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Nikola Kallay

Publications and source records attributed to Nikola Kallay.

5 recordsLinked to original sources
PubMed2006

Determination of surface potentials from the electrode potentials of a single-crystal electrode.

Determination of surface potentials Psi0 at the metal oxide/aqueous solution interface from measured electrode potentials of a metal oxide single-crystal electrode (SCrE) is described. The proposed method is based on the surface complexation model and evaluates the surface potential at the isoelectric point, i.e., at pHiep. This value is used for calculation of Psi0 values from the measured electrode potentials. Both 1-pK and 2-pK models produced the same result so that the procedure does not depend on the assumed mechanism of the surface charging. It is proposed to determine the pristine point of zero charge pHeln and the isoelectric point pHiep, and use these data to set the scale of surface potentials. The value of pHeln can be obtained at a sufficiently low ionic strength where pHpzc coincides with pHiep. The method is demonstrated on the example of the anatase single-crystal electrode. From the shifts of pHiep and pHpzc with respect to the pristine point of zero charge pHeln it was concluded that Cl- ions exhibit higher affinity for association with positively charged surface groups than ClO-4 ions. Also, preferential surface association of Na+ cations compared to both anions was detected. The slopes of the Psi0(pH) functions were found to be significantly lower in magnitude with respect to the Nernst equation, which is due to the high degree of counterion association at the surface caused by their relatively high concentration.

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PubMed2006

Surface potential of hematite in aqueous electrolyte solution: hysteresis and equilibration at the interface.

Electrostatic potential at the inner plane of the hematite aqueous interface, i.e., surface potential, was measured by means of a single-crystal hematite electrode. Acidic solutions were titrated with base and then back-titrated with acid. Surface potentials were evaluated from electrode potentials by setting the zero value at the isoelectric point. In the case of fast titrations the equilibration time was approximately 10 min, and significant hysteresis was obtained, more pronounced at higher electrolyte concentrations. Hysteresis disappeared in slow titration runs when the equilibration time was extended up to 120 min, and also when ultrasound was applied. Hysteresis was observed in the pH region close to neutrality, where the concentrations of potential-determining H+ and OH- ions are low. Equilibration was fast in acidic and basic regions. These results are explained on the basis of the kinetics of surface reactions, supported by the following rate of single-crystal electrode equilibration. It is concluded that the equilibration rate at the interface is specific for a given system and is not a general phenomenon. As several systems may undergo fast equilibration, such data may be regarded as equilibrium data and interpreted by the surface complexation model. In other cases, one should perform kinetic tests and apply extended equilibration times.

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PubMed2002

Stability of nanodispersions: a model for kinetics of aggregation of nanoparticles.

In the course of aggregation of very small colloid particles (nanoparticles) the overlap of the diffuse layers is practically complete, so that one cannot apply the common DLVO theory. Since nanopoarticles are small compared to the extent of the diffuse layer, the process is considered in the same way as for two interacting ions. Therefore, the Brønsted concept based on the Transition State Theory was applied. The charge of interacting nanoparticles was calculated by means of the Surface Complexation Model and decrease of effective charge of particles was also taken into account. Numerical simulations were performed using the parameters for hematite and rutile colloid systems. The effect of pH and electrolyte concentration on the stability coefficient of nanosystems was found to be more pronounced but similar to that for regular colloidal systems. The effect markedly depends on the nature of the solid which is characterized by equilibrium constants of surface reactions responsible for surface charge, i.e., by the point of zero charge, while the specificity of counterions is described by their association affinity, i.e., by surface association equilibrium constants. The most pronounced is the particle size effect. It was shown that extremely small particles cannot be stabilized by an electrostatic repulsion barrier. Additionally, at the same mass concentration, nanoparticles aggregate more rapidly than ordinary colloidal particles due to thier higher number concentration.

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