Measurements of transient membrane potential after current switch-off as a tool to study the electrochemical properties of supported thin nanoporous layers

We have studied the potential of chronopotentiometry after current switch-off as a tool for electrochemical characterization of thin supported nanoporous layers. Within the scope of this technique, a thin supported electrochemically active layer is polarized by direct electric current until a steady state is reached. After that, the current is switched-off in a stepwise manner, and the reading of transient membrane potential begins. A linear non-steady-state theory of the method has been developed in terms of a model-independent approach of network thermodynamics. The measurements of transient membrane potential after current switch-off have been carried out in KCl solutions of various concentrations for a commercially available nanofiltration membrane (Desal5 DK). Such membranes consist of micron-thick active (or barrier) nanoporous layers and much thicker (100-200 microm) and coarse-porous supports (the pore size usually is 0.1-5 microm). The reproducibility of the method has been found to be quite reasonable especially in not too dilute electrolyte solutions and at not too short times (> or = 10 ms). The relaxation measurements have been complemented by the measurements of the steady-state membrane potential and by sample measurements of salt rejection in the pressure-driven mode, which enabled us to carry out a self-consistent interpretation of the experimental data. This has revealed, in particular, that the ion rejection mechanism related to the fixed electric charges is not the dominant one in the case of the Desal5 DK nanofiltration membrane. Proceeding from a quantitative interpretation of relaxation patterns, we could also determine some properties of membrane support, namely, the porosity and the salt diffusivity. They have been found to have reasonable values remarkably independent of salt concentration, which confirms the self-consistency of our interpretations.     

Adsorption properties of cermet and track-etched poly(ethylene terephthalate) membranes

The adsorption properties of cermet and track-etched poly(ethylene terephthalate) (PET-TM) membranes are compared with respect to proteins and water-soluble dyes. It is shown that the cermet membranes have a noticeably higher adsorption capacity (calculated per unit surface area) than the PET-TM. In this case, the character of adsorption of these substances on both types of membranes is quite similar and determined by the combination of ionic and hydrophobic interactions. The adsorption values of basic dyes are considerably higher than acid one because of the negative charge at the membrane surfaces. The isotherm of adsorption of the basic dye rhodamine 6G on PET-TM from aqueous solution is characterized by the inflection in the concentration range of lower than 1 µmol/l due to the presence of highly active adsorption sites on the surface. The adsorption of dyes considerably lowers on adding isopropanol to the aqueous solution. Using basic protein cytochrome C as an example, it is established that its adsorption on cermet membranes can be prevented by increasing solution ionic strength.Translated from Kolloidnyi Zhurnal, Vol. 67, No. 1, 2005, pp. 124–127. Original Russian Text Copyright © 2005 by Khataibe, Khokhlova, Trusov, Mchedlishvili.     

Modification of electrochemical properties of pore wall of track-etched mica membranes

The electrochemical properties of the pore wall of track-etched mica membranes are modified (a) by covalent binding of positively and negatively charged groups, and (b) by adsorption of cationic and amonic polyelectrolytes. The electrochemical properties of the pore wall are characterized by measurements of membrane potential, electrical conductivity and streaming potential.By these methods it is possible to change the sign of the surface charge density of the pore wall and to increase its absolute value by a factor of about 30 compared with that of the unmodified pore wall. Changes of electrochemical properties of the pore wall are desirable in studies of negative osmosis and incongruent electrolyte transport in membranes with known pore structure.     

