Streaming potentials phenomena

In 1861, Georg Hermann Quincke described a phenomenon that is the converse of electroosmosis When an electrolyte solution is forced through a porous diaphragm by means of an external hydrostatic pressure P (Fig. 31.1ft), a potential difference called the streaming potential arises between indicator electrodes placed on different sides of the diaphragm. Exactly in the same sense, in 1880, Friedrich Ernst Dorn described a phenomenon that is the converse of electrophoresis During... 

In words, the application of a pressure difference in an electrolyte should produce a potential difference and a corresponding electric field. This is the phenomenon of streaming potential... 

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

An inverse phenomenon, i.e., the appearance of a steady-state potential difference, AT,., due to the action of the pressure gradient, Ap, (the streaming potential) is described by the condition 4 + 4 = 0, and consequently... 

An inverse phenomenon streaming potential), generation of an electric field inside the membrane, takes place if the solution passes through this porous medium due to an imposed hydrostatic pressure. This time it is the flow of the fluid inside the pores that induces the displacement of the mobile part of the EDL at the surface of capillaries, with respect to the charges attached to the surface. These dipoles create an electric field, which, under stationary conditions, prevents the farther displacement of the mobile charges. The resulting potential difference across the membrane, A(p, is proportional to the excessive hydrostatic pressure, AP ... 

When the flow is driven by a pressure gradient, a streaming potential is established because of the ion transport. The streaming electric field is always opposite to the flow direction, and hence the net flow in the channel is diminished. This phenomenon is commonly referred as the electroviscous effect since the liquid appears to be of higher viscosity near the surfaces. In this work, the streaming electric field strength is calculated by a simplified model [2] ... 

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. 

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. 

Electro-osmosis is another electrokinetic phenomenon-in which an electric field is applied across a charged porous membrane or a slit of two charged nonporous membranes (see figure IV - 31). Due to the applied potential difference an electric current will flow and water molecules will flow with the ions (electro-osmotic flow) generating a pressure difference. As can be derived from nonequilibrium thermodynamics (sec chapter V) the following equation can be obtained indicating that both phenomena, electro-osmose and streaming potential, are similar... 

Hi) By streaming potential A third type of electrokinetic phenomenon observed is the development of a streaming or flow potential when a hquid like water is forced to flow through a Uiss capillary. A potential develops on either side of the capillary at its two ends (see figure 3.10). 

Mention was made previously of the electroosmotic flow of water to the cathode encountered in electrophoresis in various stabilizing media. This phenomenon plays a relatively small role in free electrophoresis because of the small surface area in contact with the electrolyte solution in the U tube. However, it is a very familar subject to workers who deal with electrical forces in membranes and porous materials. The well-known streaming potential produced by forcing liquid under pressure through a porous medium is closely related to the electroosmotic flow. The potential of the surface to the liquid (f potential) can be determined by measurements of the volume (V) of liquid transported per second by electroosmosis, through the use of equation (1), where i denotes the current strength, k the specific... 

If the electric field E is applied to a system of colloidal particles in a closed cuvette where no streaming of the liquid can occur, the particles will move with velocity v. This phenomenon is termed electrophoresis. The force acting on a spherical colloidal particle with radius r in the electric field E is 4jrerE02 (for simplicity, the potential in the diffuse electric layer is identified with the electrokinetic potential). The resistance of the medium is given by the Stokes equation (2.6.2) and equals 6jtr]r. At a steady state of motion these two forces are equal and, to a first approximation, the electrophoretic mobility v/E is... 

These entities would then cause a reduction in the standing or background current. This phenomenon is known as electron capture and is observed more readily at lower electrode potentials. Essentially a current of electrons is monitored so that when an electroactive solute passes through this stream there is a decrease in the standing current. 

ADSORPTION is the adhesion or retention of a thin layer of molecules of a gas or liquid mixture brought into contact with a solid surface resulting from the force held at the surface. Because the surface may exhibit different affinities for the various components of a fluid, the composition of the adsorbed layer generally differs from that of the bulk fluid. This phenomenon offers a straightforward means of purification (removal of undesirable components from a fluid mixture) as well as a potentially useful method of bulk separation (separation of a mixture into two or more streams of enhanced value). 

Since scaling is a concentration phenomenon, it goes to reason that scale would be most likely found in the last stages of an RO system where the concentration of salts is the highest. To determine the potential for a salt to form scale, the ion product of the salt in question (taken in the reject stream) is compared with the solubility product for the salt under the conditions in the reject. 

Our study also showed that the catalyst deactivates with time-on-stream even at low conversions. The activity dropped 30% from its initial value over a few hours. The present work further investigates this deactivation phenomenon in order to evaluate more thoroughly the potential application of copper oxide catalysts for OMR. Experiments were conducted to determine the cause of deactivation and the effect of the support on deactivation rate. Zirconia has been explored as an alternative support to ZnO and/or alumina. Reaction and deactivation rate data for 18-hour OMR reactions are reported for these catalysts. 

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