Mass transport through swelling membranes

This paper deals with non-Fickian mass transport through polymeric membranes. The process is described via continuum mechanics. We introduced an appropriate function relating the stress to concentration and time, such that the model predicts a realistic stress distribution at equilibrium also. The effect of different dimensionless groups is illustrated and quantitative agreement with experimental data is shown for transport of organic solvents through PVC.     

Evidence for microwave enhanced mass transport in the annealing of nanoporous alumina membranes

Results of a comparative study of pore evolution in nanostructured alumina membranes under annealing in a gyrotron microwave system and in conventional furnace are described. Microwave heating has resulted in an enhanced mass transport leading to reduction in the surface porosity of the membranes. Evolution patterns for the shape of individual pores are discussed and compared for microwave and conventional annealing. The notably different behavior of the pores suggests that microwave radiation provides an additional driving force for mass transport. The experimentally observed enhancement of mass transport appears to be stronger than predicted by the earlier proposed models.     

Fast mass transport through carbon nanotube membranes

The May 19, 2006 issue of Science included a paper by Holt et al. on "Fast Mass Transport Through Sub-2-Nanometer Carbon Nanotubes". The paper was also featured on the cover, showing methane molecules translating inside a carbon nanotube (CNT). The authors explained how they prepared 2-6-mum thin membranes consisting of double-walled carbon nanotubes (DWNTs) all aligned perpendicular to the apparent membrane surface. These tubes are open at both ends and the space between the tubes is filled with dense Si(3)N(4). Pure gas and water fluxes were measured at room temperature with the application of a small pressure difference. Interpretation of the results led to the conclusion that the membranes showed much higher fluxes than what was estimated from Knudsen gas diffusion and Poiseuille viscous flow models. The membranes have a straight-channel morphology with a narrow pore-size distribution and exceptionally smooth pore walls. The unusual geometry and surface properties make it difficult to compare the membrane's properties with common membranes but there is no question that the mass transport in the aligned DWNTs is fast indeed. To appreciate how fast, we will consider their transport properties starting from the perspective of "conventional" porous membrane technology. Recent molecular dynamics simulations suggest that none of the classic models for gas (Knudsen) and water (Poiseuille) permeation work in a meaningful way for these nanotube membranes, and new models are needed.     

Single-molecule spectroscopy using nanoporous membranes

We describe a novel approach for optically detecting DNA translocation events through an array of solid-state nanopores that potentially allows for ultra high-throughput, parallel detection at the single-molecule level. The approach functions by electrokinetically driving DNA strands through sub micrometer-sized holes on an aluminum/silicon nitride membrane. During the translocation process, the molecules are confined to the walls of the nanofluidic channels, allowing 100% detection efficiency. Importantly, the opaque aluminum layer acts as an optical barrier between the illuminated region and the analyte reservoir. In these conditions, high-contrast imaging of single-molecule events can be performed. To demonstrate the efficiency of the approach, a 10 pM fluorescently labeled lambda-DNA solution was used as a model system to detect simultaneous translocation events using electron multiplying CCD imaging. Single-pore translocation events are also successfully detected using single-point confocal spectroscopy.     

Polarity-dependent electrochemically controlled transport of water through carbon nanotube membranes

We demonstrate here that water can be efficiently wet and pumped through superhydrophobic aligned multiwalled nanotube membranes by application of a small positive dc bias. At a critical bias ( approximately 1.7 V), with the membrane acting as anode, there is an abrupt transition from a superhydrophobic to hydrophilic state. Interestingly, this phenomenon is strongly polarity dependent; for a negative bias applied to the membrane, 2 orders of magnitude higher bias is required for the transition. The polarity and voltage-dependent wetting that we report could be used to controllably wick fluids through nanotube membranes and could find various applications.     

Anisotropic diffusion of hydrogen in nanoporous carbons

It is accepted that hydrogen transport capacity through carbons depends on the anisotropy of the empty spaces that constitute their porous structure. However, very little is known about this relationship. Computational simulation is an excellent tool to accomplish this kind of studies. Simulation requires digital representations of materials and a model describing the interaction potential among the gas molecules and the solids surfaces. In this work, it is proposed to use the analytical solutions of the truncated pore problem for modeling the potentials, and an immiscible lattice gas for obtaining the representations. The degree of anisotropy was quantified by using the mean intercept length method. The adsorption isotherms and the self-diffusion coefficients in the three orthogonal directions were found by the grand canonical and kinetic Monte Carlo methods, respectively. The results suggest the existence of a gas pressure at which a molecular saturation threshold (P _s) is reached. P _s determines if the degree of anisotropy is or not a representative variable of diffusive transport. For P ≤ P _s, the degree of anisotropy favors the molecular mobility. When P > P _s, the degree of anisotropy loses influence on mobility.     

Penetration of hydrogen through multilayer metal membranes with thin separated layers

Translated from Fiziko-khimicheskaya Mekhanika Materialov, Vol. 26, No. 5, pp. 66–68, September–October, 1990.     

