Electrochemistry of capillary systems with narrow pores. I. Overview

In capillary systems with narrow pores the Helmholtz electrochemical double layer located at the pore wall extends over the entire cross section of the pores. It loses its character as the “charge on the wall”. It will be shown that not only the electrokinetic phenomena but also the electrical conductivity and the dialysis potential of membranes with narrow pores can be understood from the same point of view, namely: the electrolyte solution in the pores of a membrane with narrow pores is considered to be an approximately homogeneous solution in contact with immobilised charges located at the pore wall. In this case the electrochemical equations contain the fixed ion concentration as a parameter instead of the ζ potential. This makes it possible to describe quantitatively to a good approximation data on the electroosmosis, the electrical conductivity, the streaming potential and the dialysis potential taken from the literature, as well as results of our own measurements, by using a single membrane constant.     

Electrochemistry of capillary systems with narrow pores. I. Overview

In capillary systems with narrow pores the Helmholtz electrochemical double layer located at the pore wall extends over the entire cross section of the pores. It loses its character as the “charge on the wall”. It will be shown that not only the electrokinetic phenomena but also the electrical conductivity and the dialysis potential of membranes with narrow pores can be understood from the same point of view, namely: the electrolyte solution in the pores of a membrane with narrow pores is considered to be an approximately homogeneous solution in contact with immobilised charges located at the pore wall. In this case the electrochemical equations contain the fixed ion concentration as a parameter instead of the ζ potential. This makes it possible to describe quantitatively to a good approximation data on the electroosmosis, the electrical conductivity, the streaming potential and the dialysis potential taken from the literature, as well as results of our own measurements, by using a single membrane constant.     

Palladium nanoparticle-decorated iron nanotubes hosted in a polycarbonate porous membrane: development, characterization, and performance as electrocatalysts of ascorbic acid

One-dimensional iron metallic nanotubes were prepared by electroless deposition within the pores of polycarbonate (PC) membranes. The longitudinal nucleation of the nanotubes along the pore walls was achieved by mounting the PC membrane between two halves of a U-shaped reaction tube. Palladium nanoparticles were post-deposited on the inner wall of the nanotubes. The composition, morphology, and structure of the Pd/Fe nanotubes were characterized by transmission electron microscopy, scanning electron microscopy, and inductively coupled plasma-atomic emission spectroscopy. A glassy carbon (GC) electrode modified with the free Pd/Fe bimetallic nanotubes (isolated after the dissolution of the host membranes) showed small improvement on the overpotential oxidation of ascorbic acid in comparison to the bare GC electrode. Alternatively, the Pd/Fe-polycarbonate membrane was covered with a sputtered gold thin layer of 10?nm from one side and mounted in a homemade electrochemical cell acting as the working electrode. The potential use of these functional membranes as catalytic surfaces for the electrochemical monitoring of ascorbic acid was investigated by cyclic voltammetry and amperometry. In the presence of a phosphate buffer solution, pH?7, Pd/Fe-polycarbonate membranes showed excellent electrocatalytic properties toward the oxidation of ascorbic acid even at potentials as low as 0?mV versus a Ag/AgCl reference electrode. In addition to the substantial lower overpotential, these electrodes offered selectivity over acetaminophen and uric acid, and a prolonged working stability without the need for maintenance. The electrodes were kept dry between different working days and retained their original activity for more than 1?week. Pd-polycarbonate and Fe-polycarbonate membranes were also developed for comparison purposes.     

Tubular Structures of Calcium Carbonate: Formation,Characterization, and Implications in Natural Mineral Environments

