The Effect of Thermal Treatment on the Structural and Electrokinetic Properties of Porous Glass Membranes

The properties of porous glass membranes prepared by acid leaching of sodium borosilicate glasses 8B and 8V and also 8B glass containing small amounts of fluorine and phosphorus (SFP) are comprehensively studied. The effect of the composition and conditions of thermal treatment of the original and porous glasses on their structural (specific surface area, structure resistance coefficient, average pore radius, volume porosity, and filtration factor) and electrokinetic characteristics (conductivity, counterion transport numbers, and electrokinetic potential) in KCl solutions at neutral pH values is studied. It is shown that an increase in thermal treatment temperature T _TT of the porous glasses from 120 to 750°C leads to a decrease in structure resistance coefficient β of 8B membranes. For membranes prepared from SFP glass, β values, efficiency coefficients, and counterion transport numbers are virtually independent of T _TT at 120–600°C and increase at T _TT = 750°C. Specific surface area and volume porosity decrease with a rise in T _TT for all studied membranes. The observed regularities of variations in the membrane characteristics are explained by the increasing fraction of large pores because of sintering of small pores with an increase in T _TT and by the different amounts of secondary silica in the pore space of porous glasses.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 3, 2005, pp. 299–307.Original Russian Text Copyright © 2005 by Volkova, Ermakova, Sidorova, Antropova, Drozdova.     

Equilibrium and Transport Characteristics of Perfluorinated Cation-Exchange Membranes Containing Carboxyl Groups

Equilibrium (the exchange capacity and moisture content) and transport (the ion transport numbers and the electrical conductance) properties of perfluorinated cation-exchange membranes containing carboxyl groups were studied as a function of pH and concentration of KCl solutions (10^–4–1 M). Dissociation constants of carboxyl groups, adsorption potentials of K⁺ ions, transport numbers and mobilities of counterions in the membranes, concentrations of fixed ions, co- and counterions, as well as the Donnan potentials were calculated.     

Preparation of ultra-and nanoporous glasses and study of their structural and electrokinetic characteristics in 1: 1 electrolyte solutions

Nano-and ultraporous glass membranes with pore radii of 4.5–150 nm are prepared from sodium borosilicate glasses of various compositions. Structural parameters (structure resistance coefficient, volume porosity, and filtration factor) and electrokinetic characteristics (conductivity, counterion transport numbers, and electrokinetic potential ζ _α ^* ) of membranes are determined at various KCl and NaCl solution concentrations (10^?4?10^?1 M) in a neutral pH region. The passage from nano-to ultraporous glasses is accompanied by an increase in |ζ _α ^* | values, which is apparently related to a decrease in the thickness of a gel layer due to the removal of ion-permeable secondary silica from pore channels. The comparison of electrokinetic characteristics of glass membranes (counterion transport numbers, efficiency coefficients, and electrokinetic potentials) measured in NaCl and KCl solutions indicates a higher specificity of K⁺ counterions as compared to Na⁺ ions.     

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.     

Colloido-chemical characteristics of nanoporous glasses with different compositions in solutions of simple and organic electrolytes. 2. Equilibrium electrochemical characteristics of membranes

The regularities of variations in the electrokinetic potential and surface charge of nanoporous glass membranes with different compositions have been studied as depending on the type of an electrolyte (sodium, potassium, ammonium, tetramethylammonium, and tetraethylammonium chlorides) and the structure of pore space. It has been shown that, in solutions containing specifically sorbed organic counterions, the range of positive values of electrokinetic potential arises due to the superequivalent absorption of counterions in the Stern layer. It has been found that the influence of the specific adsorption of counterions on the electrokinetic potential of porous glasses increases with the amount of secondary silica in the pore space. The effects of the counterion specificity, pore channel sizes, and composition of a porous glass on the value of the surface charge have been analyzed. The absolute value of the surface charge has been shown to significantly increase in the presence of organic counterions in comparison with inorganic ions throughout the examined range of background electrolyte concentrations.     

Electrokinetic Phenomena in Sulfonated Cation-Exchange Membranes with Tetraalkylammonium Ions

The electric transport of solvent ions and molecules through membrane systems containing cations of tetraalkylammonium bases is studied. The interrelation between electrokinetic characteristics of sulfonated cation-exchange membranes at different degrees of their saturation with tetrabutylammonium ions and the content of water in membranes at equilibrium is established. A new type of conductor-insulator percolation transitions is revealed for homogeneous MF-4SK perfluorinated membranes saturated with organic counterions. This transition is interpreted with allowance for the concept of the governing role of narrow channels in the membrane microstructure. The electroosmotic permeability of MF-4SK membranes in solutions of sodium and tetraethylammonium chloride, as well as in their mixtures, is studied. An experimentally observed abnormally high amount of water transferred by tetraethylammonium ions is discussed with allowance for the dynamic hydration characteristics of ions.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 4, 2005, pp. 485–493.Original Russian Text Copyright © 2005 by Kononenko, Berezina, Shkirskaya.     

