Charge and the Zeta Potential

Pechanek et al. (1982) determined ionic polysaccharides by fl migration through polyacrylamide and agarose gels and on cellulose acetate membranes the polyanions were detected by staining. At the dimensions found in gel micropores, pairs of surfaces create an adsorption potential ( ) 3.5 times that created at the same distance from a single surface (Void and Void, 1983). 

In a method of capillary electrophoresis, 1 pg of dextran was dispersed in alkaline buffer containing fluorescein, and the dextran was detected by negative fluorescence (Richmond and Yeung, 1993). 

Anion exchangers retain polysaccharide polyanions on a positively charged resin (RNHp in exchange for OH- or Cl counterions in proportion to their charge density neutral polysaccharides pass through freely. The most densely charged molecules adsorb closest to the sample placement site. 

Aldonic, uronic, and ascorbic acids, lactones, and N-acetylated amino sugars were separated on sulfonated polystyrene-divinylbenzene, a strong polyanion exchanger (Wheaton and Bauman, 1953). This method is adaptable to neutral carbohydrates without complexation or adsorption, by immersion in strong alkali to ionize the hydroxyl groups (ion chromatography). 

An initial negative slope of iq, vs ct in a dilute water dispersion (electroviscosity) of a polysaccharide is indicative of polyanionic character. Electroviscosity disappears in excess electrolyte solution and is nonexistent in neutral polymer viscosity profiles. 

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One of the applications of zeta potential measurements is in the determination of surface charges of colloids such as proteins. We look into this briefly in Section 12.10 and derive the equation connecting the surface charge to the zeta potential under some simplifying assumptions. 

In the quantitative sections of this chapter the primary emphasis has been on establishing the relationship between the electrophoretic properties of the system and the zeta potential. We saw in Chapter 11 that potential is a particularly useful quantity for the characterization of lyophobic colloids. In this context, then, the f potential is a valuable property to measure for a lyophobic colloid. For lyophilic colloids such as proteins, on the other hand, the charge of the particle is a more useful way to describe the molecule. In this section we consider briefly what information may be obtained about the charge of a particle from electrophoresis measurements. 

The inner part of the double layer may include specifically adsorbed ions. In this case, the center of the specifically adsorbed ions is located between the surface and the Stem plane. Specifically adsorbed ions (e.g., surfactants) either lower or elevate the Stem potential and the zeta potential as shown in Figure 4.31. When the specific adsorption of the surface-active or polyvalent counter ions is strong, the charge sign of the Stem potential will be reversed. The Stem potential can be greater than the surface potential if the surface-active co-ions are adsorbed. The adsorption of nonionic surfactants causes the surface of shear to be moved to a much longer distance from the Stem plane. As a result, the zeta potential will be much lower than the Stem potential. 

In brush cleaning, an alkaline chemistry such as NH4OH is often used to remove the particles such as particles of silica, alumina, glass, polystyrene latex (PSL), and silicon nitride from various wafers in the first brush. The basic chemistry is used mainly to increase the repulsive charge by the zeta potential between the particle and the substrate. 

Physical techniques for evaluating surface polarity led deMayo and coworkers to assign relative rates of reaction on silica gel particles from shifts in the absorption spectra of absorbed spiropyrans [76, 77]. Similarly, Darwent and coworkers demonstrated that kinetic salt effects correlate with surface charge and with zeta potential measurements on colloidal titanium dioxide [80]. 

The charge on emulsion stabilized by sulpha-pyridine was found to be negative. The electrokinetic mobility of the emulsion was measured and the zeta potential was calculated by the Helmholtz equation,... 

In foams with charged gas/liquid interface, various electrokinetic parameters can be obtained, such as streaming potential and zeta-potential. For example, the relationship between the volumetric flow of a liquid flowing through a capillary or membrane and the zeta-potential can be given by the Smoluchowski equation. 

Now the frequency dependence and the phase lag are determined entirely by the inertia term G, and the zeta potential is calculated from a modified form of the Smoluchowski formula (41) which takes account of the inertia effect for the larger particles, especially at the higher frequencies. The determination of size and charge is particularly simple in this case. 

The maximum is an activation barrier that prevents or reduces agglomeration. For a stable dispersion the particles must have a large charge and a high activation barrier. An experimental parameter proportional to this charge is the zeta potential. Therefore, the determination of the zeta potential is an important experimental procedure in characterizing the particles for co-deposition. 

FIGURE 6. The arrangement of water molecules and counterions near to a negatively charged membrane surface according to the Stern model. Within the Stern layer of polarized water molecules the electric potential falls linearly, and for distances further than this the potential profile follows that predicted by the Gouy-Chapman theory of electrical double layers. For ascites cells the potential drop between the surface potential and the zeta potential has been determined to be around... 

Zeta potential is the potential of the surface at the plane of shear between the particle and the surrounding medium as the particle and medium move with respect to each other. In the presence of an applied electric field, the charged surface (and the attached material) tends to move in the appropriate direction, while the counterions in the mobile part of the double-layer would have a net migration in the opposite direction. On the other hand, an electric field would be created if the charged surface and the diffuse part of the double-layer were made to move relative to each other. The plane of shear is beyond the Stem plane, and the zeta potential facilitates easy quantification of the surface charge. The pH at which the calculated zeta potential value is zero is known as the isoelectric point (lEP). 

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