N. F. Bogdanova

Electrosurface characteristics of titanium dioxide in solutions of simple electrolytes: I. Effect of nature of counterions on adsorption and electrokinetic parameters of TiO2

The adsorption and electrokinetic characteristics of different titanium dioxide samples (produced by the Merck Co. and synthesized by the sol-gel method) are studied depending on the pH, background electrolyte concentration, and the nature of counterions (halide ions and Na+, K+, Ba2+, and La3+ metal ions). It is revealed that, in the presence of an indifferent electrolyte, the points of zero charge (PZC) for the synthesized TiO2 sample and the Merck sample correspond to pH = 6.0 ± 0.1 and 5.0, respectively. It is found that the nature of halide ions has almost no influence on the magnitude of the TiO2 surface charge σ0 (in the region of its positive values) and the position of PZC. An increase in the specificity of cations with a rise in the charge causes PZC shift to the acidic region and enhances the absolute values of σ0 at both negative and positive surface charges. It is established that the positions of PZC and isoelectric point in 10−2 M solutions of the examined 1: 1 electrolytes nearly coincide with one another. The ζ potential is found to decline in the following series of counterions: Cl−, Br−, and I− due to an increase in the degree of filling of the dense part of the electrical double layer.

Influence of the structure of boundary layers and the nature of counterions on the position of the isoelectric point of silica surfaces

Dependences of electrokinetic potentials of different silica materials (nano-and ultraporous glasses, a quartz glass plane-parallel capillary, and monodisperse spherical particles of silicon oxide) on the pH of solutions containing single-, double-, and triple-charged cations have been compared. It has been shown that the degree of hydration of a single-charged cation and the structure of an interface substantially affect the position of the isoelectric point (IEP). The most hydrated Na+ ions have virtually no effect on the position of the IEP up to their concentration of 0.1 M irrespective of the thickness of an ion-permeable layer at the surface of a solid phase. A reduction in the radius of a hydrated cation (K+, Cs+) enables its penetration into an ion-permeable layer and, as a consequence, causes the IEP to shift toward larger pH values depending on the parameters of this layer. Two IEPs are observed in LaCl3 solutions: one at a pH value close to pHIEP in NaCl solutions and another at a higher pH value corresponding to the charge reversal of the Stern layer.

Electrokinetic and adsorption characteristics of (hydr)oxides and oxide nanostructures in 1:1 electrolytes

Abstract The electrokinetic and the adsorption characteristics of silica, titanium oxide, aluminium and ferric hydroxides were studied as a function of pH on the background of 1:1 electrolytes. Surface reaction constants and ion adsorption potentials were calculated within the framework of 2-p K model. The oxide Si-, Al-, Ti- and Fe-oxo-layers were synthesized onto silica substrates by the molecular chemical modification from the gas phase. The surface parameters of oxide nanostructures were compared with characteristics of substrate and corresponded (hydr)oxides. It was shown that the properties of oxide nanostructures are not dependent on the type of silica substrate. It was also obtained that the isoelectric point (IEP) of oxide nanostructure and the corresponding metal (hydr)oxide have different values except for Al.

Electrosurface Properties of Hydr(oxides) and Oxide Nanostructures in 1 : 1 Electrolyte Solutions: 2. Electrokinetic Characteristics of Boehmite, Goethite, and Silicon Dioxide

The electrophoretic mobility of particles of silicon dioxide, aluminum hydroxide, and iron hydroxide was measured as a function of pH in NaCl and KCl background solutions. Isoelectric points for the investigated objects were determined. Electrokinetic potentials were calculated with allowances for the particle shape and the polarization of the electrical double layer within the framework of the Overbeek–Boes–Wiersema model.

