Electrokinetics isoelectric point

Electrokinetic measurements consisted of measuring the viscosity with and without NaGl (Carlo Erba, Argentine) (Figures 3-a and 3-b), while the isoelectric point (figure 2) and zeta potential (figure 3-c) were measured at different pH (HCl Ciccarelli and NaOH Tetrahedrom, Argentine). 

Modifications of surface layers due to lattice substitution or adsorption of other ions present in solution may change the course of the reactions taking place at the solid/liquid interface even though the uptake may be undetectable by normal solution analytical techniques. Thus it has been shown by electrophoretic mobility measurements, (f>,7) that suspension of synthetic HAP in a solution saturated with respect to calcite displaces the isoelectric point almost 3 pH units to the value (pH = 10) found for calcite crystallites. In practice, therefore, the presence of "inert" ions may markedly influence the behavior of precipitated minerals with respect to their rates of crystallization, adsorption of foreign ions, and electrokinetic properties. 

If the solid diaphragm material adsorbs both hydrogen and hydroxyl ions it is evident that electric endosmose will cease when equal ionic adsorption has taken place, the double layer potential or electrokinetic potential being at this point zero and the diaphragm is at the isoelectric point. 

The reaction finished within 1 h at 26°C.. They used seed crystals of CdS to promote the uniformity of the final product, and analyzed the growth kinetics using Nielsen s chronomal. The isoelectric point in terms of pH was determined to be 3.7 by electrokinetic measurement. They also prepared zinc sulfide (ZnS polycrystalline spheres), whose isoelectric point in pH was 3.0 (2), lead sulfide (PbS monocrystalline cubic galena) (3), cadmium zinc sulfide (CdS/ZnS amorphous and crystalline spheres) (3), and cadmium lead sulfide (CdS/PbS crystalline polyhedra) (3), in a similar manner. 

On calcination of this prepared powder, particles having the composition ZrY(, k03.2 were obtained. The electrokinetic measurements with aqueous dispersions of the latter showed an isoelectric point at pH 6.8, characteristic of Y203. This example further substantiates the inhomogeneity within the particles, but also indicates that heating, as carried out in this case, did not produce internal uniformity. 

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. 

Two important parameters describing the EDL of a mineral are the point of zero charge (PZC) and the isoelectric point (IP). Healy et al.18) define the PZC as the concentration of PDI with the surface charge of a mineral metal oxides, PZC is determined by the concentration of PDI H+ or OH", in sparingly soluble salts by the concentration of PDI of the lattice. When both mechanisms of surface charge formation operate simultaneously, both ion species and their reaction products determine the PZC16,31). The IP is defined18) as the concentration of PDI at which the electrokinetic potential = 0. 

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. 

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-... 

The results obtained proved that very good correlations exist between the value of electrokinetic potential, HPLC results, XPS measurements, contact angle values and changes in the isoelectric point (IEP). 

Also, the enzymatic treatment causes a change in the isoelectric point (IEP) value of wool, as demonstrated by the electrokinetic measurements (Fig. 5). With an increase in enzyme concentration the IEP value shifts in the direction of more acidic pH (from 3.8 for untreated wool to 3.7 for wool after 1% enzymatic treatment, and 3.45 for wool... 

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. 

Isoelectric points require electrokinetic experiments as a function of pAg. pH, etc. which will be discussed in sec. 4.4. Theoretical problems are all but absent because phenomena like surface conduction and relaxation retardation vanish as 0. However, experimental problems may arise because the systems become... 

The isoelectric point (lEP), a characteristic property of proteins, is defined as the pH where the electrokinetic potential is zero or that pH where no migration occurs in an electric field 2, 3, 4). For gelatins the... 

Isoelectric Point The solution pH for which the electrokinetic, or zeta, potential is zero. See also Point of Zero Charge. 

The pH value at which the oxide surface carries no fixed charge, i.e. Oj = 0, is defined as the point of zero charge (PZC) . A closely related parameter, the isoelectric point (lEP), obtained from electrophoretic mobility and streaming potential data, refers to the pH value at which the electrokinetic potential equals to zero The PZC and lEP should coincide when there is no specific adsorption in the iimer region of the electric double layer at the oxide-solution interface. In the presence of the specific adsorption, the PZC and lEP values move in opposite directions as the concentration of supporting electrolyte is increased. ... 

Two parameters were introduced into the description of double electrical layer. One of them is the point of zero charge (PZC) which according to lUPAC definition [101] can be expressed as concentration of potential-determining ions PDI at which the surface charge is equal to zero ( o = 0), as well as the surface potential (V>o = 0). Another parameter is isoelectric point (lEP) defined [101] as concentration of PDI at which the electrokinetic potential is equal to zero (( = 0). 

Determination of the isoelectric point by electrokinetic methods is simple. The accuracy depends on control of the conditions such as pH and in the case of electrophoresis, on the positioning of the stationary layer [5,37]. A shortcoming of common methods is that one cannot characterise conductive samples. 

There is another possibility which was applied for interpretation of adsorption of organic ions [34,74]. One measures the effect of pH on the adsorption of cobalt ions at their different total concentrations. Simultaneous electrokinetic measurements provide the concentrations at isoelectric point so that each experimental run would yield the adsorption amounts and cobalt equilibrium concentration at zero electric potential. Regardless to different... 

With smooth, nonporous surfaces the zero-point of charge and the isoelectric point usually do not differ much from each other. However, when porous particles, e.g., of activated carbons, are measured, the surface of the grains or particles may be acidic in character due to ageing while the internal surface is stiU basic. As mentioned before, aging in narrow pores is very slow due to diffusion restrictions. The electrokinetically measured lEP is determined by the -potential of the particle surface while the PZC is determined by the much larger interior surface of the particles [21]. 

Parks [1] reports lEPs of Fe(OH)2 obtained by electro-osmosis after [1272] (or its abstract in CA). The result from [1] was then cited in numerous studies. In fact, [1272] (in Polish, with extended abstract in German the present author is a native speaker of Polish) reports the degree of oxidation of Fe(OH)2 in air at 18°C and the Fe(iii) phases formed at various pH values. The term isoelectric point was indeed used, but the value (pH 11.5-12.4) reported in [1272] was obtained as an inflection point in the degree of oxidation (pH) curve, and is unrelated to the IEP obtained from electrokinetic measurements. 

The kinetic potential is usually denoted as the zeta (0 potential and it is determined from the electrophoretic mobility of the extremely dilute particles in an electric field. More recently, the nse of electrokinetic sonic amplitude (ESA), acoustosizer (AZR), or colloid (or ultrasonic) vibration potential (CVP) has become available for the determination of the potential in rather concentrated particle suspensions. Again the potential may be measured as a function of either the metal concentration or the pH. In the latter case the point where the mobility ceases is denoted the isoelectric point (pH,Ep Fignre 8.27). It correlates particnlarly well with the stability of the sol. 

The silica sols, and indeed all oxide sols, show an increasing negative zeta potential with increasing pH as the pH is raised above the pHiep. The magnitude of the zeta potential decreases uniformly at each pH as the salt concentration is increased. There are subtle effects in the electrokinetics as the counterion is varied from Li+ to Cs+, but these effects are minor compared with the general reduction in zeta potential as the pH is moved toward the isoelectric point or as 1 1 electrolyte is added at any pH. 

This finding is consistent with the rule of thumb of electrokinetics that the isoelectric point of a mixed surface is the surface-area-weighted average of the isoelectric point values of the components (15). Hence it appears that this surface concentration regime corresponds to the completion of silica monolayer surface coverage of titania particles. This... 

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