Showing papers on "Membrane published in 2006"

Fast Mass Transport Through Sub-2-Nanometer Carbon Nanotubes

Abstract: We report gas and water flow measurements through microfabricated membranes in which aligned carbon nanotubes with diameters of less than 2 nanometers serve as pores. The measured gas flow exceeds predictions of the Knudsen diffusion model by more than an order of magnitude. The measured water flow exceeds values calculated from continuum hydrodynamics models by more than three orders of magnitude and is comparable to flow rates extrapolated from molecular dynamics simulations. The gas and water permeabilities of these nanotube-based membranes are several orders of magnitude higher than those of commercial polycarbonate membranes, despite having pore sizes an order of magnitude smaller. These membranes enable fundamental studies of mass transport in confined environments, as well as more energy-efficient nanoscale filtration.

How proteins produce cellular membrane curvature

Abstract: Biological membranes exhibit various function-related shapes, and the mechanism by which these shapes are created is largely unclear. Here, we classify possible curvature-generating mechanisms that are provided by lipids that constitute the membrane bilayer and by proteins that interact with, or are embedded in, the membrane. We describe membrane elastic properties in order to formulate the structural and energetic requirements of proteins and lipids that would enable them to work together to generate the membrane shapes seen during intracellular trafficking.

Effects of membrane cation transport on pH and microbial fuel cell performance.

Abstract: Due to the excellent proton conductivity of Nafion membranes in polymer electrolyte membrane fuel cells (PEMFCs), Nafion has been applied also in microbial fuel cells (MFCs). In literature, however, application of Nafion in MFCs has been associated with operational problems. Nafion transports cation species other than protons as well, and in MFCs concentrations of other cation species (Na+, K+, NH4+, Ca2+, and Mg2+) are typically 10(5) times higher than the proton concentration. The objective of this study, therefore, was to quantify membrane cation transport in an operating MFC and to evaluate the consequences of this transport for MFC application on wastewaters. We observed that during operation of an MFC mainly cation species other than protons were responsible for the transport of positive charge through the membrane, which resulted in accumulation of these cations and in increased conductivity in the cathode chamber. Furthermore, protons are consumed in the cathode reaction and, consequently, transport of cation species other than protons resulted in an increased pH in the cathode chamber and a decreased MFC performance. Membrane cation transport, therefore, needs to be considered in the development of future MFC systems.

Materials science of membranes for gas and vapor separation

Abstract: Contributors. Preface. 1. Transport of Gases and Vapors in Glassy and Rubbery Polymers (Scott Matteucci, Yuri Yampolskii, Benny D. Freeman and Ingo Pinnau). 2. Principles of Molecular Simulation of Gas Transport in Polymers (Doros N. Theodorou). 3. Molecular Simulation of Gas and Vapor Transport in Highly Permeable Polymers (Joel R. Fried). 4. Predicting Gas Solubility in Membranes through Non-Equilibrium Thermodynamics for Glassy Polymers (Ferruccio Doghieri, Massimiliano Quinzi, David G. Rethwisch and Giulio C. Sarti). 5. The Solution-Diffusion Model: A Unified Approach to Membrane Permeation (Johannes G. (Hans) Wijmans and Richard W. Baker ). 6. Positron Annihilation Lifetime Spectroscopy and Other Methods for Free Volume Evaluation in Polymers (Yuri Yampolskii and Victor Shantarovich). 7. Prediction of Gas Permeation Parameters of Polymers (Alexander Alentiev and Yuri Yampolskii ). 8. Synthesis and Permeation Properties of Substituted Polyacetylenes for Gas Separation and Pervaporation (Toshio Masuda and Kazukiyo Nagai). 9. Gas and Vapor Transport Properties of Perfluoropolymers (Tim C. Merkel, Ingo Pinnau, Rajeev Prabhakar and Benny D. Freeman). 10. Structure and Transport Properties of Polyimides as Materials for Gas and Vapor Membrane Separation (Kazuhiro Tanaka and Ken-Ichi Okamoto). 11. The Impact of Physical Aging of Amorphous Glassy Polymers on Gas Separation Membranes (Peter H. Pfromm). 12. Zeolite Membranes for Gas and Liquid Separations (George R. Gavalas). 13. Gas and Vapor Separation Membranes Based on Carbon Membranes (Hidetoshi Kita). 14. Polymer Membranes for Separation of Organic Liquid Mixtures (Tadashi Uragami ). 15. Zeolite Membranes for Pervaporation and Vapor Permeation (Hidetoshi Kita). 16. Solid-State Facilitated Transport Membranes for Separation of Olefins/Paraffins and Oxygen/Nitrogen ( Yong Soo Kang, Jong Hak Kim, Jongok Won and Hoon Sik Kim ). 17. Review of Facilitated Transport Membranes (Richard D. Noble and Carl A. Koval ). Index.

