Zeta potential typical value

The zeta potential of a colloidal dispersion can be a strong function of the dispersion pH. Figure 12.16 shows a qualitative depiction of the zeta potential versus pH behavior. As the pH increases along the abscissa, the zeta potential crosses over the axis of zero potential at a locus termed the isoelectric point. The pH about this point, extending out to ( = 30, would be considered to yield an unstable colloid, whereas pH values outside of this domain yield zeta potentials typically considered stable. 

Fig. 12.7 (a) Fabrication schematic of a graphene-encapsulated metal oxide, (b) Zeta potentials of APS-modified silica ( ) and graphene oxide ( ) in aqueous solutions with various pH values, (c) and (d) typical SEM, and (e) transmission electron microscopy (TEM) images of graphene-encapsulated silica spheres. Reprinted with permission from [87]. Copyright 2010, John Wiley Sons, Inc. 

Colloid vibration potentials offer a means of measuring the zeta-potential, and hence charge, on colloid particles. Values of-KT4 Vein s at frequencies of a few hundred kilohertz seem to be typical of this effect, and a range of colloids were examined, including silver, silver iodide, and arsenic trisulfide. 

This formula for the electroosmotic velocity past a plane charged surface is known as the Helmholtz-Smoluchowski equation. Note that within this picture, where the double layer thickness is very small compared with the characteristic length, say alX t> 100, the fluid moves as in plug flow. Thus the velocity slips at the wall that is, it goes from U to zero discontinuously. For a finite-thickness diffuse layer the actual velocity profile has a behavior similar to that shown in Fig. 6.5.1, where the velocity drops continuously across the layer to zero at the wall. The constant electroosmotic velocity therefore represents the velocity at the edge of the diffuse layer. A typical zeta potential is about 0.1 V. Thus for = 10 V m" with viscosity that of water, the electroosmotic velocity U 10 " ms, a very small value. 

The parameter n refers to the ratio of the zeta potential in the sample and the BGE regions. Typical values of this parameter range between 0.2 and 0.3 [50,51], so that the advective dispersion effects of mismatched slip velocities is negligible for low y but dominates at high y. An analogous scaling... 

This plot is represented by the squares ( ) in Fig. 1 (note that the value of the zeta potential is held constant at its room-temperature value). This treatment still fails to accurately predict the EOF velocity in the microchannel. For example, at 90 °C the EOF velocity predicted by this approach would be 1.5 mm/s, while the actual velocity is 2.2 nun/s, a 32 % underestimation This plot makes it clear that in some cases, the typical approach for velocity estimation is insufficient. Since the zeta potential is the only unaltered variable in the Smoluchowski equation (Eq. 1), it is assumed that the discrepancy is owing to inaccuracy in this variable. In these cases, the zeta potential should be taken to be a temperature-dependant variable rather than a constant. 

Zeta potential is defined as the electrical potential at the shear plane of the electric double layer. Measurement techniques are based on indirect readings obtained during electrokinetic experiments. Typically, the magnitude of the zeta potential varies between 0 and 200 mV where both negative and positive values are possible depending on the electrochemistry of the solid-liquid interface. 

Typically, the zeta potentials for latex or pigment particles are measured at different pH values. The zeta potential and hence the magnitude of the electric double-layer surface charge vary with the pH of the aqueous phase. The point at which the net charge is zero is termed the isoelectric point. 

In the case of aqueous suspensions of oxide materials, the stability is mainly determined by the van-der-Waals and double layer interactions (DLI) between the particles. The former is always attractive in the event of particle collision. Whether the DLI is attractive or repulsive depends on the signs of the surface charges, the absolute values of the zeta-potential, and the regulation capacity of the double layer. When all particles are identically charged, the DLI is repulsive. The most influential factors on the stability are the pH value, the total electrolyte concentration, and the valency of the ions. The pH range in which a binary suspension remains stable is typically different to the respective stability ranges of the two particle components. 

Maximizing I involves a low concentration of large monovalent ions. In this case, the zeta potential will also be large, and the colloidal solution will be stabihzed against coUapse and flocculation. Typical values of L for a NaCl solution are 30 nm at lO molT, and Inm at 0.1 moll . Similarly for an MgS04 solution, L = 15nm at lO molT and 0.5nm at 0.1 molT . 

Model e adds a supplementary interfacial layer compared to the Stem model (model d). This supplementary layer merely has the piupose to sufficiently decrease the diffuse-layer potential and to have closer agreement with measmed zeta potentials. One additional adjustable parameter is introduced (the capacitance C2). Based on Eq. (15), the typically used value of C2 = 0.2 F/m will control the overall capacitance of the compact part to the electric double layer. Contrary to model d, model e uses a Gouy-Chapman approach rather than the HNC approximation to account for the diffuse layer, but this can, of course, be varied. Otherwise, the discussion of model d also applies to model e. 

Table 10.3 shows some typical lEP (isoelectric point) values for some metal oxides and proteins. The lEP is determined by a pH titration measming the zeta potential as a function of pH. The point of zero charge is the pH at which the positive and negative charges of a zwitterionic surface are balanced. It is the same as the lEP if there is no specific adsorption of ions onto the surface. Note that some solids are surface treated (e.g. TiOa) and the lEP depends on the surface treatment (see Figure 10.14). 

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