Electrokinetic behavior

This equation is a reasonable model of electrokinetic behavior, although for theoretical studies many possible corrections must be considered. Correction must always be made for electrokinetic effects at the wall of the cell, since this wall also carries a double layer. There are corrections for the motion of solvated ions through the medium, surface and bulk conductivity of the particles, nonspherical shape of the particles, etc. The parameter zeta, determined by measuring the particle velocity and substituting in the above equation, is a measure of the potential at the so-called surface of shear, ie, the surface dividing the moving particle and its adherent layer of solution from the stationary bulk of the solution. This surface of shear ties at an indeterrninate distance from the tme particle surface. Thus, the measured zeta potential can be related only semiquantitatively to the curves of Figure 3. 

Departures of the electrokinetic behavior of real systems from that described by the equations reported occurs most often because of breakdown of two of the assumptions above because of marked surface conductivity (particularly in dilute solutions, where the bulk conductivity is low) and because of a small characteristic size of the disperse-phase elements (e.g., breakdown of the condition of bg <5 r in extremely fine-porous diaphragms). A number of more complicated equations allowing for these factors have been proposed. 

Elliott El.A., Sparks D.L. Electrokinetic behavior of a paleudult profile in relation to mineralogical composition. Soil Sci 1981 132 402-409. 

Adsorption and Electrokinetic Behavior of Hydroxyapatite. The adsorption densities of glutamic acid and lysine on hydroxyapatite are shown in Flgures b and 7. The change in slope of the adsorption isotherm at 10 M glutamic acid is considered to be due to a... 

The adsorption of amino acids on rutile and hydroxyapatite exhibits some characteristics of specific adsorption. The results can be interpreted in terms of electrostatic models of adsorption, however, if reorientation of adsorbed molecules is taken into consideration. The electrokinetic behavior of hydroxyapatite in glutamic acid is complicated because of a chemical reaction, possibly involving calcium ions. The study shows that it is necessary to take into consideration the orientation of adsorbed molecules, particularly for zwitterionic surfactants. 

Amankonah J.F. and Somasundran P. (1985) Effects of dissolved mineral species on the electrokinetic behavior of calcite and apatite. Colloids and Surfaces 15, 335-353. 

Figure 4 shows that deposition of one layer of PMBQ from solution with pH = 6 resulted in drastic changes of surface properties. This was quantitatively characterized by the shift of surface IEP (isoelectric point) from pH < 3.5 to 6. Since the isoelectric point of the PMBQ solution was higher than 12, it may be assumed that the electrokinetic behavior was still determined by the com-... 

Equation (21.62) shows that as k co, p tends to a nonzero limiting value p°°. This is a characteristic of the electrokinetic behavior of soft particles, in contrast to the case of the electrophoretic mobility of hard particles, which should reduces to zero due to the shielding effects, since the mobility expressions for rigid particles (Chapter 3) do not have p°°. The term p°° can be interpreted as resulting from the balance between the electric force acting on the fixed charges ZeN)E and the frictional force yu, namely. 

The comparison with amphoteric oxides [57-59] is also instructive. In an early review, Snoeyink and Weber [60] compared the surface functional groups on carbons and silicas but failed to point out the resulting differences in the symmetries" of their electrokinetic behavior. For amphoteric oxides, the symmetry (see Fig. 4a) is a consequence of the following equilibrium [57,61-66] ... 

Wiese, G.R. Healy, T.W. Coagulation and electrokinetic behavior of Ti02 and AI2O3 colloidal dispersions. J. Colloid Interface Sci. 1975, 51, 427-433. 

The effect of heat treatment on the electrokinetic behavior of powders has been emphasized by Kittaka [40,41]. Janssen and Stein [42] compared electrokinetic and charging curves of two samples of titania which underwent H2 and O2 treatment at... 

Mishra studied effects of addition of calcium and carbonates and phosphates (concentrations on the order of 10" mol dm ) on the C(pH) curves for natural calcite and apatite [246]. Usually the presence of calcium induced a shift in to more positive and presence of carbonates and phosphates to more negative value, i.e. according to expectations, but over certain pH ranges there was no effect or even the effect was opposite to the expected one. The electrokinetic behavior of apatite in calcite supernatant was similar to that of calcite in water, and the electrokinetic behavior of calcite in apatite supernatant was similar to that of apatite in water [247]. The potentials of calcite and apatite were positive and rather pH insensitive in the presence of >10" mol dm" of calcium, and negative and rather pH insensitive in the presence of >10 mol dm of phosphate, and the effect of carbonates on potentials of apatite was negligible. 

Hysteresis in electrokinetic behavior of calcite has been also reported [248]. The PZC of salts are summarized in Table 3.5. The PZC eolumn specifies not only the value but also the choice of independent variable (pH or concentration of the cation of the salt). The salts are sorted by the chemical symbol of the anion, and then of the cation. 

Electrokinetic behavior of reagent grade synthetic covellite CuS, pyrrhotite FesSs and millerite NiS was studied over the pH range 2-11 [260]. Pyrrhotite showed lEP at pH 6. Covellite and millerite showed negative C potential at very low and very high pH and positive potential over limited pH range 7-9 (CuS) and 8-10 (NiS). A series of (CuS)x(CdS)i.x samples [261] had negative potentials at pH 4 in 0.05... 

