Membrane Pore Structure Research Articles

Mass transport in the hierarchical porous structure of zeolite-based composite membranes

Mass transport in the hierarchical porous structure of zeolite-based composite membranes

A study of porous structure of cellular membranes in human erythrocytes

Properties of cell membrane of human erythrocytes are studied using the mechanistic formalism of membrane transport developed earlier. We estimate that an erythrocyte with a membrane surface of 176 × 10 6 nm 2 has about 1900 water-permeable pores with cross-section areas ranging from 0.07 to 0.2 nm 2.

SEM Image Analysis of Pore Structure of Extending PTFE Membrane

The pore structure of extending PTFE(ePTFE) membrane by the stretching method is much different from other polymers. The image analysis of Scanning Electron Microscope(SEM) was used to describe the fiberized PTFE molecular and the coagulated PTFE node. The interconnected notes and fibrils were defined as numerical parameters, LF, WF, LN, WN and AN. The results by description of the SEM image analysis were corresponding with the porosity Pr and density Pf in the combination method of bubblepressure and fluid permeability used in other microporous membranes. The membrane with bigger AN images of node gave larger Pf and smaller Pr, and membrane with smaller AN images showed smaller Pf and bigger Pr. However there was no certain relationship with r or rmax. It is said that the SEM image analysis can be used as one of the methods to describe the pore performance of ePTFE membrane. Moreover, the influences of molecular weight of PTFE, and the stretching ratio at mechanical operation on ePTFE pore structure could also be interpreted by the SEM image analysis. The bigger molecular weight of raw material PTFE, the higher porosity and strength of the membrane would be found. The less stretching ratio, the bigger crystal node and the shorter fibril SEM image were observed.

Surface characteristics and hemocompatibility of PAN/PVDF blend membranes

Polyacrylonitrile (PAN) was blended with polyvinylidine fluoride (PVDF) at various ratios and made into membranes. The hemocompatibility of the resulting membranes was evaluated based on human plasma proteins adsorption, platelet adhesion, thrombus formation, and blood coagulation time. The PAN/PVDF blends exhibited partial miscibility according to the inward shifting of their two glass transition temperatures. The microstructures of blend membranes examined using atomic force microscopy (AFM) indicated that the roughness increased with the PVDF content, and the phase separation was too severe to form a membrane when the PVDF content was more than 30%. The water contact angle of PAN/PVDF blend membranes increased with the PVDF content. By blending with 20wt% apolar PVDF the adsorption of blood proteins could be reduced, and hence the platelet adhesion and thrombus formation was also reduced. However, when the PVDF content was 30wt%, severe thrombogenicity was observed due probably to the more porous structure of blend membrane. These results demonstrated that the hemocompatibility would be improved for PAN/PVDF blend membranes with appropriate hydrophilicity and roughness. Copyright # 2005 John Wiley & Sons, Ltd.

Simulations of the active transport of a neutral solute based on a kinase‐channel‐phosphatase topology

Simulations of coupled interactions involving two opposite enzymatic reactions, solute diffusions, and electrostatic interactions between membrane charges and charged solutes were conducted under a fixed kinase-channel-phosphatase (KCP) topology oriented from the outside to the inside of a porous membrane structure. Depending on the kinase and phosphatase locations, we recently demonstrated that an active transport of a phosphorylated substrate may occur via the opposite topology, that is, a PCK topology. The present analysis demonstrates that, under a KCP membrane topology, which also behaves as a specific ATP-dependent transporter, the active transport of a neutral substrate may occur. This analogous active transport appears to be dependent on the phosphatase location and on the membrane surface potentials. A broad analysis of the role played by the main parameters taken into account in the model was conducted in order to define precisely the physico-chemical conditions and the membrane topology needed for the highest active transports within the shortest time.