Correlation of structural and permeation properties in sol-gel-made nanoporous membranes

The aim of the present work was to establish a fundamental link between the basic structural properties of ceramic nanoporous membranes made by the sol-gel process and their respective transport properties, for a systematic evaluation of their performance in gas and liquid applications. For this purpose, supported and unsupported gamma-Al2O3 and TiO2 membranes were prepared from different colloidal dispersions (sols) by a sol-gel dipping process followed by drying and calcination, resulting in structures of crystallites of different shape and stacking arrangement. Accordingly, the pore structure of each membrane was simulated employing process-based reconstruction techniques and the permeation properties were predicted by solving the appropriate transport equations in the generated structures. Excellent agreement was achieved between the computed and experimental permeability values in the Knudsen and viscous flow regimes, validating the considerations made regarding the basic structural characteristics and the procedure for generation of the membrane structures. Moreover, it was shown that the shape and stacking arrangement of the primary particles (crystallites) of the sol have a major impact on the formation of pathways in the membrane pore structures and control the transport and therefore also the separation properties of these materials.     

Electrochemical properties of nanoporous carbon electrodes in various nonaqueous electrolytes

Electrical double layer and electrochemical characteristics at the nanoporous carbon|acetonitrile interface with additions of Et₄NBF₄, Et₃MeNBF₄, EtMe₃NBF₄, LiClO₄, and LiBF₄ have been studied by cyclic voltammetry and impedance spectroscopy methods. A value of zero charge potential, dependent on the structure of the cations as well as on the composition of the anions, the region of ideal polarizability, and other characteristics has been established. Analysis of the complex plane plots shows that the nanoporous carbon|acetonitrile+0.1 M electrolyte (Et₄NBF₄, Et₃MeNBF₄, or EtMe₃NBF₄) interface can be simulated by the equivalent circuit, in which the two parallel conduction parts in the solid and liquid phases are interconnected by the double layer capacitance in parallel with the complex admittance of the hindered reaction of the charge transfer process or of the partial charge transfer (i.e. adsorption stage limited) process. The values of the characteristic frequency depend on the electrolyte composition and on the electrode potential, i.e. on the nature of the ions adsorbed at the surface of the nanoporous carbon electrode. In the region of moderate a.c. frequencies, the modified Randles-like equivalent circuit has been used for simulation of the complex plane plots. In the region of negative surface charge densities, the intercalation process of Li⁺ ions from LiClO₄ and LiBF₄ solutions into the surface film is possible and these data can be simulated using the modified Ho et al. model or Meyer et al. model. Electronic Publication     

Evidence of ion transport through surface conduction in alkylsilane-functionalized nanoporous ceramic membranes

Alkylsilane-modified nanoporous ceramic membranes exclude water from their pores yet exhibit transmembrane electrical conductivity in aqueous electrolyte solutions. That effect was studied using impedance spectroscopy and (29)Si NMR. Anodic aluminum oxide membranes with alkylsilane-functionalized pores exhibited a transmembrane electrical resistance that increased with the length of the hydrocarbon chain. Microstructural studies revealed that the conduction was due primarily to a small number of "hydrophilically defective" pores in membranes modified by long-chain alkylsilanes and both hydrophilic defects and surface conduction in pores modified by short-chain alkylsilanes. Hydroxyl groups in short-chain alkylsilane layers act as "water wires" to enable surface ion transport. The local concentration of hydroxyl groups decreased with alkylsilane chain length, explaining the resistance trend. This constitutes the first direct evidence that alkylsilane functionalization affects electrical as well as wetting properties.     

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.     

Characterization of the surface charge property and porosity of track-etched polymer membranes

As an important property of porous membranes, the surface charge property determines many ionic behaviors of nanopores, such as ionic conductance and selectivity. Based on the dependence of electric double layers on bulk concentrations, ionic conductance through nanopores at high and low concentrations is governed by the bulk conductance and surface charge density, respectively. Here, through the investigation of ionic conductance inside track-etched single polyethylene terephthalate (PET) nanopores under various concentrations, the surface charge density of PET membranes is extracted as ∼−0.021 C/m² at pH 10 over measurements with 40 PET nanopores. Simulations show that surface roughness can cause underestimation in surface charge density due to the inhibited electroosmotic flow. Then, the averaged pore size and porosity of track-etched multipore PET membranes are characterized by the developed ionic conductance method. Through coupled theoretical predictions in ionic conductance under high and low concentrations, the averaged pore size and porosity of porous membranes can be obtained simultaneously. Our method provides a simple and precise way to characterize the pore size and porosity of multipore membranes, especially for those with sub-100 nm pores and low porosities.     

