L. E. Ermakova

Structural transformations in thermally modified porous glasses

The changes in the structural parameters of porous glasses (pore radius, pore volume, specific surface of pores, structural resistance coefficients) upon heating are investigated as a function of the composition of initial two-phase alkali borosilicate glasses with the use of a number of independent methods, such as the adsorption techniques (water vapor adsorption, mercury porosimetry, thermal desorption of nitrogen), transmission electron microscopy, small-angle X-ray scattering, and membrane conductivity measurements. It is demonstrated that the structural transformations in thermally modified porous glasses are associated with the processes of overcondensation of pores and viscous flow in the silica network.

Adsorption and electrokinetic characteristics of micro- and macroporous glasses in 1:1 electrolytes

Abstract The influence of pore size on adsorption and electrokinetic characteristics of macroporous (pore radius r =13−160 nm) glasses in 1:1 electrolytes were investigated. The obtained results were compared with parameters of microporous glasses. The influence of electroviscous effect on liquid filtration rate was observed for macroporous glasses in contrast to microporous. The adsorption of potential-determining ions, obtained by a dynamic method, had a tendency to increase with pore size growth in result of both adsorption kinetics and electrical double layer (EDL) overlapping influence for microporous glasses. The values of surface reaction constants and ion adsorption potentials were calculated from adsorption data. These values were typical for a silica surface and practically did not depend on pore size. The counter-ion transport numbers and the contribution of surface conductance in the conductance of pore liquid decrease with an increase in pore size and electrolyte concentration and was due to diminishing of the EDL influence. The electrokinetic potentials of macroporous glasses, calculated with correction for EDL overlapping and surface conductance, did not depend on pore size at a constant surface charge. The isoelectric point of micro- and macroporous glasses in indifferent NaCl solutions correspond to pH IEP =1.1. The adsorption and electrokinetic data were used to calculate the values of total end electrokinetically mobile charge, the convective component of surface conductance, the Donnan and EDL potentials, the ion concentration and mobilities in pore solution in the framework of the homogeneous and heterogeneous models. The analysis of experimental and calculated parameters make it possible to propose that, besides Hittorfs, there are other mechanisms of surface conductance, connected with protonic conductance of undissociated silanol groups and with contribution of molecularly sorbed electrolyte in the IEP region.

Electrochemistry of weakly charged membranes

Measurements of the structural and electrochemical parameters of reverse-osmosis, ultrafiltration and microfiltration membranes were carried out in solutions of 10−1-10−4 mol/l NaCl. It was found that the ion exchange capacity did not exceed 10−5 mol/g in neutral solutions. The measured transport numbers of counterions for ultrafilters and reverse-osmosis membranes are close to unity in a 10−4 mol/l solution and decrease with increasing electrolyte concentration. The electrokinetic potentials of various membranes do not exceed −30 mV taking into account the overlap of the electrical double layer. The structural and electrochemical parameters of the selective layer of asymmetric membranes were found from a comparison of adsorption and electrokinetic measurements. The electrical potentials of the membranes were used to calculate the electrostatic component of the rejection coefficient.

Colloido-chemical characteristics of porous glasses with different compositions in KNO3 solutions. 1. Structural and electrokinetic characteristics of membranes

The structural (specific surface area, liquid-filtration coefficient, average pore radius, volume porosity, and structural-resistance coefficient) and electrokinetic (counterion transport numbers, specific electrical conductivity, and electrokinetic potential) characteristics of porous glasses with different compositions have been determined in potassium nitrate solutions with concentrations of 10−3–10−1 M. All the membranes under investigation have been shown to exhibit the dependences of efficiency coefficients and counterion transport numbers on electrolyte concentration and pore size that are predicted by the theory of an electrical double layer. It has been established that, at a constant electrolyte concentration, the absolute values of electrokinetic potential increase with the average pore radius because of variations in the slipping-plane position.

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.

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.

Electrochemistry of porous glass membranes in electrolyte solutions

A comparative study of the influence of pore radius, concentration, counter-ion (H+, Na+, K+, Ba2+, La3+) charge on electrochemical properties of porous glass (PG) membranes with pore radii 1.3–13.2 nm was carried out by the methods of potentiometric titration, streaming and membrane potential and conductance. “Secondary” silica was found in pores for all membranes except for PG-13.2. This showed up in a concentration dependence of structural resistance coefficient, a non constancy of liquid velocity at constant pressure and a difference between total and through porosity. It was shown that the surface charge of PGs increased with the growth of counter-ion charge, pH and electrolyte concentration. Surface reaction constants, calculated from adsorption data were within the range of values reported in the literature for silanol groups. Cation transport numbers and efficiency coefficients, which characterise the contribution of EDL, decreased with increase in electrolyte concentration, pore radius and cation charge. The PG specific surface conductance, was of the order of 10−10 mhO. Electrokinetic potentials ζ∗ of PG-13.2 calculated with correction for EDL overlapping showed a normal concentration dependence, and ζ∗ was close to the ζ-potentials usually observed for flat glass surfaces.

Colloido-chemical characteristics of nanoporous glasses with different compositions in solutions of simple and organic electrolytes. 1. Structural and transport characteristics of membranes

The regularities of variations in the structural (bulk porosity, structural resistance coefficient, and average pore radius) and transport (specific electrical conductivity and ion transport numbers) characteristics of nanoporous glass membranes with different compositions have been studied as depending on their preparation conditions and the type of electrolyte (sodium, potassium, ammonium, tetramethylammonium, and tetraethylammonium chlorides). An additional alkaline treatment of the porous glasses, which increases the average pore radius and modifies the morphology of pore channels, leads to variations in the electrotransport characteristics of the glasses. In solutions containing inorganic counterions, the dependences of the transport characteristics of the porous glasses on the solution concentration and pore size have a classic character. The high specificity of tetraalkylammonium ions in relation to the surface gives rise to a region of negative values of the specific surface conductivity and complex dependences of efficiency coefficients and transport numbers on electrolyte concentration and pore channel size. The duration of the thermal treatment of initial sodium-borosilicate glass containing small amounts of phosphorus oxide and fluoride ions, as well as the conditions of its leaching, has almost no effect on the properties of the membranes in tetraethylammonium chloride solutions.

Colloido-chemical characteristics of porous glasses with different compositions in KNO3 solutions. 2. Electrosurface characteristics of porous glass particles

The surface charge and electrophoretic mobility of particles of porous glasses with different compositions have been studied in potassium nitrate solutions with concentrations of 10−3–10−2 M. It has been shown that the surface charge in KNO3 solutions virtually coincides with the charge in KCl solutions and has negative values in the studied pH range of 4–9. The concentrations and mobilities of ions in glass pores, Donnan potentials, and the convection component of electrical conductivity have been calculated within the framework of a homogeneous model. Electrokinetic potentials of the glasses under investigation have been calculated with regard to the effect of the electrolytic conductivity of the porous particles and electroosmotic transport in the pore space on the electrophoretic mobility.