Characterizations of Pd–Ag membrane prepared by sequential electroless deposition

Palladium alloy membranes were prepared by sequential electroless plating steps on porous stainless steel (PSS). Full characterizations of Pd–Ag/PSS composite membranes were performed using scanning electron microscopy (SEM), energy dispersive spectroscopic analysis (EDS), atomic force microscopy (AFM) and X-ray diffractometry (XRD). AFM images show that in the electroless plating process lower skin layer roughness and lower deposition rate were related. Although electroless plating was conducted in a sequential process, the deposit over and inside of porous substrate was homogenous as analyzed by EDS analysis. Therefore, the deposited Pd–Ag alloy membrane retained the same composition over and through the porous substrate. The porous structure of composite membranes was also characterized by the gas permeability technique. From the experimental results, it can be concluded that a combined mechanism of Knudsen diffusion and viscous flow was conducted in the porous membrane.

A Review of Biofouling and its Control in Membrane Separation Bioreactors

Membrane separation technology is increasingly becoming an important innovation in biological wastewater treatment. Biofouling of the membrane is a major factor affecting the efficient and economic operation of membrane separation bioreactors (MBRs). This review summarizes the state-of-the-art progress in understanding the mechanisms and factors affecting membrane biofouling and the strategies for biofouling control. Biofouling mechanisms include the adsorption of soluble and suspended extracellular polymers on membrane surfaces and in membrane pores, the clogging of membrane pore structure by fine colloidal particles and cell debris, and the adhesion and deposition of sludge cake on membrane surfaces. Design and operating conditions of membrane modules and materials, hydrodynamic conditions in MBRs, process and environmental conditions of activated sludge systems, and the physicochemical properties of the wastewater are the dominant factors determining membrane biofouling. Current strategies to control biofouling include periodic relaxation, backwashing, chemical cleaning, and possible manipulation of hydrodynamic conditions and sludge properties. Achieving full integration of MBRs in wastewater treatment technology requires further research and development. Fundamental information on the bacteria, colloid, and membrane interaction, developed through multimethod and multiscale approaches, is particularly needed.

Potentials of Ceramic Membranes as Catalytic Three-Phase Reactors

The results obtained in this study provide an experimental verification of gas–liquid interface shifting within the membrane wall of both single-channel and multi-channel membrane contactors and its influence on the performance of catalytic three-phase membrane reactors. The primary objective was to demonstrate that it is possible to displace liquid occupying membrane pores and control the gas–liquid interface position within the membranes of different geometry by applying sufficient trans-membrane pressure difference. Liquid displacement from the membrane wall and consequent reduction of the diffusion path of the gaseous reactant enabled more efficient operation of membrane contactors. The second part of the investigations concerned mass transfer and reaction studies conducted at different operating conditions. Ascendance of the gas–liquid interface position on the effectiveness of the catalytic membrane reactor was studied by conducting catalytic liquidphase nitrite hydrogenation. A catalytic membrane was prepared via metallic palladium depositions within the porous structure of tubular membrane using the incipient wetness impregnation technique. The performed experimental study established effects of various process parameters (such as diffusion path of the reactant, liquid phase re-circulation rate, use of a static mixer) on the overall performance of the membrane reactor.

Separation of dilute electrolytes in poly(amino acid) functionalized microporous membranes: model evaluation and experimental results

