Solid phase zeta potential

Fig. 1. Variation of the eiectric potential near a surface in the presence of an electrolyte solution, (a) Electrical double layer at the surface of a solid positively charged, in contact with an electrolyte solution, (b) The variation of the electrical potential when the measurement is made at an increasing distance from the surface, and when the liquid phase is mobile at a given flow rate. The zeta potential [) can be calculated from the streaming potential, which can be measured according to the method described by Thubikar et al. [4].

For an interpretation of the adsorption process it is important to know the so called zeta-potential ( ) that can be calculated from electrokinetic measurements. It may be defined as the potential difference at the shear plane (near to the outer Helmholtz plane) between the diffuse layer outside the slipping plane and the bulk phase, when the solid and liquid phases are moved tangentially to each other. The location of the slipping plane is not exactly known, but it can be assumed that the shear plane is only very little further... 

The presence of high valence ions of the same sign as the solid surface results in a numerical increase of the charge density which would normally be accompanied by an increase of the zeta-potential it is possible, however, for the thickness of the double layer to decrease sufficiently at the same time for the zeta-potential to decrease in magnitude. This effect has been observed in certain instances. If the ionic concentration of the liquid phase is increased sufficiently it is possible for the thickness of the double layer to diminish to such an extent that it eventually collapses and reforms with the charges reversed. Even if the collapse does not occur, the double layer will become very thin so that at high ionic concentrations the zeta-potential should be small this may account for the fact, which is evident from Fig. 127, that the zeta-potential tends to approach zero in the presence of relatively large amounts of electrolyte. 

As discussed before, EOF is proportional to the zeta potential and porosity of the soil and the dielectric constant of the pore fluid. It has been reported that the zeta potential of the soil became more positive in the presence of cationic surfactants. However, anionic surfactants decreased the zeta potential, and nonionic surfactants caused slight increase (Kaya and Yukselen, 2(X)5). The influence of surfactants on zeta potential is closely related to the sorption of surfactants in sods. The sorption of surfactants onto the solid phase is affected by various factors, including solid and surfactant types and environmental conditions such as pH, temperature, and electrolytes (Rosen, 1989). 

Zeta potential - a measure of the charge on the solids, as opposed to the PCD which measures charge in the water phase a useful troubleshooting tool when retention aid performance changes. 

Electrokinetic phenomena arise when the mobile layer of the EDL interacts with an externally applied electric field resulting in relative motion between the solid and liquid phases. There are three types of electrokinetic phenomena relevant to microfluidics electroosmotic flow, streaming potential, and electrophoresis. In aU of these cases, the zeta potential is a key parameter that defines either the fluid flow or particle motion. Since it is not possible to probe the zeta potential directly, measurements are based on indirect readings obtained from electrokinetic experiments. The following discussion focuses on modem methods of measuring the zeta potential using electroosmotic flow, electrophoresis, and streaming potential. 

When a (solid) surface moves in a liquid, or vice versa, there is always a layer of liquid adjacent to the surface that moves with the same velocity as the surface. The distance from the surface over which this stagnant liquid layer extends or, in other words, the location of the boundary between the mobile and the stationary phases, the so-called plane of shear or slip plane, is not exactly known. For smooth surfaces, the plane of shear is within a few liquid (water) molecules from the surface (see Figure 9.4), that is, well within the electrical double layer. The stagnant layer is probably somewhat thicker than the Stern layer, so that the plane of shear is located in the diffuse part of the electrical double layer. It follows that the potential at the plane of shear, that is, the electrokinetic potential or the zeta potential is somewhat lower than the Stern potential /j. Because the largest part of the potential drop in the... 

During the relative motion of dispersed particles with the electric double layer against the dispersion medium, the Stern layer and part of the diffusion layer move with the particle while the rest of the diffusion layer moves with the fluid. A potential thus arises in the interface with the liquid, which is called the electrokinetic potential or ((-potential (zeta-potential). Its size depends on the type of electrolyte and the ability to adsorb ions. The existence of electrical charge on the dispersed phase particles (the existence of electric double layer) significantly affects the stability of many dispersed systems. It is also associated with the phenomena that occur when one phase moves relative to another (in liquid-gas, liquid-liquid and liquid solid systems) or with the behaviour of dispersed systems under an external electric potential gradient. 

Figure 27. The zeta potential and surface charge density of HgQgCdQgTe as a function of pH. The solid line is the best fit theoretical curve to the acid dissociation model with pK = 12.7, pK = 2.4, pK = 7.6, and y = 0,30. The pK values correspond to the dissociation constants of the different phases of tellurous acid and y is proportional to the total acid site density. The surface oxide chemistry is HTeOg" (positive zeta potential, H2Te03 (zero zeta potential), HTeOg (first step in the zeta potential), and TeOa" (second step in the zeta potential). (With permission of American Institute of Physics.)...

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