Aliphatic/Aromatic Polyimide Ionomers as a Proton Conductive Membrane for Fuel Cell Applications

Abstract: To produce a proton conductive and durable polymer electrolyte membrane for fuel cell applications, a series of sulfonated polyimide ionomers containing aliphatic groups both in the main and in the side chains have been synthesized. The title polyimide ionomers 1 with the ion exchange capacity of 1.78−2.33 mequiv/g were obtained by a typical polycondensation reaction as transparent, ductile, and flexible membranes. The proton conductivity of 1 was slightly lower than that of the perfluorinated ionomer (Nafion) below 100 °C, but comparable at higher temperature and 100% RH. The highest conductivity of 0.18 S cm-1 was obtained for 1 at 140 °C. Ionomer 1 with high IEC and branched chemical structure exhibited improved proton conducting behavior without sacrificing membrane stability. Microscopic analyses revealed that smaller (<5 nm) and well-dispersed hydrophilic domains contribute to better proton conducting properties. Hydrogen and oxygen permeability of 1 was 1−2 orders of magnitude lower than that of Na...

Plasticization-Enhanced Hydrogen Purification Using Polymeric Membranes

Abstract: Polymer membranes are attractive for molecular-scale separations such as hydrogen purification because of inherently low energy requirements. However, membrane materials with outstanding hydrogen separation performance in feed streams containing high-pressure carbon dioxide and impurities such as hydrogen sulfide and water are not available. We report highly permeable, reverse-selective membrane materials for hydrogen purification, as exemplified by molecularly engineered, highly branched, cross-linked poly(ethylene oxide). In contrast to the performance of conventional materials, we demonstrate that plasticization can be harnessed to improve separation performance.

Nanoporous Membranes with Ultrahigh Selectivity and Flux for the Filtration of Viruses

Abstract: The filtration, separation, and isolation of viruses are critical issues for controlling blood-borne viral infections and for viral research. Membrane-based technology has been identified as a useful method for the separation of biomaterials including viruses, owing to its efficiency, ease of implementation, and cost effectiveness. Several types of membranes have been employed for virus filtration. For example, microfiltration (MF) membranes show a relatively high flux and good retention of viruses on the membrane due to the presence of electrostatic interactions under appropriate conditions. However, the pore size of MF membranes is typically much larger than the size of the virus particles, which limits their applicability to biomaterials that are tens of nanometers in size. Ultrafiltration membranes with smaller pore sizes have also been employed for the separation of viruses. However, they have not been very effective, since the virus particles permeate into a small number of abnormally largesized pores. Track-etched polycarbonate (PC) and anodized aluminum oxide (AAO) membranes with a uniform pore size have also been studied for the separation of viruses. While the pore size distributions are narrow for these membranes, both types of membranes show a very low flux for virus separation. Thus, a new type of membrane, providing both high selectivity and high flux, is needed to filter viruses. Here, we introduce a new membrane with an asymmetric film geometry, which shows both high selectivity and flux. This membrane consists of a thin nanoporous layer, prepared from a blockcopolymer template, and a support membrane that provides mechanical strength. This asymmetric membrane shows ultrahigh selectivity while still maintaining a high flux for the separation of human rhinovirus type 14 (HRV14), which has a diameter of ∼ 30 nm and is a major pathogen for the common cold in humans. Since the pore diameter in the top layer can be tuned from 10 to 40 nm, the cutoff size of the membrane filter can be precisely controlled. With these pore sizes, this membrane allows biomolecules such as proteins present in the unfiltered solution to pass through the membrane, while only the viruses are screened. This unique characteristic of the new membrane filter eliminates the risk of contamination from viruses while processing biotherapeutic proteins such as vaccines and hormones. Therefore, this new membrane can be used to develop new types of bloodfiltering systems, such as a haemodialysis membrane that is free of the risk of viral infection. Moreover, this membrane provides an easy means to increase the concentration of the virus, thereby making it possible to investigate virus cultivation and the morphologies of unknown viruses. Figure 1 shows a schematic depiction of the fabrication of asymmetric nanoporous membranes. The top separation layer (∼ 80 nm thick) is made from a thin film of a mixture of polystyrene-block-poly(methyl methacrylate) copolymer (PS-b-PMMA), with cylindrical microdomains of PMMA, on a ∼ 100 nm thick sacrificial silicon oxide layer. As previously reported, when the PMMA homopolymer is added to PS-b-PMMA, the cylindrical nanodomains orient normal to the surface in films of up to ∼ 300 nm thickness on surfaces where the interfacial interactions have been balanced (Fig. 1a). This thin film can be removed from the substrate by using a buffered HF solution to dissolve the oxide layer. The film is then transferred onto the MF polysulfone (PSU) membrane, which acts as a support (Fig. 1b). The adhesion between the block-copolymer-mixture film and the PSU membrane is sufficient to maintain the mechanical integrity of the system during the fabrication and filtration experiments. Porous thin films of the upper layer can be prepared by selectively removing the PMMA homopolymer from the cylindrical PMMA microdomains with acetic acid (Fig. 1c). This produces a well-ordered array consisting of ∼ 15 nm diameter pores with a narrow pore size distribution (see Fig. S1, Supporting Information), which completely prevents the HRV14 virus (colored green) from penetrating into the pores, while proteins, such as bovine serum albumin (BSA) (colored yelC O M M U N IC A IO N S