Chemical gardens are self-assembled tubular precipitates formed by a combination of osmosis, buoyancy, and chemical reaction, and thought to be capable of catalyzing prebiotic condensation reactions. In many cases, the tube wall is a bilayer structure with the properties of a diaphragm and/or a membrane. The interest in silica gardens as microreactors for materials science has increased over the past decade because of their ability to create long-lasting electrochemical potential. In this study, we have grown single macroscopic tubes based on calcium carbonate and monitored their time-dependent behavior by in situ measurements of pH, ionic concentrations inside and outside the tubular membranes, and electrochemical potential differences. Furthermore, we have characterized the composition and structure of the tubular membranes by using ex situ X-ray diffraction, infrared and Raman spectroscopy, as well as scanning electron microscopy. Based on the collected data, we propose a physicochemical mechanism for the formation and ripening of these peculiar CaCO₃ structures and compare the results to those of other chemical garden systems. We find that the wall of the macroscopic calcium carbonate tubes is a bilayer of texturally distinct but compositionally similar calcite showing high crystallinity. The resulting high density of the material prevents macroscopic calcium carbonate gardens from developing significant electrochemical potential differences. In the light of these observations, possible implications in materials science and prebiotic (geo)chemistry are discussed.     

Studies on the electrochemical and permeation characteristics of asymmetric charged porous membranes

Asymmetric charged porous membranes were prepared by imbedding 10% (W/W) ion-exchange resin in cellulose acetate binder. Membrane potential and conductance measurements have been carried out in sodium chloride solutions at different concentrations to investigate the relationship between concentration of fixed charges and electrochemical properties of developed nonselective cation- and anion-exchange membranes. Counterion transport number and permselectivity of these membranes were found to vary due to the presence of ion-exchange resin. The hydrodynamic and electroosmotic permeability of sodium chloride solutions has been studied in order to compute equivalent pore radius. For cation- and anion-exchange membranes good agreement was observed between pore radius values estimated from hydrodynamic and electroosmotic permeability coefficient separately, while for nonselective membranes no correlation was found. Membrane conductance data, along with values of concentration of fixed charges, were used for the estimation of the tortuosity factor, salt permeability coefficient, and frictional coefficient between solute and membrane matrix employing an interpretation by nonequilibrium thermodynamic principles based on frictional forces. Moreover, surface morphological studies of these membranes also have been carried out and the membranes were found to be reasonably homogeneous.     

Electrochemistry of capillary systems with narrow pores. IV. Dialysis potentials

The dialysis potentials of different collodion membranes with graded pore sizes and electrochemical activities have been measured in dilute aqueous KCl solutions as functions of concentration. It is possible to predict the value of the diffusion potential within a few millivolts on the basis of electrical conductivity data obtained with the same membranes. In general, the measured values are lower than those calculated. It is assumed that this difference is caused by the membranes having a distribution of pore sizes.     

Preparation of organic-inorganic composite anion-exchange membranes via aqueous dispersion polymerization and their characterization

Organic-inorganic composite membranes based on poly(vinyl alcohol)/SiO(2) were prepared via an aqueous dispersion polymerization route and anion-exchange groups were introduced in the membrane matrix by the chemical grafting of 4-vinylpyridine with the desired content. These membranes were extensively characterized for their surface morphology, thermal stability, water content, and surface-charge properties using SEM, TEM, FTIR, TGA, water uptake, and ion-exchange capacity measurements. Counterion transport numbers across these membranes were estimated from membrane potential data. Membrane conductance measurements were also performed and these data were used for the estimation of values of counterion diffusion coefficients in the membrane phase. Physicochemical and electrochemical properties of these membranes and equivalent pore radius (estimated from electroosmotic flux measurements) were found to be highly dependent on the 4-vinylpyridine (4-VP) content in the membrane phase. It was also observed that for better selectivity and membrane conductivity of anion-exchange membranes complete optimization of the loading of 4-VP in the membrane phase is necessary. Furthermore, among these, membrane with 25% loading with 4-VP exhibited very good selectivity, water content, and ion-exchange capacity along with moderate membrane conductivity, which may be used for their application in electro-driven separation at elevated temperatures or for other electrochemical processes.     

Electrochemistry of capillary systems with narrow pores. IV. Dialysis potentials

The dialysis potentials of different collodion membranes with graded pore sizes and electrochemical activities have been measured in dilute aqueous KCl solutions as functions of concentration. It is possible to predict the value of the diffusion potential within a few millivolts on the basis of electrical conductivity data obtained with the same membranes. In general, the measured values are lower than those calculated. It is assumed that this difference is caused by the membranes having a distribution of pore sizes.     

