Interpretation of the Zeta Potential

For all electrokinetic phenomena discussed in this chapter (except the one described in Section 10.3.3), the zeta potential is the electric potential at the plane of shear. Interpretation of should therefore start with addressing the question regarding the location of the plane of shear. As mentioned in Section 10.1, for smooth, rigid surfaces the plane of shear is situated only a little further out from the surface than the Stem layer so that tir, which is difficult to establish experimentally, may be approximated by However, for irregularly shaped and hairy surfaces, the plane of shear is usually farther away from the surface and is correspondingly lower. 

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 

FIGURE 10.10 Variation of the electrokinetic potential with the concentration of chargedetermining ions for different concentrations of indifferent electrolyte (a) absence and (b) presence of specific adsorption of counterions. The arrows indicate decreasing concentrations of indifferent electrolyte. 

In Section 9.1, we defined the point of zero charge as -log (i -. the pH, if H+ and OH are the charge-determining ions) at which the surface charge density, a , is zero. In the absence of specific ion adsorption and at smooth surfaces o (= = 0 and, 

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