Pressure-driven transport of dilute electrolytes in microporous membranes containing terminally-anchored charged poly(amino acids) (PAA) has been investigated through both experimental characterization and model evaluation. The membrane pore structure was modified via single-point covalent attachment of either negatively (poly( l-glutamic acid) or PLGA) or positively-charged polypeptides (poly( l-arginine) or PLA and poly( l-lysine) or PLL) allowing for separations using microporous materials (i.e. cellulosic, silica/polyethylene composites). Thus, efficient exclusion of ionic species can be achieved in open membrane platforms with considerably lower pressure requirements than conventional NF. For instance, the solute rejection of 0.5 mM solutions of environmentally-toxic species, such as divalent oxyanions of As(V) and Cr(VI), using a PLGA functionalized silica support (pore size ∼100 nm) was >80% at 0.7 bar. The effects of solute type, concentration, pH, polypeptide loading and pore coverage of the attached macromolecule on the observed solute rejection and hydraulic permeability have been examined. In addition, immobilization of PLGA allows for conformation-based alteration of membrane separation properties upon changes in pH. These morphological transitions were investigated through application of permeability data to a two-region pore model describing solvent transport. This allows for theoretical evaluation of the effective thickness of the polypeptide-containing pore region. Ion transport was then modeled using a two-dimensional approach based on the extended-Nernst Planck equations coupled with Donnan equilibrium principles. The required parameters are the effective membrane surface charge density and the PAA pore coverage as determined through permeability studies. This analysis allows for evaluation of the fixed membrane charge density based on solute rejection data, as well as, estimation of the electrostatic properties of the immobilized poly(amino acids) (i.e. p K a shifts, pH dependent thickness of polymer containing pore region).

Ordered mesoporous and macroporous inorganic films and membranes

This article reviews synthesis, structures and emerging applications of ordered inorganic meso- and macroporous films and membranes. Thin films or membranes of these groups of porous materials can be prepared by various techniques commonly used for film formation from liquid solution, including solvent evaporation, in situ growth from solution and other alternative methods. Although these techniques allow control over the thickness and pore structures in thin films or membranes, they currently offer limited control over the pore alignment relative to the substrate surface. The ordered mesoporous and macroporous thin films and membranes are highly promising for a range of potential applications in separations, chemical sensing, heterogenous catalysis, microelectronics and photonics as insulating layers of low dielectric constant and photonic bandgap materials. Furthermore, functionalization of the internal pore surfaces in these films and deposition of functional nanoparticles within the pores offer numerous new possibilities for molecular engineering of catalytic, photonic, and magnetic materials for advanced nanotechnological applications based on quantum confinement effect.

Voltage-dependent Anion-selective Channels VDAC2 and VDAC3 Are Abundant Proteins in Bovine Outer Dense Fibers, a Cytoskeletal Component of the Sperm Flagellum

Outer dense fibers (ODF) are specific subcellular components of the sperm flagellum. The functions of ODF have not yet been clearly elucidated. We have investigated the protein composition of purified ODF from bovine spermatozoa and found that one of the most abundant proteins is a 30-32-kDa polypeptide. This protein was analyzed by sequencing peptides derived following limited proteolysis. Peptide sequences were found to match VDAC2 and VDAC3. VDACs (voltage-dependent, anion-selective channels) or eukaryotic porins are a group of proteins first identified in the mitochondrial outer membrane that are able to form hydrophilic pore structures in membranes. In mammals, three VDAC isoforms (VDAC1, -2, -3) have been identified by cDNA cloning and sequencing. Antibodies against synthetic peptides specific for the three mammal VDAC isoforms were generated in rabbits. Their specificity was demonstrated by immunoblotting using recombinant VDAC1, -2, and -3. In protein extracts of bovine spermatozoa, VDAC1, -2, and -3 were detected by specific antibodies, while only VDAC2 and -3 were found as solubilized proteins derived from purified bovine ODFs. Immunofluorescence microscopy of spermatozoa revealed that anti-VDAC2 and anti-VDAC3 antibodies clearly bound to the sperm flagellum, in particular to the ODF. Transmission electron immunomicroscopy supported the finding that VDAC2 protein is abundant in the ODF. Since the ODF does not have any known membranous structure, it is tempting to speculate that VDAC2 and VDAC3 might have an alternative structural organization and different functions in ODF than in mitochondria.