Structural and Electrosurface Properties of Porous Glasses of Various Composition in 1 : 1 Electrolyte Solutions

Complex studies of structural (the specific surface area, the volume porosity, the structural resistance coefficient, and the average pore radius), adsorption and electrokinetic (the electrical conductivity, the ion transport numbers, and the electrokinetic potential) characteristics as functions of pH and the concentration of KCl solution were carried out on porous glasses (PGs) with or without lead oxide and leached under various conditions. It was established that temperature of the leaching solution affects the colloidochemical parameters of PGs, while the addition of salt to the leaching solution exerts practically no influence on the PG behavior. It was shown that the addition of lead oxide results in the formation of membranes with thinner pores and higher surface charge.     

Electrokinetic Characteristics of Porous Glasses in Solutions of Sodium and Iron(III) Chlorides

The structural (structural resistance coefficient, bulk porosity, average pore radius, and specific surface area) and electrokinetic (surface conductivity and electrokinetic potential) characteristics of high-silica micro- and macroporous glasses produced from two-phase sodium borosilicate glass have been compared in solutions of an indifferent electrolyte (sodium chloride) and iron(III) chloride containing iron ions, which have a high specificity to silica surfaces. It has been shown that, in the presence of iron ions, the electrokinetic behavior of porous glasses is governed by two factors. First, the superequivalent adsorption of these ions in the Stern layer leads to positive values of the electrokinetic potential, and, second, their mobility in the pore space decreases, thereby resulting in the appearance of equilibrium solution concentration ranges, in which the specific conductivity of a pore solution becomes lower than that of a free solution.     

Electric transport properties of ultra-and nanoporous glasses in electrolyte solutions

Electric transport characteristics (conductivity, specific surface conductance, and transport numbers of counterions) for nano-and ultraporous glass membranes with pore radii of 1.3–160 nm are studied and compared for chloride solutions containing single-, double-, and triple-charged cations.     

Colloid chemical characteristics of nanoporous glasses with different compositions in solutions of simple and organic electrolytes. 3. Calculation of electrochemical characteristics of membranes in terms of a homogeneous model

The electrosurface characteristics of nanoporous glass membranes–ion concentrations in pores with taking into account the specificity of counterions, electrokinetically mobile charge, the convective component of pore solution electrical conductivity, electroosmotic mobility of a liquid in the field of streaming potential and ion mobilities in pore space–were calculated within the homogeneous model. The effects of the type of counterion (sodium, potassium, ammonium, tetramethylammonium, and tetraethylammonium ions), solution concentration, glass composition, and pore size on the equilibrium and transport characteristics of membrane systems have been analyzed. A method for the determining of electrolyte activity coefficients in the membranes has been proposed.     

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 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.     

Green,ultrafine cellulose-based porous nanofibrous membranes for efficient heavy metal removal through incorporation of chitosan by various electrospinning ways

Rapid global industrialization has worsened the heavy metal contamination of aquatic ecosystems globally. In this study, green, ultrafine cellulose-based porous nanofibrous membranes for efficient heavy metal removal were obtained by incorporating chitosan (CS) and using conventional and core–shell electrospinning ways. The relationship between the parameters of the electrospinning solution, the micro-morphology and porosity, the chemically active sites, the thermal stability, and the adsorption performance of the biocomposite nanofibrous membranes were analyzed. The adsorption effects of the copper ions, including the initial concentration, solution pH, and interaction time, were investigated. The results show that the average diameters of the conventional and core–shell ultrafine nanofibers with 50% and 30% CS loading are 56.22 nm and 37.28 nm, respectively. The core–shell cellulose acetate (CA)/CS biocomposite nanofibrous membranes showed the weaker thermal stability with a 48.2 °C lower maximum thermal decomposition temperature and induced the surface aggregation of more copper ions compared to the conventional one. A more uniform distribution of the chemical adsorption sites is obtained by conventional single-nozzle electrospinning than by core–shell electrospinning, which effectively promotes the adsorption performance of copper ions and decreases the surface shrinkage of the nanofibrous membranes during adsorption. The 30% CS conventional nanofibrous membranes at an aqueous solution pH of 5 showed the optimum adsorption capacity of copper ions (86.4 mg/g). The smart combination of renewable biomass with effective chemical adsorption sites, electrospinning technology that produces an interwoven porous structure, and an adsorption method with low cost and facile operation shows a promising prospect for water treatment.