Proton transport properties in hybrid membranes investigated by ac-electrogravimetry

In this paper, a hybrid organic/inorganic membrane, based on poly(VDF-co-HFP) polymer associated to mesostructured modified silica and made through sol–gel techniques, was characterized by using ac-electrogravimetry. In order to perform experiments, a polypyrrole–heteropolyanion doped mediator film was inserted between the working electrode of the microbalance and the hybrid membrane. This mediator film, which is characterized by mixed conducting properties, is necessary to provide proton transfer between the different interfaces and in that way, the proton transport inside the hybrid film which is only an ionic conducting material. Proton transfer and transport was characterized through ac-electrogravimetry and an original theoretical approach was developed for extracting the attractive parameters. The change of the exchanged species concentration and the diffusion coefficient of the protons in the hybrid membrane, were estimated, for the first time, according to the applied potential. These studies pointed out that the low conductivity value in this hybrid membrane is related to i) the low diffusion coefficient of proton (10^? 7 cm² s^? 1) in this membrane associated, ii) the low concentration of proton in the membrane (0.4 meq g^? 1).     

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.     

Peculiarities of the formation of track-etched membranes by the data of atomic force microscopy and X-ray scattering

Atomic force microscopy (AFM) is employed to obtain information on the main stages of the preparation of ultrafiltration track-etched membranes based on poly(ethylene terephthalate) (PET). The surface structure of initial commercial samples of PET films and the same films irradiated with various fluxes of accelerated heavy ions and subsequently treated with alkaline that results in the formation of pores owing to track etching is studied. It is shown that the order of the aforementioned stages of the formation of track-etched membranes with various porosities (from 0.02 to 6%) at a typical pore size of about 50 nm leads only to slight changes in surface structural parameters and does not fundamentally affect the polymer structure formed by spherulites with sizes that are comparable with the pore sizes. In this case, nearly the same content of the crystalline phase in the initial film and track-etched membrane are identified by large-angle X-ray scattering. The picture of X-ray scattering by track-etched membranes at small angles fully corresponds to the scattering on cylindrical pores with a diameter of about 50 nm. The analysis of the set of roughness profiles of the surfaces of initial films and track-etched membranes obtained by the AFM technique makes it possible to determine and introduce—in addition to standard parameters of the surface, the mean value of roughness and its standard deviation the correlation length characterizing the mean distance at which the memory of the roughness value is lost. It is shown that treatments resulting in the formation of track-etched membranes favor an increase in the values of roughness and practically do not affect the mean correlation length, thus supporting the conclusion of the invariance of the main structural parameters of the PET surface.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 2, 2005, pp. 248–258.Original Russian Text Copyright © 2005 by Solovieva, Timofeeva, Erina, Vstovsky, Krivandin, Shatalova, Apel, Mchedlishvili, Timashev.     

Crystalline orientation and molecular transport properties in nanoporous syndiotactic polystyrene films

The orientation of the crystalline δ nanoporous phase in syndiotactic polystyrene films, obtained by different procedures, have been characterized. For both solution cast and biaxially stretched films a high degree of uniplanar orientation, corresponding to the tendency of the ac crystallographic planes, to be parallel to the film plane has been observed and rationalized. According to molecular dynamics simulations of diffusion of small molecules into the δ nanoporous phase, this uniplanar orientation would minimize the molecular diffusivity through the nanoporous crystalline phase.     

