Electrokinetics charged interfaces

For well-dispersed colloid systems, particle electrophoresis has been the classic method of characterization with respect to electrostatic interactions. However, outside the colloidal realm, i.e., in the rest of the known world, the measurement of other electrokinetic phenomena must be used to characterize surfaces in this respect. The term electrokinetic refers to a number of effects induced by externally applied forces at a charged interface. These effects include electrophoresis, streaming potential, and electro-osmosis. 

FIGURE 9.5 Net proton charge, Qq, and electrokinetic charge, Q, of the protein ribonucle-ase in aqueous solution. T = 25°C ionic strength 0.05 M KNO3. (Adapted from Norde, W. and Lyklema, J., J. Colloid Interface Sci., 66, 266, 1978.)... 

As has been mentioned before and is discnssed in more detail in Chapter 10, for smooth surfaces t r may be identified with the electrokinetic potential. It appears that under most conditions t r is mnch smaller than t To, that is, the potential decays for the largest part across the Stem layer. Anticipating the discnssions in forthcoming chapters, for the stability of (hydrophobic) coUoids as well as in adsorption and adhesion processes at charged interfaces, t r plays a more relevant role than /o. 

Electrokinetic phenomena designate the transport phenomena involving electrolytes near charged interfaces. 

Electrokinetic phenomena refers to dynamic processes that occur when viscous or electrical forces are applied to a charged interface. The most common of these phenomena are electrophoresis, electroosmosis, streaming potential and sedimentation potential. 

Electric double layers are formed in heterogeneous electrochemical systems at interfaces between the electrolyte solution and other condncting or nonconducting phases this implies that charges of opposite sign accumnlate at the surfaces of the adjacent phases. When an electric held is present in the solntion phase which acts along snch an interface, forces arise that produce (when this is possible) a relative motion of the phases in opposite directions. The associated phenomena historically came to be known as electrokinetic phenomena or electrokinetic processes. These terms are not very fortunate, since a similar term, electrochemical kinetics, commonly has a different meaning (see Part 11). 

The adsorption of ions at insulator surfaces or ionization of surface groups can lead to the formation of an electrical double layer with the diffuse layer present in solution. The ions contained in the diffuse layer are mobile while the layer of adsorbed ions is immobile. The presence of this mobile space charge is the source of the electrokinetic phenomena.t Electrokinetic phenomena are typical for insulator systems or for a poorly conductive electrolyte containing a suspension or an emulsion, but they can also occur at metal-electrolyte solution interfaces. 

Zhang, H. Zhang, X.-N. (1992) Contribution of iron and aluminum oxides to electrokinetic characteristics of variable charge soils in relation to surface charge. Pedosphere 2 31-42 Zhang, J. Buffle, J. (1995) Kinetics of hematite aggregation by polyacrylic acid Importance of charge neutralization. J. Colloid Interface Sci. 174 500-509... 

Electroosmosis refers to the movement of the liquid adjacent to a charged snrface, in contact with a polar liquid, under the influence of an electric field applied parallel to the solid-liquid interface. The bulk fluid of liquid originated by this electrokinetic process is termed electroosmotic flow. It may be prodnced either in open or in packed or in monolithic capillary columns, as well as in planar electrophoretic systems employing a variety of snpports, such as paper or hydrophilic polymers. The origin of electroosmosis is the electrical donble layer generated at the plane of share between the snrface of either the planar support or the inner wall of the capillary tube and the surronnding solntion, as a consequence of the nneven distribntion of ions within the solid/liquid interface. 

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 ... 

The concentration of potential-determining ions at which the zeta potential is zero (C = 0) is called the isoelectric point (iep). The isoelectric point is determined by electrokinetic measurements. We have to distinguish it from the point of zero charge (pzc). At the point of zero charge the surface charge is zero. The zeta potential refers to the hydrodynamic interface while the surface charge is defined for the solid-liquid interface. 

Electrokinetic measurements at 25°C on silver iodide in 10 3 mol dm-3 aqueous potassium nitrate give d /d(pAg) = -35 mV at the zero point of charge. Assuming no specific adsorption of K+ or NO3 ions and no potential drop within the solid, estimate the capacity of the inner part of the electric double layer. Taking the thickness of the inner part of the double layer to be 0.4 nm, what value for the dielectric constant near to the interface does this imply Comment on the result. 

The electrical double layer at the metal oxide/electrolyte solution interface can be described by characteristic parameters such as surface charge and electrokinetic potential. Metal oxide surface charge is created by the adsorption of electrolyte ions and potential determining ions (H+ and OH-).9 This phenomenon is described by ionization and complexation reactions of surface hydroxyl groups, and each of these reactions can be characterized by suitable constants such as pKa , pKa2, pKAn and pKct. The values of the point of zero charge (pHpzc), the isoelectric point (pH ep), and all surface reaction constants for the measured oxides are collected in Table 1. 

The data about the electrophoretic behaviour of bubbles in aqueous electrolytes, the first concerning electrophoretic mobilities and zeta-potential, can be regarded as a main direct source of information about surface charge at solution/air interface. As cited by many earlier authors, the electrokinetic behaviour of a gas bubble in aqueous solutions has been studied for over a century [e.g. 174-181]. However, the mechanism of creation of surface charge and the effect of inorganic salts, etc. are not completely clear. Recently Li and Somasunderan [182,183] and Kelsall et al. [184,185] have reported some new results in this field. 

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. 

In foams with charged gas/liquid interfaces, as in other disperse systems, various electrokinetic phenomena are possible to occur. Such are the change in the transport numbers of ions, electroosmosis, streaming potential and surface conductivity. While these phenomena are largely studied in disperse systems with solid disperse phase, the first electrokinetic observations in foams have been reported only recently. 

Values of lhe electrokinetic potential and surface charge density at the surfactant solution/gas interface... 

Electric double layers at phase boundaries pervade the entire realm of Interface and colloid science. Especially in aqueous systems, double layers tend to form spontaneously. Hence, special precautions have to be taken to ensure the absence of charges on the surfaces of particles. Insight into the properties of double layers is mandatory, in describing for Instance electrosorption, ion exchange, electrokinetics (chapter 4), charged monolayers (Volume III), colloid stability, polyelectrolytes and proteins, and micelle formation of ionic surfactants, topics that are intended to be treated in later Volumes. The present chapter is meant to Introduce the basic features. 

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