Microwave plasma in preparation of new membranes

The presented paper is a peer-review of plasma action on porous polymer membranes. Considering the plasma medium the discussion is split in two parts: i) the first shows some effects caused by action of non-polymerizing gases, i.e. polymer etching and/or alteration of surface character, and, ii) the second covers action of plasma reagents resulted in the deposition of polymer layers. The authors' attention is focused mainly on alteration of the membranepore structure and surfaces character. Both kinds of plasma can turn porous membrane to new porous species with different surface character and pore size. It is possible also to prepare solid membranes where deposited polymer completely plugs the pores. The presented discussion is illustrated by the results of the authors' own research on preparation of brand-new polymer membranes.

In situ determination of the adsorption characteristics of a zeolite membrane

A methodology based on adsorption-branch porosimetry is described for in situ measurement of the adsorption of condensable gases within the pore structure of inorganic membranes. The method is applied to the study of n-hexane and p-xylene adsorption in a high-silica, MFI zeolite membrane. The results, interpreted in terms of a simple model for competitive adsorption effects on the permeance of a non-adsorbing gas, yield Langmuir adsorption constants and Henry’s law constants for n-hexane and p-xylene that are in excellent agreement with measurements on bulk materials. The method is proposed for the fundamental study of fouling characteristics of inorganic membranes, especially in cases where a true bulk surrogate is not available.

Blend membranes prepared from cellulose and soy protein isolate in NaOH/thiourea aqueous solution

We have successfully prepared a series of blend membranes from cellulose and soy protein isolate (SPI) in NaOH/thiourea aqueous solution by coagulating with 5 wt % H2SO4 aqueous solution. The structure and properties of the membranes were characterized by Fourier transform infrared spectroscopy, ultraviolet-visible spectrometry, dynamic mechanical thermal analysis, scanning electron microscopy (SEM), transmission electron microscopy, and tensile testing. The effects of SPI content (WSPI) on the structure and properties of the blend membranes were investigated. The results revealed that SPI and cellulose are miscible in a good or a certain extent when the SPI content is less than 40 wt %. The pore structure and properties of the blend membranes were significantly improved by incorporation of SPI into cellulose. With an increase in WSPI from 10 to 50 wt %, the apparent size of the pore (2re) measured by SEM for the blend membranes increased from 115 nm to 2.43 μm, and the pore size (2rf) measured by the flow rate method increased from 43 to 59 nm. The tensile strength (σb) and thermal stability of the blend membranes with lower than 40 wt % of WSPI are higher than that of the pure cellulose membrane, owing to the strong interaction between SPI and cellulose. The values of tensile strength and elongation at break for the blend membranes with 10 wt % of WSPI reached 136 MPa and 12%, respectively. The blend membranes containing protein can be used in water because of keeping σ of 10 to 37 MPa. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 748–757, 2004

Determining Characteristics of the Porous Structure of Electrochemically Synthesized Ultrafiltration Membranes from Their Electroresistance

Basic characteristics of the porous structure (overall porosity, pore radii) of ultrafiltration membranes formed by electropolymerization of monomers in aqueous environment are studied. The overall porosity of such membranes is higher than that of membranes prepared by traditional techniques and ensures their high productivity. Pore radii of membranes estimated by different methods are compared. The most accurate method is based on the determination of the electroresistance of ultrafiltration membranes and the velocity of the liquid passing through the membranes. This method permits the examination of membranes in real conditions of their operation (swollen state, pressure of solution). Determined are coefficients of consistency of results calculated from the electroresistance of films with the electron and tunneling microscopy data and by testing substances with the known particle radii and calculations from the overall porosity. The pore radii are shown to depend on the electropolymerization mode and the monomer composition.

Nonequilibrium molecular dynamics simulation of gas-mixtures transport in carbon-nanopore membranes.

The numerical simulation of nonequilibrium cotransport of H2 and alkane gas mixtures with very different molecular sizes through a porous carbon membrane structure was implemented. Simulated permeabilities and selectivities in binary diffusive systems (and in one ternary system), at pressure about tens of atmospheres and at operating condition of room temperature or higher, can be predicted from single-gas permeabilities. This suggests that the effect is geometrical and an approximate model of the transport is proposed. It can be used for an estimation of the separation factor of a membrane. Simulations are compared with experimental results of two- and three-component codiffusion and counterdiffusion in a carbon membrane. It is shown that diffusion in a porous molecular network and in a carbon nanotube are completely different. The uniqueness of this work lies in the comparison of simulated, approximate, and experimental results, which enables us to identify the important parameters.