Electrosurface characteristics of titanium dioxide in solutions of simple electrolytes: II. Calculation of electrical double layer parameters of TiO2 from adsorption and electrokinetic measurements

A set of experimental data is used to calculate parameters of a 2-pK model for charging titanium dioxide surface in NaCl solutions, as well as the capacities, charges, and potentials of electrical double layers in terms of different models. It is shown that, according to the Graham model, good agreement between the experimental and calculated surface charges can only be achieved at variable capacity of the second capacitor.

Adsorption of Ions on the Surface of Tin Dioxide and Its Electrokinetic Characteristics in 1 : 1 Electrolyte Solutions

Adsorption and electrokinetic characteristics of tin dioxide (cassiterite) are studied as a function of pH in aqueous NaCl and KCl solutions. The dissociation constants of surface groups, formation constants of ion pairs, and the adsorption potentials of ions are calculated in terms of the 2-pK model; capacities and potentials of the electrical double layer are determined; the dissociation degree of surface groups in the point of zero charge and in the isoelectric point are also estimated. It is shown that the 2-pK model in combination with the Graham model can be applied in the studied systems for describing the experimental results.

Electrokinetic Properties of Aluminum Oxide Nanodispersions in NaCl Solutions

Using a Zetasizer Nano analyzer, electrophoretic mobility and particle sizes are studied as depending on the pH (3–11) and concentration (10−3–10−1 M) of NaCl background solutions for two samples of aluminum oxide, namely, γ-oxide of the Degussa brand and a sample synthesized by aluminum chloride hydrolysis. The values of the electrokinetic ζ potential are calculated through the Smoluchowski equation and with allowance for the effect of the polarization of electrical double layer on the electrophoretic mobility within the framework of the Overbeek-Boes-Wiersema model (ζW). It is established that, in the region of the isoelectric point (IEP), at (pH-pHIEP) = ±1.5, the |ζW|-pH dependences are linear and their slopes are almost equal for both aluminum oxide samples. The difference between the positions of IEPs for γ-oxide and synthesized oxide sample may be related to different degrees of their surface hydration.

Aggregation stability of a positively charged γ-Al2O3 sol prepared from an air-dry nanopowder

Photometric data on the aggregation stability of Al2O3 hydrosols prepared from Aeroxide Alu C nanopowder in NaCl solutions at pH 4.5 and 5.5 have been discussed within the framework of the generalized Derjaguin-Landau-Verwey-Overbeek theory. Analysis of the pair interaction potentials in an ensemble of particles of a sol primordially containing both primary nanoparticles and their aggregates has led to the conclusion that the coagulation proceeds via the barrierless mechanism in the secondary potential minimum.

Tin– and Titanium–Oxygen Nanostructures on the Various Hydr(oxides) Surfaces

Tin– and titanium–oxygen nanostructures were synthesized by molecular layer-by-layer deposition from the gaseous phase onto various substrates (SnO2, SiO2, AlOOH) using (in the course of synthesis) hydroxyl and methoxy surface groups. The electrokinetic (electrokinetic potential, isoelectric point) and adsorption (adsorption of potential-determining ions, the point of zero charge) properties of prepared composites were studied as a function of pH of NaCl background solutions. Synthesis conditions that make it possible to obtain (on the boehmite substrate) nanostructures with electrosurface characteristics similar to the properties of corresponding bulk oxides were determined.

Electrosurface Characteristics of (Hydr)oxides and Oxide Nanostructures in 1 : 1 Electrolyte Solutions. 3. Calculation of the Electrical Double Layer Parameters for Boehmite, Goethite, and Silicon Oxide According to Adsorption and Electrokinetic Data

The charge components, potentials, and capacities of the electrical double layer are calculated for aluminum, iron, and silicon (hydr)oxides on the basis of adsorption and electrokinetic measurements conducted in NaCl and KCl solutions. The dissociation degrees of surface groups in the point of zero charge and in the isoelectric point are estimated on the basis of the surface reaction constants. The 2-pK model in combination with the Graham model is shown to be applicable for describing the experimental results obtained for the systems in question.