     

Mechanical and transport property modifications of perfluorosulfonate ionomer membranes prepared with mixed organic and inorganic counterions

Preparation of solution‐processed perfluorosulfonate ionomer membranes containing both small alkali metal and large alkylammonium counterions has been shown to have a profound impact on the mechanical and transport properties of the resulting acidified ionomer. The use of mixed counterions is shown to be an effective means of tailoring the thermomechanical properties of the membrane as evidenced by compositionally dependent relaxations in dynamic mechanical analysis. In agreement with our recent assignments, the α‐relaxation is found to be systematically dependent on the strength of electrostatic interactions, whereas the T_g of Nafion® (i.e., the β‐relaxation) is susceptible to plasticization. Investigations of ionic aggregation using solid‐state ²³Na NMR and small‐angle X‐ray scattering provided information suggesting the presence of mixed aggregates containing populations of both sodium and tetrabutylammonium ions. In contrast to the general perception that proton conductivity tracks with water content, membranes prepared at a 50:50 sodium/tetrabutylammonium counterion composition, followed by conversion to the H⁺‐form, showed a minimum in water content yet relatively high proton conductivity. This behavior suggests that specific interactions during processing affect the organization of the ionic domains and yield persistent structures that can significantly influence membrane transport properties. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2267–2277, 2006     

Electrical conductivity of porous glasses saturated with sulfuric acid solutions

Electrical conductivity of porous glasses saturated with sulfuric acid solutions was studied. At impregnating solution concentrations in the range 20–40 wt %, the glasses have high conductivities a σ > 0.1 ω^?1 cm^?1. The dependences of the electrical conductivity on parameters of the porous structure of membranes are considered.     

Transport Properties of Selective Membranes Reversible to Nitrogen-Containing Organic Base Cations: Permeability and Ion Flow

Main transport properties were studied for selective membranes with low dielectric constants based on liquid ion exchangers involving nitrogen-containing organic base cations. Permeabilities and ion flows through a membrane were calculated for major and interfering ions. Dependences of the transport properties of membranes on the concentrations of the ion exchanger and near-membrane solution and their potentiometric characteristics are presented. It was demonstrated that the transport properties of liquid membranes are determined by two main factors: the transfer of counterions through the phase boundary by the extraction–exchange mechanism and the leaching of the ion exchanger from the membrane.     

Structural and Electrosurface Properties of Iron-Containing Porous Glasses in NaCl Solutions. I. Structural and Transport Characteristics of Porous Glasses

The structural (structural resistance coefficient, volume porosity, average pore radius, and specific surface area) and transport (specific electrical conductivity and counterion transport numbers) characteristics of high-silica micro- and macroporous glasses with different compositions (magnetite-free and magnetite- containing glasses) have been compared in solutions of an indifferent electrolyte (sodium chloride). It has been shown that the incorporation of iron(III) oxide into basic sodium-borosilicate glass changes the structure of the pore space of both microporous glasses produced by acidic leaching and macroporous glasses obtained from the microporous samples by additional alkaline treatment. Moreover, it has been found that the transport characteristics of microporous glasses with different compositions are similar, while, for magnetite- phase-containing macroporous glasses, the specific conductivity of a pore solution and counterion transport numbers are increased.     

Nuclear magnetic relaxation and phase transitions of diethylene glycol in Vycor porous glasses

The times of longitudinal T ₁ and transverse T ₂ magnetic relaxation of protons of diethylene glycol in the bulk phase and in Vycor porous glasses with average pore radii of 4, 11, and 32 nm are measured by the pulse NMR method in the 172–350 K temperature range. It is found that, for all samples of porous glasses, the crystallization of diethylene glycol is not observed if its content corresponds to the monolayer surface filling. The minimum content of diethylene glycol, which makes it possible to cause its crystallization in porous glass, is determined. By analyzing the dependence of the characteristics of a component with T _2b = 20−40 μs in the transverse magnetization decay on the content of diethylene glycol in porous glasses, the volume of nanopores comparable in size with diethylene glycol molecule is estimated.     

Structural and Electrosurface Characteristics of Sulfonated Cation-Exchange Perfluoropolymer Membranes in 1 : 1 Electrolyte Solutions

The equilibrium (the exchange capacity, the structural resistance coefficient, and contact angles) and transport (the conductivity) characteristics of differently obtained sulfonated cation-exchange perfluoropolymeric membranes in 1 : 1 electrolyte solutions were investigated. It was shown that the transformation of membranes from the Na⁺form to the K⁺form sharply decreases their moisture content, which is accompanied by an increase in the structural resistance coefficient and the counterion concentration in membranes. Experimental data were used for calculating the electrochemical characteristics of membranes: efficiency coefficients, ion mobilities and transfer numbers of ions in the intramembrane liquid, as well as Donnan potentials. Measurements of the wettability of fluoroplastic ion-exchange membranes with water and electrolyte solutions showed that the presence of strongly acidic ionogenic groups significantly decreases contact angles as compared with that of the polytetrafluoroethylene surface. It was also established that, for the investigated systems, the contact angle is virtually independent of the composition of the liquid phase.