Electrokinetic phenomena and the zeta potential

Calculation of the zeta-potential from particle mobility depends on the particle size and shape, as well as the electrolyte concentration, and several theories are available for this purpose. 

Von Smoluchowski (Classical] Treatment The von Smoluchowsld treatment [16] applies to the case where the particle radius R is much larger than the double layer thickness (l/jc), that is 1. This generally apphes to particles that are greater than 0.5 mp (when the 1 1 electrolyte concentration is lower than 10 moldm , that is R 10), 

The Hiickel Equation The Htickel equation [17] applies for the case kR 1, 

The above equation applies for small particles ( 100 nm) and thick double layers (low electrolyte concentration). 

Henry s Treatment Henry s treatment [18] apphes to intermediate cases where kR is not too small or too large, Henry derived the following expression (which can be applied at all kR values), 

It has already been said that a practical way to characterize the double layer is to measure the zeta potential, C- Several techniques are possible these come within the category of electrokinetic phenomena. 

In electrophoresis a potential gradient is applied to a dispersion and the movement of the charged particles relative to the stationary bulk phase is measured. Sedimentation potential is the reverse of the above the 

Kruyt has related the electro-osmotic velocity of flow to the double layer potential at the shear plane and to the applied external field. The double layer must satisfy the relation 

When an external field E is applied, a volume of the bulk liquid of thickness dx is subject to a force 

A number of layers adjacent to the shear layer will be moving at different velocities and will exert viscous drag on the latter. Consider a layer at position X from the surface the drag force on this layer is 

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It is very difficult and scarce to find literature to study the electrokinetic phenomena of proteins or macromolecules in solution therefore limit us to the basic concepts of electrokinetic changes observed, they are conformational change because of the presence of salts and the zeta potential change in pH. 

When one thinks of electrokinetic phenomena in the context of a first-level course on colloid and surface chemistry, the first thought that probably comes to mind is the use of such phenomena to measure zeta potentials and charges of colloidal species. But, as we have already seen in Chapter 1 and as we see later in this chapter, electrokinetic phenomena play a significant role in many other applications. We take a look at one such application here and see why the topics we consider in this chapter and in others are important in that context. 

The surface of shear is the location within the electrical double layer at which the various electrokinetic phenomena measure the potential. We saw in Chapter 11 how the double layer extends outward from a charged wall. The potential at any particular distance from the wall can, in principle, be expressed in terms of the potential at the wall and the electrolyte content of the solution. In terms of electrokinetic phenomena, the question is How far from the interface is the surface of shear situated and what implications does this have on the relation between measured zeta potential and the surface potential ... 

Hunter, R. J., Zeta Potentials in Colloid Science Principles and Applications, Academic Press, London, 1981. (Advanced. This is a research-level monograph on electrokinetic phenomena and electro viscous and viscoelectric effects and contains extensive details on the subject.)... 

If a liquid moves tangential to a charged surface, then so-called electrokinetic phenomena arise [101]. Electrokinetic phenomena can be divided into four categories Electrophoresis, electro-osmosis, streaming potential, and sedimentation potential [102], In all these phenomena the zeta potential plays a crucial role. The classic theory of electrokinetic effects was proposed by Smoluchowski2 [103],... 

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. 

Quantitative measurements of electrokinetic phenomena permit the calculation of the zeta potential by use of the appropriate equations. However, in the deduction of the equations approximations are made this is because in the interfacial region physical properties such as concentration, viscosity, conductivity, and dielectric constant differ from their values in bulk solution, which is not taken into account. Corrections to compensate for these approximations have been introduced, as well as consideration of non-spherical particles and particles of dimensions comparable to the diffuse layer thickness. This should be consulted in the specialized literature. 

This material on the measurement of zeta potential comes from three excellent books by Adamson [ 2, p. 340], Heimenz [44], and Hunter [41]. The potential at this shear plane, the zeta potential, is measured using one of several electrokinetic phenomena which have in common the relative motion of a charged surface (e.g., a ceramic particle) and the bulk solution as elaborated in Table 9.10. When the electric field is applied, the charged surface experiences a force. When the surface moves, an electric field is induced in the solution. 