Porous Membranes of Montmorillonite/Poly(vinylidene fluoride‐trifluorethylene) for Li‐Ion Battery Separators

Porous membranes of poly(vinylidene fluoride‐trifluoroethylene) with different contents of montmorillonite (MMT) particles were prepared. The filler content does not affect the porous morphology but leads to an increase in the average pore size, porosity and electrolyte uptake up to 16 μm, 85 % and 325 %, respectively, for a membrane with 16 wt% of MMT particles. The mechanical properties, ionic conductivity and its temperature stability are improved by the presence of clays. The electrochemical stability reveals a stable operation window up to 5 V. The overall characteristics of the membranes for battery separators are optimized for the 4 wt% MMT filler content.     

Structure and electrochemical properties of polymeric composite membranes with a selective layer

The structure and electrochemical properties of polyethylene terephthalate track-etched membranes were studied. The membranes were covered with a solution of a styrene-butylmethacrylate copolymer on one side. The formation of a selective layer of copolymer in the pores of the starting membranes led to composite membranes characterized by asymmetric conductivity in electrolyte solutions—a rectification effect similar to the p-n transition in semiconductors. The asymmetry resulted from a considerable decrease in the pore diameter in the deposited copolymer layer, changing the pore geometry, and was also due to the existence of an interface in the pores between the starting membrane and the copolymer layer, having different levels of hydrophilicity.     

Synthesis of graphitic ordered macroporous carbon with a three-dimensional interconnected pore structure for electrochemical applications

In this study, ordered macroporous carbon with a three-dimensional (3D) interconnected pore structure and a graphitic pore wall was prepared by chemical vapor deposition (CVD) of benzene using inverse silica opal as the template. Field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman spectrometry, nitrogen adsorption, and thermogravimetric analysis techniques were used to characterize the carbon samples. The electrochemical properties of the carbon materials as a carbon-based anode for lithium-ion batteries and as a Pt catalyst support for room-temperature methanol electrochemical oxidation were examined. It was observed that the CVD method is a simple route to fabrication of desired carbon nanostructures, affording a carbon with graphitic pore walls and uniform pores. The graphitic nature of the carbon enhances the rate performance and cyclability in lithium-ion batteries. The specific capacity was found to be further improved when SnO(2) nanoparticles were supported on the carbon. The specific activity of Pt catalyst supported on the carbon materials for room-temperature methanol electrochemical oxidation was observed to be higher than that of a commercial Pt catalyst (E-TEK).     

Preparation and characterization of N-methylene phosphonic and quaternized chitosan composite membranes for electrolyte separations

Chitosan was functionalized either by introducing a phosphonic acid group or by quaternization of existing primary ammonium groups in order to make it a water-soluble material. Functionalized chitosans and poly(vinyl alcohol) (PVA)-based nanoporous charged membranes were prepared in aqueous media and gelated in methanol at 10 degrees C to tailor their pore structure. These membranes were extensively characterized for their physicochemical, electrochemical, and permeation characteristics using FTIR, TGA, DSC, water content, ion-exchange capacity, ionic transport properties, and membrane permeability studies. N-Methylene phosphonic chitosan (NMPC)/PVA-based membranes exhibited mild cation selectivity and quaternized chitosan (QC)/PVA composite membranes had mild anion selectivity, while a blend of NMPC-QC/PVA membranes exhibited weak cation selectivity because of formation of zwitterionic structure. Viscosity measurements and interaction studies for individual and mixed solutions of NMPC and QC were carried out for the prediction of charge interactions between -PO3H2 and -N+(CH3)3 groups and effect on molecular weight due to functionalization. Elaborate electrochemical and permeation experiments were conducted in order to predict suitability of these membranes for the separation of mono- and bivalent electrolytes based on their hydrated ionic radius, and it was found that among all the synthesized membranes, PC/QC-30 had the highest relative permeability, which may extend its suitability for electrolyte separations. Observations were correlated with equivalent pore radius of the different membranes as estimated by membrane permeability measurements.     