Sol–gel preparation of mesoporous photocatalytic TiO2 films and TiO2/Al2O3 composite membranes for environmental applications

Abstract: This study describes the application of novel chemistry methods for the fabrication of robust nanostructured titanium oxide (TiO2) photocatalysts. Such materials can be applied in the development of efficient photocatalytic systems for the treatment of water. Mesoporous photocatalytic TiO2 films and membranes were synthesized via a simple synthesis method that involves dip-coating of appropriate substrates into an organic/inorganic sol composed of isopropanol, acetic acid, titanium tetraisopropoxide, and polyoxyethylenesorbitan monooleate surfactant (Tween 80) followed by calcination of the coating at 500 8C. Controlled hydrolysis and condensation reactions were achieved through in-taking of water molecules released from the esterification reaction of acetic acid with isopropanol. The subsequent stable incorporation of Ti–O–Ti network onto self-assembled surfactants resulted in TiO2 photocatalysts with enhanced structural and catalytic properties. The properties included high surface area (147 m 2 /g) and porosity (46%), narrow pore size distribution ranging from 2 to 8 nm, homogeneity without cracks and pinholes, active anatase crystal phase, and small crystallite size (9 nm). These TiO2 photocatalysts were highly efficient for the destruction of methylene blue and creatinine in water. High water permeability and sharp polyethylene glycol retention of the prepared photocatalytic TiO2/Al2O3 composite membranes evidenced the good structural properties of TiO2 films. In addition, the multi-coating procedure made it possible to effectively control the physical properties of TiO2 layer such as the coating thickness, amount of TiO2, photocatalytic activity, water permeability and organic retention. # 2005 Elsevier B.V. All rights reserved.

Temperature-Controlled Assembly and Release from Polymer Vesicles of Poly(ethylene oxide)-block- poly(N-isopropylacrylamide)

Abstract: Vesicles, together with spherical micelles and cylindrical micelles, are the three most common and stable morphologies of amphiphiles in water. Unlike micelles, vesicles can entrap hydrophilic molecules within the vesicle lumen and also integrate hydrophobic molecules within the membrane core. The development of vesicle-forming materials has therefore attracted wide interest for applications ranging from cosmetics to drug delivery. Biological vesicles and membranes that selfassemble from amphiphilic phospholipids are central to cell compartmentation and function; various environmental factors and even fusion can trigger release of contents. However, lipids typically have molecular weights less than 1 kDa so encapsulation, retention, and overall stability of natural vesicles are often limited. In comparison with amphiphilic block copolymers, molecular weight, composition, and chemical functionalities can be tuned for tailored polymer vesicles or “polymersomes”, [1–4] with opportunities to improve control and stability for various applications such as sensors [5] and drug delivery. [6,7] Oxidation [7] and pH [8–11] responsive polymersomes have recently been reported for controlled encapsulation and release. Thermal transitions provide another means for stimulated release. In liposomes, membrane permeability is already known to be strongly perturbed at phase transitions, [12] and this has been exploited for hypothermic delivery of anticancer drugs by coupling to local heating. [13] Thermoresponsive polymersomes have yet to be explored, and the most thoroughly studied copolymer systems are relatively temperature insensitive, possessing either very high glass transitions (e.g., polystyrene) or subzero glass transitions (e.g., polybutadiene). [2] In addi

Relating organic fouling of reverse osmosis membranes to intermolecular adhesion forces

Abstract: Organic fouling of reverse osmosis (RO) membranes and its relation to foulant--foulant intermolecular adhesion forces has been investigated. Alginate and Suwannee River natural organic matter were used as model organic foulants. Atomic force microscopy was utilized to determine the adhesion force between bulk organic foulants and foulants deposited on the membrane surface under various solution chemistries. The measured adhesion force was related to the RO fouling rate determined from fouling experiments under solution chemistries similar to those used in the AFM measurements. A remarkable correlation was obtained between the measured adhesion force and the fouling rate under the solution chemistries investigated. Fouling was more severe at solution chemistries that resulted in larger adhesion forces, namely, lower pH, higher ionic strength, presence of calcium ions (but not magnesium ions), and higher mass ratio of alginate to Suwannee River natural organic matter. The significant adhesion force measured with alginate in the presence of calcium ions indicated the formation of a crossed-linked alginate gel layer during fouling through intermolecular bridging among alginate molecules.