Many studies resulted in common crossover points of titration curves even for electrolyte concentrations > I mol dm. On the other hand, the lEP is substantially different form the pristine value at electrolyte concentration as low as 0.1 mo dm. Typical electrokinetic behavior of metal oxides at very high ionic strengths is illustrated in Fig. 3.101. The lEP is gradually shifted to high pH values when the electrolyte concentration increases, and at very high ionic strengths the ( potential is always positive and rather insensitive to pH. These results have been recently confirmed by rheological measurements (cf. Table 3.7). 

The effect of specific adsorption on electrokinetic behavior of materials is usually presented in form of C(pH) curves at constant initial (total) concentration of a specifically adsorbing salt. The electrophoretic mobility rather than the potential is often plotted as a function of the pH. The mobility (directly measured quantity) is a complicated function involving the C potential on the one hand and particle size and shape, and concentrations of ionic species in the solution on the other (cf. Figs. 3.80 and 3.81), and exact calculation of the potential in real systems (polydispersed and irregularly shaped particles) is practically impossihle. This is a serious difficulty in quantitative interpretation of electrokinetic data obtained in the presence of specific adsorption. On the other hand, the zero electrophoretic mobility corresponds to zero C potential, and the shifts in the lEP along the pH axis can be determined with accuracy on the order of 0.1 pH unit. 

The effect of specific adsorption of anions (phosphate) on the electrokinetic behavior of alumina is shown in Figs. 4.15-4.18 (experimental data from Ref. [36]). All data points correspond to the same solid to liquid ratio. The electrokinetic curve obtained at initial phosphate concentration of 2 x 10" mol dm (Fig. 4.15) does not differ from the electrokinetic curve at pristine conditions (not shown). The presence of 10 mol dm phosphate induces a substantial shift in the lEP, and this shift is more pronounced at higher phosphate concentrations. This behavior is typical for specific adsorption of anions. The results from Fig. 4.15 and a few analogous sets of data points obtained at different initial phosphate concentrations (10" to 10 mol dm ) are re-plotted in Fig. 4.16 in the coordinates total phosphate concentration in solution - electrophoretic mobility. This representation gives a random cloud of points. Also the electrophoretic mobility plotted as the function of phosphate surface concentration (not shown) does not reveal any regularity. On the other hand the electrophoretic mobility plotted as the function of [HPOj ] (Fig. 4.17) or as the function of [PO ] (Fig. 4.18) produces one master curve containing all data points... 

The electrokinetic behavior of silica in the presence of specific adsorption of cations has been a subject of many studies, and it is different from the behavior of most other materials. A typical set of electrokinetic curves of silica obtained at various initial concentrations of cations is shown in Fig. 4.19. At low concentrations the strongly interacting cations do not affect the electrokinetic potential. At higher concentrations the negative is depressed or even reversed to positive, but only over a limited pH range (in Fig. 4.19 the eation effect is most pronounced at pH ss6) while at very low and very high pH the effect of specific adsorption of cations on the electrokinetic curves is insignificant. This may result in multiple lEP. Multiple lEP have been also reported for titania, while for most other materials the lEP is shifted... 

A streaming current detector based on a completely different principle than the above instruments is presented in [299], The dispersion is in a narrow space between a vertical cylindrical vessel and a coaxial piston, which moves back and forth along the axis. The potential between two gold electrodes on the wall of the cylinder at different heights is measured, and its zero value is identified with the IEP. The apparatus own response corresponds to the electrokinetic behavior of the piston and cell materials. In the presence of a colloid, the piston and the cell are assumed to be covered with colloidal particles. The above design has been utilized in some commercial instruments ... 

A significant cation effect (with potentials in KCl higher than those in NaCI) in the acidic range is reported in [521], Unusual effects of the nature of 1-1 electrolytes on the electrokinetic behavior of a titania pigment (surface-modified with silica, alumina, and organic groups) are reported in [516]. 

A hysteresis in the electrokinetic behavior of alumina and hematite was found in [288] the absolute value of the potential at constant pH increased with T, but no return to lower potential on cooling was observed. An increase in the absolute value of the potential at constant pH with T was reported in [2370], Uptake of cations from a 1-1 electrolyte by silica and alumina at constant Gp was rather insensitive to temperature [1842], The surface potential of alumina was studied in [3057] as a function of temperature (ISFET response). The temperature effect on tlie streaming potential is reviewed in [3058], The PZCs at very high temperatures reported in [3047] were obtained by extrapolation of experimental results obtained at moderate temperatures. 

Thus, a few results indicate that the electrokinetic behavior of metal oxides at high concentrations of 1-1 electrolytes is similar to that at low ionic strengths that is, the increase in electrolyte concentration depresses the absolute value of the potential, and the IEP remains unaffected. In a few other studies, a shift in the IEP to high pH was observed at concentrations of 1-1 electrolytes of about 0.1 M. Such a shift suggests specific adsorption of cations. 

Gence, N. and Ozbay, N., pH dependence of electrokinetic behavior of dolomite and magnesite in aqueous electrolyte solutions, Appl. Surf. Sci., 252. 8057. 2006. 

Bouhamed, H., Boufi, S., and Magnin, A., Alumina interaction with AMPS-MPEG random copolymers. 1. Adsorption and electrokinetic behavior, J. Colloid Interf. Sci., 261, 264, 2003. 

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