Nikola Kallay

Application of mass titration to the point of zero charge determination

The mass titration method for determination of the zero point of charge (PZC) was proposed by Noh and Schwarz (J. Colloid Interface Sci. 130, 157 (1989)). This method was developed for (hydr)oxides and is limited to pure samples. It is based on the fact that pH approaches the limiting value (pH∞) by addition of solid powder to an aqueous medium. The limiting pH∞ value, at a high solid content, is then equal to pHpzc. This article is an extension of the original work and concerns the samples contaminated with an acid or base which are not irreversibly bound

The metal oxide - electrolyte solution interface revisited

Abstract The Nernstian approach to the description of the metal/electrolyte interface is compared with descriptions based on the surface complexation models. Differences are from the assumptions made about the dependence of chemical potential of the potential determining ions (p.d.i.) at the solid surface on the composition. The Nernstian approach is based on the constancy of the chemical potential of p.d.i. at/in the solid surface while the surface complexation model takes into account the variable composition of the surface through the surface complexation of a limited number of surface sites. The Nernst approach leads to a linear relationship between the surface potential and the logarithm of p.d.i. activity with slope RT(ln 10)/F, independent of ionic strength. On the other hand, values of the surface potentials calculated using the surface complexation model are always lower in magnitude and decrease as the ionic strength increases. The nature of the association of counter ions with the charged surface sites and the problems connected with the introduction of planar capacitors characterised by the constant capacitance are discussed. It is concluded that the present experimental data do not warrant the application of elaborate modelling and that further efforts to understand the nature of the electrolyte/hydrous oxide surface should focus on better and more diverse experimental studies.

Particle adhesion in model systems. Part 14.—Experimental evaluation of multilayer deposition

The deposition of uniform, spherical colloidal haematite particles on glass beads was studied as a function of the ionic strength using the packed column technique. The data were interpreted in terms of a recently developed multilayer adhesion model, which is of particular interest for systems at high ionic strength. The evaluated parameters suggest that the adhesion behaviour can be explained in terms of colloid stability and particle to particle interaction energies.

Particle adhesion and removal in model systems. XI. Kinetics of attachment and detachment for hematite—glass systems

Abstract The kinetics of deposition and detachment of colloidal hematite particles in a column packed with glass beads was examined as a function of pH and ionic strength. An expression was derived that takes into consideration the excluded area effect in the deposition process, which was then applied to the experimental data. The results showed that the excluded area was not significantly affected by the parameters studied. Since both the pH and the ionic strength control the repulsion among the adhered particles, it is apparent that their packing on the surface is not significantly influenced by the electrostatic force. The applied analysis also yields the diffusional rate constant for the attachment process, which was found to be somewhat lower than theoretically predicted. An approach was developed which takes into account the distribution of rate constants for the detachment of adhered particles. Such an analysis allows for a more realistic account of observed phenomena, since the deposited particles are not bound to the surface with equal strength.

Adsorption at solid/solution interfaces II. Surface charge and potential of spherical colloidal titania

Abstract Electrophoretic mobilities and the adsorption of potential determining ions were measured on a sample of colloidal anatase (TiO 2 ) consisting of spherical particles of narrow size distribution, dispersed in aqueous solutions of varying pH at constant concentrations of NaNO 3 . It was found that the isoelectric point (IEP) was at pH 4.4 and the point of zero charge (PZC) at pH 6.5. This difference indicates that certain anions in addition to OH − were responsible for the surface charge of TiO 2 . An analysis was developed which made it possible to evaluate the essential parameters (surface potential as a function of pH, the total number of surface-active sites, the equilibrium constants for the adsorption of potential determining ions, and the PZC of the sample in its pure form) characterizing the surface properties without the need to assume a double-layer model.

Solubility and enthalpy of precipitation of magnesium, calcium, strontium, and barium dodecyl sulfates

The solubilities of Mg, Ca, Sr, and Ba dodecyl sulfates in excess of either of the precipitation components (metal nitrate or sodium dodecyl sulfate) have been analyzed and correlated with the calorimetric data. It was shown that the precipitation boundaries could be quantitatively explained if the interactions of all ionic species were taken into consideration. Specifically, the appropriate activity coefficients and the counterion bondings by the micelles were introduced in the expressions describing the solubility boundaries. The values of the solubility products, and of enthalpies and entropies of precipitation, for the four salts are evaluated and reported.

Particle adhesion and removal in model systems: X. The effect of electrolytes on particle detachment

Abstract The effect of neutral electrolytes on particle detachment is analyzed theoretically and experimentally. It was shown that the removal of spherical colloidal hematite particles from glass surface in basic media is enhanced with increasing concentration of NaNO3. Under these conditions both surfaces (of particles and of glass) are negatively charged. This finding could be explained if the double layer energy for the case of constant potential is combined with Dahnekes expression for particle escape. The opposite effect is predicted if the constant charge model is assumed. This analysis indicates that the relaxation of the double layer during the particle separation is responsible for the observed phenomena.

pH and the surface tension of water.

Despite the strong adsorption of hydroxide ions, the surface tension of water is almost independent of pH between pH 1 and 13 when the pH is adjusted by addition of HCl or NaOH. This is consistent with the Gibbs adsorption isotherm which measures the surface excess of all species in the double layer, if hydronium ions and hydroxide ions are adsorbed and sodium and chloride ions are not. The surface tension becomes pH dependent around pH 7 in millimolar NaCl or KCl solutions, for now sodium ions can replace hydronium ions as counterions to the adsorbed hydroxide ions.

A method for continuous preparation of uniform colloidal hematite particles

Abstract A continuous method for the preparation of uniform colloidal metal (hydrous) oxide particles by thermal hydrolysis is described. A solution of proper concentration passes through a thermostated column at a rate which ensures required reaction time. Spherical hematite particles, with a diameter of 0.080 ± 0.013 μm were generated by this technique. The sol so obtained is comparable to that prepared by the batch technique.

Particle adhesion and removal in model systems. Part 6.—Kinetics of deposition of haematite particles on steel

The kinetics of adhesion of uniform colloidal spherical particles of haematite on steel was studied as a function of pH, ionic strength and particle size. It was shown that deposition occurs when the collector and the particles have charges of opposite sign and that the rate of adhesion can be greatly affected by the extent of surface coverage by the adhered particles. The deposition of solids of the same charge as the substrate can be induced if a sufficient amount of an indifferent electrolyte is added. Data were interpreted in terms of existing theories of diffusional deposition; the model of Ruckenstein and of Pfeffer and Happel explained the adhesion of small particles (0.15 µm in diameter). The effect of the electrolyte was considered by using the interaction-force boundary-layer concept combined with the double-layer and dispersion terms.