M. P. Sidorova

Electrokinetic and adsorption properties of SiO2 in polymer and surfactant solutions

Abstract A combined investigation of the electrokinetic and adsorption properties of SiO 2 in solutions of weak polyelectrolytes (poly(ethylenimine), PEI, and poly(acrylic acid), PAA) and of an uncharged surfactant (C 8 H 15 C 6 H 4 O(C 2 H 4 O) m H, OP-7) was carried out. Electro-kinetic measurements were performed on a plane-parallel fused quartz capillary, and adsorption measurements were carried out on Aerosil with a specific surface area of 175 m 2 g −1 . When OP-7 is adsorbed on SiO 2 , the surface charge and specific surface conductivity at neutral pH practically do not change. The detected decrease in the absolute value of the ζ potential in OP-7 solutions is caused by the displacement of the slipping plane into the bulk of the solution. In polyelectrolyte solutions containing polyanions, slight adsorption leads to a definite increase in the ζ potential and the surface conductivity. Specific interactions between the polycation and the SiO 2 surface is manifested in the negative surface charge increase, the displacement of the isoelectric point, the change of the sign of ζ and a decrease in the surface conductance. We show the competing adsorption of uncharged surfactant molecules and polyions in mixtures of polyelectrolyte and a surfactant.

Adsorption of potential-determining ions on porous glasses of different compositions

The adsorption characteristics are studied for nano- and ultraporous glasses (PGs) produced from sodium borosilicate glasses and a glass containing small amounts of fluoride ions and phosphorus oxide by acid (HCl) leaching and additional alkaline (KOH) and thermal treatment. The surface charges σ0 of PGs are determined by continuous potentiometric titration in 10−3−1 M NaCl, KCl, and (C2H5)4NCl solutions. Only negative surface charges of PGs are observed for all investigated systems. The |σ0| value is predetermined by the following factors: the composition of PG, the pore radius in the nanometer region (r ≤ 13 nm), the specificity of counterions, the content of secondary silica in the pore space, and the temperature of the additional thermal treatment of the membranes. The introduction of fluoride ions and phosphorus oxide into sodium borosilicate glass, an increase in the pore sizes and the amount of the secondary silica in PGs, and a rise in the specificity of counterions enhance the |σ0| values, which decrease with a rise in the temperature of the thermal treatment due to the surface dehydration and dehydroxylation. For ultraporous glasses (r > 13 nm), the surface charge is almost independent of the pore radius.

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.

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.

Stability of Tungsten(VI) Oxide Dispersions in Electrolyte Solutions

The aggregation stability of tungsten oxide hydrosols is studied by the photometric and electrooptical methods. Coagulation thresholds determined in solutions of 1 : 1, 2 : 1, and 3 : 1 electrolytes obey the Schulze-Hardy rule. The coagulation is shown to occur by the barrier mechanism predominantly through adhering of small particles to large ones without considerable changes in the sizes of the latter.

Electrosurface properties of tungsten(VI) oxide in electrolyte solutions

Electrosurface characteristics of tungsten oxide in solutions of 1: 1 electrolytes are studied in a wide range of concentration and pH values. The dependence of the position of isoelectric point on the concentration of background electrolyte and the cation nature is revealed. The equilibrium constants of surface reactions, adsorption potentials of ions, specific surface conductivities, and degrees of dissociation of surface groups in the point of zero charge are calculated.

Electrochemical properties and stability of emulsifier-free polystyrene latexes in 1: 1 electrolyte solutions

The electrokinetic and adsorption characteristics of monodisperse emulsifier-free latexes of polystyrene (particle sizes of 0.25–0.40 μm) with surface carboxyl and sulfo groups are comprehensively studied depending on pH and the concentration of background NaCl solutions. The constants of surface carboxyl group dissociation and surface complexation, as well as the adsorption potentials of OH− and Na+ ions, are calculated. The stability of latex suspensions is investigated and the coagulating concentrations of NaCl and HCl solutions are determined. An analysis of the curves plotted for the pair interaction between latex particles at different concentrations of NaCl solutions suggests that the system possesses an additional stability factor, which is probably associated with polymer chains that protrude over the particle surface into a solution.

Regularities of latex filtration through microfiltration membranes

Filtration of suspensions of emulsifier-free monodisperse polystyrene latexes with particle sizes of 0.25, 0.3, and 0.4 μm through acetylcellulose microfilters is studied as depending on the composition of liquid phases, the rate and time of filtration, and the particle-to-pore size ratios. The effect of particle-membrane interactions, which are governed by the electrostatic repulsion and molecular attraction forces, on particle rejection by membranes is considered. It is shown that, when analyzing the mechanism for the rejection, it is necessary to take into account the electrophoretic motion of particles in the field of the streaming potential arising in the course of suspension filtration.

Comparison of electrochemical characteristics of acetylcellulose microfiltration membranes and a model system

The electrokinetic properties of commercial acetylcellulose microfiltration membranes are compared to a model system, i.e., an acetylcellulose film applied to a plane-parallel capillary made of quartz glass. It is shown that the absolute magnitudes of the electrokinetic potentials of the model system are higher than those of the examined microfilters, seemingly because the latter are modified to increase their mechanical strength and elasticity. The dissociation constants of carboxyl groups and the adsorption potentials of potential-determining ions are found from the electrokinetic data on the model system.

Structural and electrokinetic characteristics of track-etched initial membranes and membranes modified with perfluorinated sulfocationic ionomer

Structural and electrokinetic characteristics of track-etched microfilters with pore radii of 1 μm and microfilters modified with perfluorinated sulfocationic ionomer are studied as functions of pH and concentration of KCl solutions. Colloidochemical properties of modified microfilters and perfluorinated sulfocationic MF-4SK membrane are compared.