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Point of Zero Charge and Surface Charge Density of TiO2 in Aqueous Electrolyte Solution as Obtained by Potentiometric Mass Titration

Mass titration method was developed as a suitable tool for determination of the point of zero charge (p.z.c.) and surface charge density ( 0) of metal oxide colloid particles at different ionic strengths. In the course of mass titration, subsequent portions of a metal oxide powder are added to an aqueous electrolyte solution, and pH of the equilibrated dispersion is measured. The pH of the system changes gradually and approaches a constant value of pH∞ , which is in the case of a metal oxide free of impurities equal to the point of zero charge pHpzc. Counterion association shifts the pHpzc either to the acidic region (preferential adsorption of cations) or to the basic region (preferential adsorption of anions). Mass titration therefore enables detection of the difference between association affinities of counterions (cations and anions), which is important information about the equilibrium within the electrical interfacial layer. Such an analysis is not possible by the conventional acid-base potentiometric titration of the dispersion, since the location of the p.z.c. is based on the common intersection point (c.i.p.) of the data obtained at different ionic strengths. It is shown that c.i.p. may be used for locating the p.z.c. only in a “ symmetric case“ , when affinities of the anions and cations to associate with oppositely charged surface groups are equal. Analysis of the mass titration was performed on the basis of experimental data obtained with colloidal titania dispersed in an aqueous sodium chloride solution, and also by numerical simulation based on the Surface Complexation Model and the 2-pK mechanism of surface reactions. Increase in the NaCl concentration shifted the point of zero charge to the basic region, while the isoelectric point was shifted to the acidic region, indicating higher association affinity of chloride ions compared to sodium ions.

mass titration; point of zero charge; surface charge density; counterion association; titan dioxide

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