Electrokinetic charge density

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. 

FIGURE 15.18 Variation in the electrokinetic charge density resulting from plateau-adsorption of lysozyme (o) and a-lactalhumin ( ) on negatively charged polystyrene particles. The arrows indicate the isoelectric points of the two proteins. (Adapted from Norde, W. et al., Polym. Adv. Technol., 6, 518, 1995.)... 

However, the behavior of large colloidal particles (compared to ions) is more complex, and their mobility is expected to depend, among other parameters in Equations 8.42 and 8.43, on the concentration of the supporting electrolyte ions in solution. Therefore, the mobility of NPs can be calculated using an approximate analytical relation reported by Ohshima et al." Also, the average charge on NPs, Zp, can be determined from their electrokinetic charge density o, which in turn is related to the zeta potential of the particles, in aqueous solutions at 25°C ... 

The first term is the contribution from the particle surface charge this is simply the Carasso gel layer result, but written in terms of the electrokinetic charge density rather than the zeta potential. The second term is the contribution from the polyelectrolyte charge, for the case when the charge density Pp is uniform across the polymer layer. 

Potentiometric titrations have been used to determine the surface charge density, Ojj(pC.cm ), in the absence and presence of the species to be adsorbed and the concentration of the surface groups [SOH neutral hydroxyls, SOH protonated hydroxyls, SO deprotonated hydroxyls]. Full details have been given elsewhere [14-17],Microelectrophoretic mobility measurements were used to determine the electrokinetic charge density, o (pC.cm ), in the absence and presence of the species to be adsorbed. Full details are given elsewhere [18]. 

The charge in the diffuse layer can be considered equivalent to the Gouy charge density qd placed at a distance K-1 from the OHP. This gives rise to a parallel-plate condenser model. The potential at one plate—deep in the solution side—is taken at zero, while the potential at the other plate—which coincides with the OHP—is [f0. This latter potential is often referred to in the study of electrokinetic phenomena as the zeta ( ) potential. Thus,... 

Surface charge density and surface potential are of primary interest. For insulating surfaces, charge can be determined by potentiometric or conductometric titration, though this is a tedious procedure. For metals, the relationship between surface charge and potential can be determined by measuring the capacitance. Finally we discuss electrokinetic effects. Elec-trokinetic experiments yield the potential at the outer Helmholtz plane. 

Electrokinetic phenomena are only directly related to the nature of the mobile part of the electric double layer and may, therefore, be interpreted only in terms of the zeta potential or the charge density at the surface of shear. No direct information is given about the potentials tf/0 and charge density at the surface of the material in question. 

The surface charge density at the slipping plane or electrokinetic surface charge density may be calculated from the potential by means of the Gouy theory (17) which for a plane surface gives ... 

As already stated in Section 3.3.2, the precise values of the potential of the diffuse electric layer (po can be obtained by the method of equilibrium foam film . The results correlate well with the values of the electrokinetic potential, measured by the method of the rotating bubble [65], Table 8.1 presents the -potential values and the surface charge density Oo for foams from various surfactant kinds [65]. 

Values of lhe electrokinetic potential and surface charge density at the surfactant solution/gas interface... 

Some entries have empty columns 2, 3 or 4 because relevant information was not given explicitly. Some studies report only negative (or less often only positive) surface charge densities (or electrokinetic potentials) over the studied pH range. This suggests that the pHo is beyond the studied pH range or does not exist at all. Such results are summarized in separate tables in Seetion III. 

Sonnefeld, J., Lobbus, M., and Vogelsberger, W.. Determination of electric double layer parameters for spherical silica particles under application of the triple layer model using surface charge density data and results of electrokinetic sonic amplitude measurements, Colloids Surf. A, 195, 215, 2001. 

Hartley, P.G. and Scales, P.J., Electrostatic properties of polyelectrolyte modified surfaces studied by direct force measurement, Langmuir, 14. 6948, 1998. Rodrigues, E.A., Monteiro, P.J.M., and Sposito, G., Surface charge density of silica suspended in water-acetone mixtures, J. Colloid Interf. Sci.. 211, 408, 1999. Abraham, T. et al.. Asphaltene-silica interactions in aqueous solutions Direct force measurements combined with electrokinetic studies, Ind. Eng. Chem. Res., 41, 2170, 2002. 

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