Metal oxides isoelectric points

Mular, A.L. and Roberts, R.B., A simplified method to determine isoelectric points of oxides. Can. Miner. Metal. Bull., 69, 438, 1966. 

Lin, X.Y. Farhi, E., and Arribart, H., Determination of the isoelectric point of planar oxide surfaces by a particle adhesion method, J. Adhesion, 51, 181, 1995. McCafferty, E. and Wightman, J.P., Determination of the surface isoelectric point of oxide films on metals by contact angle titration, J. Colloid Interf. Sci., 194, 344, 1997. 

Hydrolysis. The surfaces of metal oxides and hydroxides can take up or release H+ or OH- ions and become chaiged. Potentials as high as 100 mV may be sustained in aqueous solutions. For aqueous solutions this is a function of the pH the zeta potential for the particle is positive if the solution pH is below the particle s isoelectric pH (pH ), and negative if the pH is above pH. Isoelectric points for metal oxides are presented in several publications (22,23). Reactions of hydroxyl groups at a surface, Q, with acid and base may be written as follows ... 

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. 

The acidic surfaces of quartz and tungsten oxide are noted in this table, as well as, the basic surfaces of alumina and magnesium oxide. It should also be noted that the method of powder fabrication is important in establishing the structure of the surface and, therefore, the isoelectric point of the powder surfiice. The lEP for a simple oxide is inversely proportional to the ratio of the valence, z, to the radius, Rcaom > of the metal cation making up the oxide. The regression equation for lEP data from Parks [53] is as follows ... 

Kosmulski M, Maczka E, Rosenholm JB. Isoelectric points of metal oxides at high ionic strengths. J Phys Chem B 2002 106 2918-2921. 

F ure 9.21. The net charge at the hydrous oxide surface is established by the proton balance (adsorption of or OH and their complexes) at the interface and specifically bound cations or anions. This charge can be determined from an alkalimetric-acidi-metric titration curve and from a measurement of the extent of adsorption of specifically adsort)ed ions. Specifically adsorbed cations (anions) increase (decrease) the pH of the point of zero charge (pzc) or the isoelectric point but lower (raise) the pH of the zero net proton condition (pznpc). Addition of a ligand, at constant pH, increases surface protonation while the addition of a metal ion (i.e., specifically adsorbed) lowers surface protonation. (Adapted from Hohl et al., 1980.)... 

Figure 10 Isoelectric points (points of zero charge, PZC) of metal oxides. (Data are presented in Parks, J. Phys. Chem., 66 (1962) 967.)...

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