Phospholipids and the origin of cationic gating charges in voltage sensors

Abstract: Cells communicate with their external environment through physical and chemical processes that take place in the cell-surrounding membrane. The membrane serves as a barrier as well as a special environment in which membrane proteins are able to carry out important processes. Certain membrane proteins have the ability to detect the membrane voltage and regulate ion conduction or enzyme activity. Such voltage-dependent processes rely on the action of protein domains known as voltage sensors, which are embedded inside the cell membrane and contain an excess of positively charged amino acids, which react to an electric field. How does the membrane create an environment suitable for voltage sensors? Here we show under a variety of conditions that the function of a voltage-dependent K+ channel is dependent on the negatively charged phosphodiester of phospholipid molecules. A non-voltage-dependent K+ channel does not exhibit the same dependence. The data lead us to propose that the phospholipid membrane, by providing stabilizing interactions between positively charged voltage-sensor arginine residues and negatively charged lipid phosphodiester groups, provides an appropriate environment for the energetic stability and operation of the voltage-sensing machinery. We suggest that the usage of arginine residues in voltage sensors is an adaptation to the phospholipid composition of cell membranes.

Catalytic membranes prepared using layer-by-layer adsorption of polyelectrolyte/metal nanoparticle films in porous supports.

Abstract: Layer-by-layer adsorption of polyelectrolytes and gold nanoparticles within porous supports provides a convenient method for forming catalytic membranes. The polyelectrolyte film effectively immobilizes the gold nanoparticles without inhibiting access to catalytic sites, as shown by the similar rate constants for nanoparticle-catalyzed 4-nitrophenol reduction in solution and in membranes. Modified alumina membranes reduce >99% of 0.4 mM 4-nitrophenol at linear flow rates of 0.98 cm/s, and the modification process is also applicable to track-etched polycarbonate supports.

Molecular basis for dimethylsulfoxide (DMSO) action on lipid membranes

Abstract: Dimethylsulfoxide (DMSO) is an aprotic solvent that has the ability to induce cell fusion and cell differentiation and enhance the permeability of lipid membranes. It is also an effective cryoprotectant. Insights into how this molecule modulates membrane structure and function would be invaluable toward regulating the above processes and for developing chemical means for enhancing or hindering the absorption of biologically active molecules, in particular into or via the skin. We show here by means of molecular simulations that DMSO can induce water pores in dipalmitoyl-phosphatidylcholine bilayers and propose this to be a possible pathway for the enhancement of penetration of actives through lipid membranes. DMSO also causes the membrane to become floppier, which would enhance permeability, facilitate membrane fusion, and enable the cell membrane to accommodate osmotic and mechanical stresses during cryopreservation.

Conducting Polymer‐Based Solid‐State Ion‐Selective Electrodes

Abstract: Conducting polymers, i.e., electroactive conjugated polymers, are useful both as ion-to-electron transducers and as sensing membranes in solid-state ion-selective electrodes. Recent achievements over the last few years have resulted in significant improvements of the analytical performance of solid-contact ion-selective electrodes (solid-contact ISEs) based on conducting polymers as ion-to-electron transducer combined with polymeric ion-selective membranes. A significant amount of research has also been devoted to solid-state ISEs based on conducting polymers as the sensing membrane. This review gives a brief summary of the progress in the area in recent years.

Copper(II)-selective potentiometric sensors based on porphyrins in PVC matrix

Abstract: Cu2+ selective sensors have been fabricated from polyvinyl chloride (PVC) matrix membranes containing neutral carrier porphyrin (A and B) ionophores. The addition of sodium tetraphenylborate and the plasticizer DOP has been found to substantially improve the performance of the sensors. The membranes of various compositions of the two porphyrins were investigated and it was found that the best performance was obtained for the membrane of composition B:PVC:NaTPB:DOP in the ratio 5:150:2:150. The sensor shows a linear potential response for Cu2+ over a wide concentration range 4.4 × 10−6 to 1.0 × 10−1 M with Nernstian compliance (29.3 mV decade−1 of activity) between pH 2.8 and 7.9 and a fast response time of ∼8 s. The potentiometric selectivity coefficient values as determined by fixed interference method indicate excellent selectivity for Cu2+ ions over interfering cations. The sensor exhibits adequate shelf life (∼4 months) with good reproducibility (standard deviation ± 0.2 mV). The sensor can also be used in partially non-aqueous media having up to 20% (v/v) methanol, ethanol or acetone content with no significant change in the value of slope or working concentration range. The sensor has been used in the potentiometric titration of Cu2+ with EDTA. The utility of the sensor has been tested by determining copper in vegetable foliar and swimming pool water sample successfully.