Electrochemical Properties of Phospholipid Monolayers at Liquid–Liquid Interfaces

Biomembrane models built at the interface between two immiscible electrolytes (ITIES) are useful systems to study phenomena of biological relevance by means of their electrochemical processes. The unique properties of ITIES allow one either to control or measure the potential difference across the biomimetic membranes. Herein we focus on phospholipid monolayers adsorbed at liquid–liquid interfaces, and besides discussing recent developments on the subject, we describe electrochemical techniques that can be used to get insight on the interfacial processes and electrostatic properties of phospholipid membranes at the ITIES. In particular, we examine the electrochemical and physicochemical properties of (modified) phospholipid monolayers and their interaction with other biologically relevant compounds. The use of liquid–liquid electrochemistry as a powerful tool to characterize drug properties is outlined. Although this review is not a survey of all the work in the field, it provides a comprehensive referencing to current research.     

Streaming potential in cylindrical pores of poly(ethylene terephthalate) track-etched membranes: variation of apparent zeta potential with pore radius

Streaming potential variation with pressure measured through poly(ethylene terephthalate) track-etched membranes of different pore sizes led to the determination of an apparent interfacial potential zetaa in the presence of 10-2 M KCl. The variation of zetaa with the pore radius r0 is interpreted by (i) the electric double layer overlap effect and (ii) the presence of a conductive gel layer. We propose a method which integrates both effects by assuming a simple model for the conductive gel at the pore wall. We observed the following three domains of pore size: (i) r0 > 70 nm, where surface effects are negligible; (ii) approximately 17 nm < r0 < 70 nm, where the pore/solution interface could be described as a conductive gel of thickness around 1 nm; (iii) r0 < approximately 17 nm, which corresponds to the region strongly damaged by the ion beam and is not analyzed here. The first one (zeta = -36.2 mV) corresponds to the raw material when etching has completely removed the ion beam predamaged region, which corresponds to the second intermediate domain (zeta = -47.3 mV). There the conductance of the gel layer deduced from the treatment of streaming potential data was found to be compatible with the number of ionic sites independently determined by the electron spin resonance technique.     

Asymmetrical nanopores in track membranes: Fabrication,the effect of nanopore shape and electric charge of pore walls,promising applications

The properties of asymmetrical nanopores prepared by chemical etching of tracks of accelerated heavy ions are studied. Procedures are developed for controlling the size and shape of pores within wide limits. The presence of charged functional groups on pore walls is an intrinsic property of track membranes, which makes them a convenient object for studying electrokinetic phenomena in nanocapillaries. In electrolyte solutions, the asymmetrical “track” membranes demonstrate the diode effect. Two methods for fabricating asymmetrical nanopores in polyethylene terephthalate films are proposed and introduced into practice. Specific features of both methods, their advantages and drawbacks are considered. In addition to the brief survey of available information on diode-like track membranes, the new results on the mechanism of pore formation and the peculiarities of their geometry and electrokinetic properties are discussed. The emerging and potential applications of track membranes with asymmetrical pores are discussed briefly.     

A universal model for nanoporous carbon supercapacitors applicable to diverse pore regimes, carbon materials, and electrolytes