A method to tune the ionic current rectification of track-etched nanopores by using surfactant

A method is reported here to tune the ionic current rectification behavior through a conical nanopore fabricated with the track-etching technique. In order to change the surface charge property of the pore wall, we added the cationic surfactant hexadecyl trimethylammonium bromide (CTAB) into the working electrolyte of 0.1 M KCl. By controlling the modified region and the concentration of CTAB, the ionic current rectification degree of the nanopore could be tuned over the wide range of 0.2-65 at the voltage of ±0.9 V. The mechanism of the changes in current rectification behavior was analyzed by numerically solving the Poison-Nernst-Planck (PNP) equations.     

Tangential streaming potential as a tool in modeling of ion transport through nanoporous membranes

Tangential streaming potential (TSP) measurements have been carried out so as to assess the electrokinetic properties of the active layer of organic nanofiltration (NF) membranes. Due to the porous structure of NF membranes, cares must be taken to convert the experimental data into zeta potential. Indeed, an assumption that is implicitly made in Smoluchowski's theory (or in related approaches accounting for the surface conduction phenomenon) is that both streaming and conduction currents involved in the streaming potential process flow through an identical path. Such an assumption does not hold with porous membranes since the conduction current is expected to flow wherever the electric conductivity differs from zero. Consequently, a non-negligible share of the conduction current is likely to flow through the membrane body filled with the electrolyte solution. This phenomenon has been taken into account by carrying out a series of TSP measurements at various channel heights. Experiments have been conducted with various electrolyte solutions. The inferred zeta potentials have been further converted into membrane volume charge densities which have been used to predict the membrane performances in terms of rejection rates. The conventional NF theory, i.e. based on a steric/Donnan exclusion mechanism, has been found to be unable to describe the experimental rejection rates. Using the volume charge density of the membrane as an adjustable parameter, it has been shown that the conventional theory even predicts the opposite sign for the membrane charge. On the other hand, the experimental rejection rates have been well described by including dielectric effects in the exclusion mechanism. In this case, a noticeable lowering of the effective dielectric constant of the electrolyte solution inside pores has been predicted (with respect to the bulk value).     

硫醇－磷脂混合双层膜的电化学性质及其与蜂毒素的相互 作用研究

报道了硫醇－磷脂混合双层膜的循环伏安和电化学交流阻抗行为研究,并用电化学方法考察了蜂毒素与其相互作用,实验中通过冷冻表面沾有磷脂溶液的硫醇单层膜制备混合双层膜,研究表明双层膜在电极表面形成致密的绝缘层,阻碍了电极表面的电子传递,在双层膜体系上引入的蜂毒素可在膜表面上形成孔洞,破坏膜的绝缘性,降低膜电阻,增加膜电容,使带负电的探针Fe(CN)6^3-的氧化还原反应速度加快。     

Electrochemical properties and structure of ion-exchange membranes upon thermochemical treatment

The effect of temperature-induced morphological changes on the electrochemical and physicochemical properties of the heterogeneous sulfo cation-exchange membrane MK-40 in aqueous, alkaline, and acidic media is subjected to comparative analysis. The deviation between the surface and volume microstructure of swollen membrane samples after their chemical conditioning and thermochemical treatment are visualized by a scanning electron microscope. The porosity and the fraction of the ion-exchange component in membranes subjected to heating in water and aggressive media are observed to increase more noticeably in their surface layer as compared with their volume. The maximum effect thus modified structural characteristics on the transport (conductivity, diffusion permeability, selectivity) and physicochemical (exchange capacity, water content, density, linear sizes) properties is observed for MK-40 membrane samples when heated in a sulfuric acid solution. The effect of thermal destruction of inert polymers (divinylbenzene, caprone) involved in the membrane composition on the transport characteristics is revealed.     

Electrochemical properties and spectrophotometry of membranes based on holmium tetraphenylporphyrinchloride

Protolytic properties of holmium tetraphenylporphyrinchloride were studied by the method of two-phase spectrophotometric titration with potentiometric monitoring of pH of the medium. Apparent constants of the heterophase reaction of the exchange of an anion for the hydroxide ion were determined. It was found that the membranes based on holmium tetraphenylporphyrinchloride with dibutyl phthalate as a solvent-softener are selective to ions of alkaline earth metals (Ca²⁺ and Ba²⁺) in the region of pH > 7 over the concentration range from 1 up to 1×10⁻⁴ M. Selectivity coefficients were determined by two independent methods: the method of biion potentials and the method of mixed solutions. The found selectivity series coincides with traditional Hofmeister series for cations.     