Experimental verification of ceramic membrane potentials for supporting three-phase catalytic reactions

An influence of gas–liquid (G–L) interface position within the membrane wall, which is a key parameter determining the gas–liquid mass transfer, and its effect on the efficiency of a ceramic membrane as a three-phase reactor was studied. This paper demonstrates that it is possible to displace liquid from the membrane wall by applying a sufficient trans-membrane pressure difference and control the position of gas–liquid interface. By supplanting the liquid-phase from the membrane pores it is possible to reduce the diffusion path of gas molecules, which resulted in considerable increase of the gas–liquid mass transfer. Ascendance of the gas–liquid interface position on the effectiveness of the catalytic membrane reactor was studied by conducting catalytic liquid-phase nitrite hydrogenation. A catalytic membrane was prepared via metallic palladium depositions within the porous structure of tubular membrane using the incipient wetness impregnation technique. Performed experimental study established the effects of the process parameters (diffusion path of the reactant, re-circulation rate …) on the overall performance of the membrane reactor. The obtained results were evaluated and compared to those obtained in conventional slurry reactor.

Hemodialysis membrane prepared from cellulose/N‐methylmorpholine‐N‐oxide solution. II. Comparative studies on the permeation characteristics of membranes prepared from N‐methylmorpholine‐N‐oxide and cuprammonium solutions

AbstractTwo kinds of regenerated cellulose membranes for hemodialysis were prepared from casting solutions of N‐methylmorpholine‐N‐oxide (NMMO) and cuprammonium (denoted NMMO membranes and cuprammonium membranes, respectively). The concentration of cellulose in the casting solution investigated was 6–8 wt %. The permeation characteristics of both membrane series were compared in terms of the ultrafiltration rate (UFR) of pure water, the sieving coefficient (SC) of dextran, and the solute permeabilities of urea, creatinine, and vitamin B12. The UFR and SC of the NMMO membranes were strongly affected by the cellulose concentration of the casting solution, and NMMO was a preferable solvent for the production of cellulose membranes with high performance; the cuprammonium solution gave low‐performance membranes. The pore structures of both types of membranes were estimated with the Hagen–Poiseuille law. The results showed that the NMMO membranes had larger pore radius and smaller pore numbers than the cuprammonium membranes. The differences in the membrane pore structures led to the differences in the performance between the two membrane series. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 333–339, 2003

Permeation mechanism of water through microporous SiO2–ZrO2 membranes for separation of aqueous solutions of organic solvents by pervaporation

Pervaporation was conducted with a microporous SiO 2 –ZrO 2 membrane at 50 and 75 °C for separation of aqueous solutions of organic solvents such as acetone, isopropyl alcohol (IPA) and dioxane. The SiO 2 –ZrO 2 membrane showed quite high fluxes of around 300 mol/(m 2 h) with large separation factors of 3800 for dioxane/water mixture and of 2800 for IPA/water mixture at 80 mol% of organic solvents at 75 °C, for example. A simple permeation model, which was proposed previously [Kitao et al., Key Eng. Mater. 61/62 (1991) 175], has been modified with the temperature and concentration polarization in order to understand the water transport characteristics across the membrane in details. Good agreements were obtained between the simulated results and the observed data. The water permeation through the micropores has been found not simply proportional to the partial water vapor pressure of the feed but complicatedly dependent on the capillary suction pressure, the osmotic pressure and on the partial water vapor pressure, as well. The effects of the membrane thickness, pore structures and film resistances at the membrane surface on the water flux are discussed by the model simulations.

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 γ-Al 2O 3 and TiO 2 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.