Supercapacitors, commonly called electric double-layer capacitors (EDLCs), are emerging as a novel type of energy-storage device with the potential to substitute batteries in applications that require high power densities. In response to the latest experimental breakthrough in nanoporous carbon supercapacitors, we propose a heuristic theoretical model that takes pore curvature into account as a replacement for the EDLC model, which is based on a traditional parallel-plate capacitor. When the pore size is in the mesopore regime (2-50 nm), counterions enter mesoporous carbon materials and approach the pore wall to form an electric double-cylinder capacitor (EDCC); in the micropore regime (<2 nm), solvated/desolvated counterions line up along the pore axis to form an electric wire-in-cylinder capacitor (EWCC). In the macropore regime (>50 nm) at which pores are large enough so that pore curvature is no longer significant, the EDCC model can be reduced naturally to the EDLC model. We present density functional theory calculations and detailed analyses of available experimental data in various pore regimes, which show the significant effects of pore curvature on the supercapacitor properties of nanoporous carbon materials. It is shown that the EDCC/EWCC model is universal for carbon supercapacitors with diverse carbon materials, including activated carbon materials, template carbon materials, and novel carbide-derived carbon materials, and with diverse electrolytes, including organic electrolytes, such as tetraethylammonium tetrafluoroborate (TEABF(4)) and tetraethylammonium methylsulfonate (TEAMS) in acetonitrile, aqueous H(2)SO(4) and KOH electrolytes, and even an ionic liquid electrolyte, such as 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI). The EDCC/EWCC model allows the supercapacitor properties to be correlated with pore size, specific surface area, Debye length, electrolyte concentration and dielectric constant, and solute ion size It may lend support for the systematic optimization of the properties of carbon supercapacitors through experiments. On the basis of the insight obtained from the new model, we also discuss the effects of the kinetic solvation/desolvation process, multimodal (versus unimodal) pore size distribution, and exohedral (versus endohedral) capacitors on the electrochemical properties of supercapacitors.     

Electrokinetic characteristics of initial porous glasses and those modified with titanium- and aluminum-oxide particles

The structural (volume porosity, structural resistance coefficient, and average pore radius) and electrokinetic (specific electrical conductivity, ion-transport numbers, and electrokinetic potential) characteristics of macroporous glass membranes obtained from two-phase sodium-borosilicate glasses with different times of thermal treatment have been studied in solutions of hydrochloric acid and potassium chloride. The properties of the initial membranes have been compared with the characteristics of the same membranes modified by filtering through them suspensions of aluminum- and titanium-oxide nanoparticles with different weight concentrations. It has been shown that, at low degrees of pore channel surface coverage with nanoparticles (<0.1), the structural parameters of the membranes remain almost unchanged. In addition, it has been found that the presence of positively charged nanoparticles on the negatively charged surface increases the surface conductivity and the absolute value of the electrokinetic potential.     

Evaluation of asymmetrical structure dialysis membrane by tortuous capillary pore diffusion model

The tortuous capillary pore diffusion model (TCPDM) has been used for estimating diffusive and pure water permeability from simple structure parameters such as pore diameter, surface porosity, wall thickness and tortuosity. The validity of this model for evaluation of homogeneous membrane has been already confirmed. Recently, there is a trend toward the use of asymmetrical dialysis membranes made of synthetic polymer such as poly(acrylonitrile) (PAN), polysulfone (PS) and a polyethersulfone polyarylate (PEPA) blend polymer. The purpose of the present study is to apply the TCPDM to evaluation of commercially available hollow-fiber dialysis membranes with asymmetrical structures by simplifying them to a double-layer membrane. The TCPDM is capable of estimating pore tortuosity of asymmetrical dialysis membranes having skin and supporting layers from data on membrane thickness, pore diameter, pure water permeability and water content. Values for diffusive permeability obtained by the TCPDM are in a good agreement with experimental data. This TCPDM model is useful for evaluation of not only homogeneous membrane but also asymmetrical membrane.     

Preparation,properties, and application of polypropylene micro/nanofiber membranes

Nanofiber membranes have huge potential applications in many areas due to their unique properties. However, the thermoplastic micro/nanofiber membranes were rarely reported. In this paper, polypropylene (PP) nanofibers were prepared by melt extrusion of immiscible blends of PP, cellulose acetate butyrate (CAB), and subsequent removal of the CAB matrix. The wet‐laid application was used to make PP nanofiber membranes and PP‐g‐MAH/nonwoven micro/nanofiber membrane. The properties of membranes including morphology, apparent density, porosity, contact‐angle, pore size distribution, and water flux were characterized. The results showed that the consequent membranes were provided with optimistic porosity and pore size distribution. Moreover, they were all with high pure water fluxes, which were superior to that of PP microporous membrane. They performed an excellent separation performance of TiO₂ suspension and dyeing wastewater. The work revealed this method could be an efficient one to make thermoplastic polymer micro/nanofiber membranes, and they would have a brilliant potential application for water treatment. Copyright © 2010 John Wiley & Sons, Ltd.