Modification of properties of ion-exchange membranes. IV. Change of transport properties of cation-exchange membranes by various polyelectrolytes

A change in electrodialytic transport properties of various cation exchange membranes was observed after the membranes had been adsorbed or ion-exchanged with various cationic polyelectrolytes. Transport properties measured in this report were the relative transport number of calcium ion to sodium ion P^Ca_Na, the current efficiency of cations, and the electric resistance of the membrane during the electrodialysis. Weakly basic and strongly basic cationic polyelectrolytes of various molecular weights were used. Though P^Ca_Na of any cation exchange membrane decreased by the adsorption or ion exchange of any cationic polyelectrolyte, the degree of the decrease in P^Ca_Na changed with species and molecular weight of polyelectrolytes and species of cation-exchange membranes. Weakly basic polyelectrolytes and low molecular weight, strongly basic polyelectrolytes were effective in producing a marked decrease in P^Na_Ca of any cation exchange membrane. The effect of strongly basic polyelectrolytes of high molecular weight on P^Ca_Na was weak in most cases. However, if it was possible to make the polyelectrolyte adhere to the surface of the membrane to form a compact coiled structure, any cationic polyelectrolyte was effective in producing a remarkable decrease in P^Ca_Na of any cation-exchange membrane.     

Electrochemical properties of photocurable membranes for all-solid-state chemical sensors

The development of ion-selective electrodes with inner solid contact is described using photocurable acrylated polyurethane matrices and electron-ion exchanger (EI), which provides a reversible transition from electrical conductivity in the metal to ionic conductivity in the membrane phase. The application of a photocurable polymer matrix gives the possibility to use modern photolithographic techniques for the formation of all-solid-state chemical sensors. The influence of the polymer matrix and of the preparation of the membrane on the electro-chemical properties of calcium-selective membrane sensors is shown. For carbonate-selective membranes the possibility of improvement of electrochemical characteristics by incorporation of the anionic additive tetrakis (4-chlorophenyl) borate was studied.     

Fabrication and gas separation properties of polybenzimidazole (PBI)/nanoporous silicates hybrid membranes

Composites of polybenzimidazole (PBI) with proton-exchanged AMH-3 and swollen AMH-3 were prepared, characterized by electron microscopy and X-ray scattering and tested for hydrogen/carbon dioxide ideal selectivity. Proton-exchanged AMH-3 was prepared under mild conditions by the ion exchange of Sr and Na cations in the original AMH-3 using aqueous solution of dl-histidine. Swollen AMH-3 was prepared by sequential intercalation of dodecylamine following the ion exchange in the presence of dl-histidine. Both silicate materials were introduced into a continuous phase of PBI as a selective phase. Mixed matrix nanocomposite membranes, prepared under certain casting conditions, with only 3 wt% of swollen AMH-3 present substantial increase of hydrogen/carbon dioxide ideal selectivity at 35 °C, i.e., more than by a factor of 2 compared to pure PBI membranes (40 vs. 15). Similar ideal selectivity was observed using higher loadings (e.g., 14%) of proton-exchanged AMH-3 particles suggesting that transport of hydrogen is faster than carbon dioxide in AMH-3-derived silicates. However, the ideal selectivity of mixed matrix membranes approaches that of pure polymer as the operating temperature increases to 100 °C and 200 °C. The composite membranes with AMH-3-derived materials were compared with MCM-22/PBI membranes. Composite membranes incorporating MCM-22 plate-like crystals show no selectivity enhancements possibly due to the presence of larger pores in MCM-22.