Electrokinetic phenomena, zeta potential

The potential governing these electrokinetic effects is clearly at the boundary (the face of shear) between the stationary phase (the fixed double layer) and the moving phase (the solution). This potential is called the electrokinetic potential or the zeta potential. An electrokinetic phenomenon in soil involves coupling between electrical, chemical, and hydraulic gradients. 

When an alternating voltage is applied to a colloid, the particles move back and forth with a velocity that depends on their size and zeta potential and on the frequency of the applied field. As they move, the particles generate sound waves. This phenomenon is called the electroacoustic effect, which can be measured and what was named electrokinetic sonic amplitude (ESA) [5],... 

Finally, it should be evident that the principal utility of electrokinetic data is qualitative and comparative. The demonstration that an electrokinetic phenomenon exists is a demonstration that an electrified interface exists. This broad implication is conclusive. In a quantitative sense, however, measurements of electrokinetic phenomena on the same kind of interface under different conditions have only a comparative value. Changes in the zeta potential as the composition of the interfacial region is varied through adsorption can be a useful guide to molecular interpretation even though C itself has little objective significance. 

Acoustics has a related field that is usually referred to as electroacoustics (8). Electroacoustics can provide particle size distribution as well as zeta potential. This relatively new technique is more complex than acoustics because an additional electric field is involved. As a result, both hardware and theory become more complicated. There are even two different versions of electroacoustics depending on what field is used as a driving force. Electrokinetic sonic amplitude (ESA) involves the generation of sound energy caused by the driving force of an applied electric field. Colloid vibration current (CVC) is the phenomenon where sound energy is applied to a system and a resultant eleetrie field or eurrent is created by the vibration of the colloid electric double layers. 

This document is organized in the following manner first, a brief introduction to the electric double layer and its significance to electrokinetic phenomenon. Second, the principles behind zeta potential measurements based on the three types of electrokinetic phenomenon are discussed, with special attention to the advantages and disadvantages of each method. Finally, a few key research results for each of the methods are presented. The purpose of this entry is to provide the reader with a brief introduction to the fundamentals of zeta potential measurements for a more detailed description of zeta potential measurements, electric double layer, and electrokinetic phenomenon, the reader should consult the various texts that discuss these topics in depth [1—4]. 

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. 

In Equation 19.12, Cq = 8.854 x j-i qi -1 jg jjjg dielectric constant in vacuum, e is the relative dielectric permittivity of the solvent (e = 78.5 for water at room temperature 298 K), and are the electrokinetic zeta potential defined at the shear plane (see Figure 19.3), r is the dynamic viscosity of the solvent (q = 8.91 x 10 kgm" s for water at room temperature 298 K), and E is the externally applied electric field. The first equation in Equation 19.12 represents the fluid motion in a stationary channel under the action of an externally appUed electric field. The motion is called electro-osmosis and the velocity is v. The second equation in Equation 19.12 gives the velocity v, of charged suspended colloidal particle (or a dissolved molecule) driven by the same electric field. This phenomenon is called electrophoresis. The EDL thickness 1/k depends on the concentration of background electrolyte [18,19,25,26]. 

The Electrokinetic Sonic Amplitude (ESA) effect in this context refers to the generation of ultrasound by the application of an alternating electric field to a colloid. Previous reviews on the ESA have mainly focused on the determination of particle size and zeta potential from the ESA. While this is certainly a very important application of the ESA phenomenon, there is more information in the ESA spectmm than just particle size and zeta. It can be used, for instance, to determine the thickness of adsorbed polymer layers or the surface conductance under the shear plane. It is these other applications that will be our main interest here. To begin we will give an alternative explanation for the ESA phenomenon, one that allows a deeper understanding of the underlying physics. 

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