed and dissolved proteins have been shown to have nearly identical mobilities. 

Equations (21.102)-(21.111) are all related to the approximate expression (21.55) for the electrophoretic mobility /i. It can easily be shown that the following Onsager s relation is satisfied between electroosmosis and streaming potential. 

Szymezyk, A. et al., Study of electrokinetic properlies of plate ceramic membranes by electroosmosis and streaming potential. Desalination, 119, 309, 1998. 

In the cases of the immobile solid phase (electroosmosis and streaming potential see Figures 5.66a and 5.66b), we can identify ... 

Similarly to the reciprocity that exists between electroosmosis and streaming potential (See Chapter V,5), there is a reciprocity between the ultrasonic waves and the alternating electric field generated by them. That is, the application of the alternating electric field to disperse systems results in the generation of an ultrasound signal, referred to as the electrosonic amplitude (ESA). The ESA is related to the dynamic electrophoretic mobility via a relationship similar to eq. (V.6), namely... 

Both electroosmosis and streaming potential relate to the motion of electrolyte solutions and are therefore considered in the following section. However, we shall reserve the detailed discussion of streaming potential for the next chapter in connection with the treatment of sedimentation potential, which together with electrophoresis deals with the motion of dissolved or suspended charged particles. 

Kim K.J., Fane A.G., Nystrom M., Pihlajamaki A., Bowen W.R., Mukhtar H. (1996b), Evaluation of electroosmosis and streaming potential for measurement of electric charges of polymeric membranes. Journal of Membrane Science, 116, 149-159. 

Of these four, the phenomenon of greatest practical interest is electrophoresis. Over the years, several relatively easy techniques for the study and application of electrophoresis have been developed and today these are important tools in many areas of science and technology, including colloid science, polymer science, biology, and medicine. Of lesser practical importance, and less intensely studied, are electroosmosis and streaming potential. Sedimentation potential has received relatively little attention because of experimental difficulties. 

In the case of water flow across a semipermeable membrane separating two electrolyte solutions, two electroki-netic phenomena may be demonstrated, namely, electroosmosis and streaming potential. The former associated with Jv, volume flow, is driven by an electric current, /, (because of Aijr), whereas the latter is owing to pressure difference (AP). In sum, we have... 

Szymczyk, A., Fievet, P., Mullet, M., Reggiani, J. C., and Pagetti, J. (1998). Comparison of two elec-trokinetic methods—electroosmosis and streaming potential—to determine the zeta-potential of plane ceramic membranes. J. Membr. Sci 143, 189. 

There are four related electrokinetic phenomena which are generally defined as follows electrophoresis—the movement of a charged surface (i.e., suspended particle) relative to astationaiy hquid induced by an applied ectrical field, sedimentation potential— the electric field which is crested when charged particles move relative to a stationary hquid, electroosmosis—the movement of a liquid relative to a stationaiy charged surface (i.e., capiUaty wall), and streaming potential—the electric field which is created when liquid is made to flow relative to a stationary charged surface. The effects summarized by Eq. (22-26) form the basis of these electrokinetic phenomena. 

FIGURE 31.1 Schematic design of cells for studying electroosmosis (a) and streaming potentials (b), the velocity of electroosmotic transport can be measured in terms of the rate of displacement of the meniscus in the capillary tube (in the right-hand part of the cell). 

The movement of a charged particle with respect to an adjacent liquid phase is the basic principle underlying four electrokinetic phenomena electrophoresis, electroosmosis, sedimentation potential, and streaming potential. 

The efficiencies of electrokinetic energy conversion for two operations modes, namely electroosmosis 7jeo and streaming potential Tjsp, are expressed as... 

FIGURE 5.7. The four principal classes of electrophoretic phenomena most often studied include (a) electrophoresis, (b) electroosmosis, (c) streaming potential, and (d) sedimentation potential. 

It may be appreciated that electrokinetic phenomena are determined by electric properties at the plane of shear rather than at the real surface. In the following sections of this chapter, the relation between the measured property and is further analyzed. This is done for electroosmosis, electrophoresis, streaming current, and streaming potential. The sedimentation potential will not be discussed any further, because in practice this phenomenon does not play an important role. The electrokinetic charge density may then be derived from using the theory for the diffuse electrical double layer. 

It should be realized that is derived from the measured quantity by invoking a model. Then the applicability of the underlying assumptions may be questioned. It is therefore reconunended to compare the values of determined by different electrokinetic methods. The results of electroosmosis and streaming current or potential may be readily compared because they can be obtained using the same capillary... 

It has been long established that there essentially is total equivalence between the electrokinetic phenomena (see Figure 7.1) and that -potentials can theoretically be obtained through electroosmosis or streaming potential as well as through electrophoretic determinations. 

The situation in electroosmosis may be reversed when the solution is caused to flow down the tube, and an induced potential, the streaming potential, is measured. The derivation, again due to Smoluchowski [69], begins with the assumption of Poiseuille flow such that the velocity at a radius x from the center of the tube is... 

Interrelations Between the Electrokinetic Processes Equation (31.4) for electroosmosis and Eq. (31.10) for the streaming potential, as well as the analogous equations for the other two electrokinetic processes, yield the relation... 

The streaming potential (Dorn effect) relates to a movement of liquid that generates electric potential, and electroosmosis occurs when a direct electric potential causes movement of the liquid. The sedimentation potential relates to sedimentation (directed movement) of charged particles that generates electric potential, and electrophoresis occurs when a direct electric potential causes a movement of charged particles. 

Electrokinetic phenomena such as electroosmosis, streaming potential, and viscoelectric effects (Chapter 12)... 

In this section we describe electroosmosis and in the following section the streaming potential. These two electrokinetic techniques also permit the evaluation of f, but are subject to objection 1. In Section 12.8 we examine in greater detail the location of the surface of shear, which is the essence of objection 2 above. 

FIG. 12.7 Schematic illustrations of the apparatus used to measure (a) electroosmosis and (b) streaming potential. 

The conditions under which Equation (64) for streaming potential and Equation (54) for electroosmosis were derived are comparable inasmuch as each applies to the case of large kRc. Comparison of Equations (54) and (64) in the limit of large Rc shows that... 

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. 

Zeta potential was the first, experimentally available value characterizing edl. The potential of the solid particles in the electrolyte solutions may be determined on the basis of one of the four following phenomena microelectrophoresis, streaming potential, sedimentation potential and electroosmosis. The most popular of them and the best described theoretically and methodically is the electrophoresis. Other papers, concerning the electrophoretic mobility, stationary level determination and the theory of the charged particles transportation in the electric field are still published. 

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