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. 

Electrokinetic phenomena arise when the mobile layer of the EDL interacts with an externally applied electric field resulting in relative motion between the solid and liquid phases. There are three types of electrokinetic phenomena relevant to microfluidics electroosmotic flow, streaming potential, and electrophoresis. In aU of these cases, the zeta potential is a key parameter that defines either the fluid flow or particle motion. Since it is not possible to probe the zeta potential directly, measurements are based on indirect readings obtained from electrokinetic experiments. The following discussion focuses on modem methods of measuring the zeta potential using electroosmotic flow, electrophoresis, and streaming potential. 

For all electrokinetic phenomena discussed in this chapter (except the one described in Section 10.3.3), the zeta potential is the electric potential at the plane of shear. Interpretation of should therefore start with addressing the question regarding the location of the plane of shear. As mentioned in Section 10.1, for smooth, rigid surfaces the plane of shear is situated only a little further out from the surface than the Stem layer so that tir, which is difficult to establish experimentally, may be approximated by However, for irregularly shaped and hairy surfaces, the plane of shear is usually farther away from the surface and is correspondingly lower. 

In the preceding examples, the measurement of elec-trokinetic phenomena has been shown to be useful in characterizing a surface in terms of source of charge and surface interfacial properties. The literature abounds with similar examples. Other than accounting for surface charge, the zeta potential derived from electrokinetic measurements have also been useful as the sole parameter for characterizing the electrostatic contribution to... 

Among the most important characteristics associated with electrokinetic phenomena are the electrophoretic mobility and zeta potential (or -potential). Electrophoretic mobility, is the steady-state drift velocity, v, a charged particle obtains per unit of applied uniform electric field E... 

The zeta potential is the parameter used to characterize a nanostructure s surface charge. Although zeta potential is not measurable directly, it can be calculated using theoretical models applied to the data provided by experimentally determined electrophoretic mobility or dynamic electrophoretic mobility, electrokinetic phenomena and electroacoustic phenomena being the usual sources of data for the calculation of zeta potential. Keeping the zeta potential far from a neutral value is important in order to avoid stability problems in systems once electrostatic repulsion is favored. Indeed, zeta potentials close to neutral result in a tendency to aggregate, and a high zeta potential (in module) is usually associated with stable systems. 

It appears that, in all modes of calculation, the electrical potential in the slipping-plane between the fixed and the flowing liquid is determinative for the electrokinetic phenomena. This potential is usually called the zeta-potential (C). 

The electrokinetic potential (zeta potential, Q is the potential drop across the mobile part of the double layer (Fig. 3.2c) that is responsible for electrokinetic phenomena, for example, elecrophoresis (= motion of colloidal particles in an electric field). It is assumed that the liquid adhering to the solid (particle) surface and the mobile liquid are separated by a shear plane (slipping plane). The electrokinetic charge is the charge on the shear plane. 

Fig. 3.25 presents the aqueous solutions in the absence of a surfactant at constant ionic strength (HC1 + KC1) [186,197], It can be seen that at pH > 5.5, op-potential becomes constant and equal to about 30 mV. At pH < 5.5 the potential sharply decreases and becomes zero at pH 4.5, i.e. an isoelectric state at the solution surface is reached. As it is known, the isoelectric point corresponds to a pH value at which the electrokinetic phenomena are not observed. Since in the absence of the potential of the diffuse electric layer, the electrokinetic potential (zeta-potential) should also be equal to zero, the isoelectric point can be used to determine pH value at which isoelectric state is controlled by the change in pH. This is very interesting, for it means that the charge at the surface of the aqueous solutions is mainly due to the adsorption of H+ and OH" ions. Estimation of the adsorption potential of these ions in the Stem layer (under the assumption that the amounts of both ions absorbed are equal) showed that the adsorption potential of OH" ions is higher. It follows that ( -potential at the solution/air interface appears as a result of adsorption of OH" ions. 

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