Zeta potential, definition

Not all of the ions in the diffuse layer are necessarily mobile. Sometimes the distinction is made between the location of the tme interface, an intermediate interface called the Stem layer (5) where there are immobilized diffuse layer ions, and a surface of shear where the bulk fluid begins to move freely. The potential at the surface of shear is called the zeta potential. The only methods available to measure the zeta potential involve moving the surface relative to the bulk. Because the zeta potential is defined as the potential at the surface where the bulk fluid may move under shear, this is by definition the potential that is measured by these techniques (3). 

It follows from the definition cited that the size of the zeta potential depends on the structure of the diffuse part of the ionic EDL. At the outer limit of the Helmholtz layer (at X = X2) the potential is j/2, in the notation adopted in Chapter 10. Beyond this point the potential asymptotically approaches zero with increasing distance from the surface. The slip plane in all likelihood is somewhat farther away from the electrode than the outer Helmholtz layer. Hence, the valne of agrees in sign with the value of /2 but is somewhat lower in absolute value. 

The definition of streaming potential was presented in the previous section. Here, we derive the relation between the streaming potential and the zeta potential and discuss some of the issues that must be considered in comparing zeta potentials obtained by different electrokinetic measurements. 

The EOF is influenced by factors that can change the zeta potential and hence the thickness of the double layer (S), which is related to the zeta potential (Eq. (4.2)) (see glossary of terms for symbol definitions) ... 

The rise of the negative zeta potential due to acrylates or to CMC should theoretically hinder the absorption of dirt particles, which generally have nq ative charges. There is no definite proof of this event however, it is known that finishes havii a poritive charge (aminoacrylates, melamine resins, etc.) are decided harmful in this regard. 

There are several effects due to the existence of the double layer on the surface of most particles suspended in liquids, and they can all be used to measure the so-called zeta potential. A simplified summary of the effects is given in Table 1.1 and further explanation and definitions are given in section 4.4.3. 

The definition of zeta-potential is an electrokinetic phenomenon characterized by the relative motion of two phases (droplet in the continuous phase) wherein two types of forces are involved electro forces and resistance forces. 

The zeta potential is the potential at the surface between a stationary solution and a moving charged colloid particle. This surface defines the plane of shear. Its definition is somewhat imprecise because the moving charged particle will have a certain number of counterions attached to it (for example ions in the Stern layer, plus some bound solvent molecules), the combined flowing object being termed the electrokinetic unit. The stability of colloidal suspensions is often interpreted in terms of the zeta potential, because, as we shall see, it is more readily accessible than the surface potential (Eq. 3.7), which describes the repulsive interaction between electric double layers. 

The electric potential of a particle varies gradually from the value of the surface potential (Vo) to zero. For the majority of particles without a complex surface structure, the variation is monotonic. Thus, electric potential at the shear plane is in between vj/o and zero (Figure 6.2). This potential is called the zeta potential ( potential). Zeta potential is different from surface potential /o but is a measurable amount. Since the shear plane is located at the frontier of the particle surface, any interfacial interaction of particle with other species (ions, neighboring particles, etc.) in medium will be encountered at the shear plane. Zeta potential actually has more direct influence compared with surface potential. Zeta potential is determined by many factors, mainly 1) surface potential, 2) potential curve, which is determined by the concentrations and valences of the co- and counter ions in the system and the dielectric constant of the continuous phase, and 3) location of the shear plane. There is no definite relationship between surface potential and zeta potential. For example, in different environments and particle surface conditions, the same zeta potential may correspond to different surface potentials and the same surface potential may result in different zeta potentials. Figure 6.2 describes the relation between... 

Some systems do show excellent agreement between zeta and the surface potential, whilst others differ significantly. Since no definitive results have yet been obtained, it is perhaps best to assume as a first approximation that these two potentials are similar. 

As discussed above the chemical potential of a constituent represents the differential of the Gibbs energy with respect to the amount of the constituent when the pressure, temperature and the amounts of all other constituents are kept constant. When a stmcture element is created, the number of complementary stmcture elements can not be kept constant due to the requirement of a definite site ratio, and it is therefore not possible to assign a tme chemical potential to a stmcture element. However, Kroger et al. have shown that one may get around this difficulty by assigning a virtual chemical potential C (zeta) to each separate stmcture element and that the virtual chemical potential behaves like a tme chemical potential and may be related in a in a similar way to activity ... 

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