Electrophoretic mobility, effect adsorption

Electron spin resonance. 353 Electrophoretic mobility effect of ionic strength on, 242 effect of particle size on, 247 EMF method, 84 Enthalpy of adsorption, 318-320 at constant cto, 320 EpHL method, 82 Equiadsorption point, 73 Equilibrium constant, 52, 588 Ethers, surface charging in the presence of, 10... 

These effects can be illustrated more quantitatively. The drop in the magnitude of the potential of mica with increasing salt is illustrated in Fig. V-7 here yp is reduced in the immobile layer by ion adsorption and specific ion effects are evident. In Fig. V-8, the pH is potential determining and alters the electrophoretic mobility. Carbon blacks are industrially important materials having various acid-base surface impurities depending on their source and heat treatment. 

Figure 6.15 The effect of adsorption of polyethyleneimines of different molecular weight on electrophoretic mobility of cellulose (PEI 10 = DP of 10 and PEI 500 = DP of500 figures in brackets are negative).

Only a few systematic studies have been carried out on the mechanism of interaction of organic surfactants and macromolecules. Mishra et al. (12) studied the effect of sulfonates (dodecyl), carboxylic acids (oleic and tridecanoic), and amines (dodecyl and dodecyltrimethyl) on the electrophoretic mobility of hydroxyapatite. Vogel et al. (13) studied the release of phosphate and calcium ions during the adsorption of benzene polycarboxylic acids onto apatite. Jurlaanse et al.(14) also observed a similar release of calcium and phosphate ions during the adsorption of polypeptides on dental enamel. Adsorption of polyphosphonate on hydroxyapatite and the associated release of phosphate ions was investigated by Rawls et al. (15). They found that phosphate ions were released into solution in amounts exceeding the quantity of phosphonate adsorbed. 

To evaluate + for each metal ion, values of p8 are required at each concentration. While this can often be evaluated from electrophoretic mobility data, the high ionic strengths—Le., pH < 2—preclude meaningful measurement of mobilities. However, it can be seen that when ij/s and cf)+ are equal and opposite then adsorption is reduced to zero. The adsorption of Na+ is reduced to zero at the z.p.c. since, in this case, + is negligibly small. With Ni2+ and Cu2+ the pH must be reduced—i.e., made more positive—by 1.3 pH units to effect zero adsorption. Since near the z.p.c. ips and i//0, the total double layer potential, are approximately equal and given by the Nernst Equation, then... 

Figures 3b, 4 and 5a show that this is the only mechanism by which BTA" ions are adsorbed on the silica surface. The slope of the cation-exchange curve is almost equal to unity (Fig.4a) since the effectiveness of adsorption reaches about 0.8 /imol-m . Contrary to the surfactant, the polar head is not able to reverse the surface charge from negative to positive (Fig.4b). The electrophoretic mobility scarcely depends on the amount adsorbed, except for the terminal part of the adsorption interval where its negative value decreases a little. As the pH curve shows the same tendency (Fig.3b), the final rise in the neutralization efficiency of the polar head can be ascribed to effect of the double layer compression induced by the increasing ionic strength in the bulk phase.

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... 

In Figure 2 are represented the electro-optical effect a, the electrophoretic mobility Ue, and the relaxation time r of the particle disorientation after the switching off of the electric field as a function of the initial polyelectrolyte concentration. One observes that the a and r variations correspond to the variation of f/e, i.e., the electrostatic attraction of the polyelectrolyte to the oppositely charged surface, which is the main driving force for the adsorption, governs the electro-optical behavior and stability of the suspension containing this polyelectrolyte. 

Varoqui, R., Effect of polymer adsorption on the electrophoretic mobility of colloids, Nouv. J. Chim.,... 

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