Influences of interfacial resistances on gas transport through carbon nanotube membranes

Carbon nanotubes have significant promise as gas separation membranes. Gas permeation through nanopores involves mass transfer resistances from molecules entering and leaving pores (so-called surface resistances) and diffusion within the pores. We use molecular simulations to give the first estimates of surface resistances for gas transport through nanotubes. For CH4 transport through (20,0) carbon nanotubes at 300 K, surface resistances are small for nanotubes 5-10 mum in length but can be significant for shorter nanotubes.     

Hydrogen transport through thin palladium-copper alloy composite membranes at low temperatures

Hydrogen permeation performance of three thin palladium-copper composite membranes with different thicknesses had been studied between 398 K and 753 K. Hydrogen permeance was obtained up to 2.7 × 10^− 6 mol/(m² s Pa) with an ideal selectivity over 1000 at 753 K. The hydrogen permeation exhibited two different activation energies over the temperature range: lower activation energy of about 9.8 kJ/mol above 548 K, while higher activation energy of about 26.4 kJ/mol below 548 K. After permeation tests, the alloy membranes were characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, energy dispersive X-ray analysis and in situ X-ray diffraction. Palladium segregation on the surface of these palladium-copper alloys may induce changes of hydrogen permeation performance and thus influence the activation energies.     

Towards nanoporous membranes based on ABC triblock terpolymers

Block copolymers represent an exciting class of complex materials as they self-assemble into highly regular structures of nanoscopic dimensions. When prepared as thin films, such structures can be used for a variety of applications including lithographic masks or nanoporous membranes. Reported here are nanostructures in thin films of structurally analogous polybutadiene-block-poly(2-vinyl pyridine)-block-poly(tert-butyl methacrylate) (BVT) and polystyrene-block-poly(2-vinyl pyridine)-block-poly(tert-butyl methacrylate) (SVT) triblock terpolymers, which are synthesized via sequential living anionic polymerization. The morphological behavior of annealed SVT and BVT films is investigated by scanning force and electron microscopies. The difference in the terpolymer composition results in the formation of an ordered perforated lamella phase in SVT films and hexagonally packed core/shell cylinders in BVT films. Further, the BVT films show high potential for the fabrication of composite membranes using track-etched poly(ethylene terephthalate) macroporous filters as a support.     

Conductivity of nanoporous InP membranes investigated using terahertz spectroscopy

We have investigated the terahertz conductivity of extrinsic and photoexcited electrons in nanoporous indium phosphide (InP) at different pore densities and orientations. The form of electronic transport in the film was found to differ significantly from that for bulk InP. While photo-generated electrons showed Drude-like transport, the behaviour for extrinsic electrons deviated significantly from the Drude model. Time-resolved photoconductivity measurements found that carrier recombination was slow, with lifetimes exceeding 1?ns for all porosities and orientations. When considered together, these findings suggest that the surfaces created by the nanopores strongly alter the dynamics of both extrinsic and photoexcited electrons.     

Surface modification of nanoporous alumina membranes by plasma polymerization

The deposition of plasma polymer coatings onto porous alumina (PA) membranes was investigated with the aim of adjusting the surface chemistry and the pore size of the membranes. PA membranes from commercial sources with a range of pore diameters (20, 100 and 200?nm) were used and modified by plasma polymerization using n-heptylamine (HA) monomer, which resulted in a chemically reactive polymer surface with amino groups. Heptylamine plasma polymer (HAPP) layers with a thickness less than the pore diameter do not span the pores but reduce their diameter. Accordingly, by adjusting the deposition time and thus the thickness of the plasma polymer coating, it is feasible to produce any desired pore diameter. The structural and chemical properties of modified membranes were studied by scanning electron microscopy (SEM), atomic force microscopy (AFM) and x-ray electron spectroscopy (XPS). The resultant PA membranes with specific surface chemistry and controlled pore size are applicable for molecular separation, cell culture, bioreactors, biosensing, drug delivery, and engineering complex composite membranes.     

Suspended nanoporous membranes as interfaces for neuronal biohybrid systems

A biohybrid system composed of neuronal cells and silicon-supported nanoporous membranes has been designed to facilitate control of the biochemical environment of neuronal networks with cellular resolution. The membranes may exhibit variable pore sizes and interpore distances and are interfaced to a microfluidic device. Different porosity parameters give rise to changes in the transconductance of the nanopores and can therefore be used to control diffusion of molecules through the membranes. It was shown that the porous membranes are biocompatible with primary vertebrate as well as insect neurons. Our results indicate that nanoporous membranes may be used to interface with biological materials in a biohybrid system, for example as an artificial chemical synapse interface.     

Electrophoretic concentration of proteins at laser-patterned nanoporous membranes in microchips

Laser-patterning of nanoporous membranes at the junction of a cross channel in a microchip is used to integrate protein concentration with an electrokinetic injection scheme. Upon application of voltage, linear electrophoretic concentration of charged proteins is achieved at the membrane surface because buffer ions can easily pass through the membrane while proteins larger than the molecular weight cutoff of the membrane (>5700) are retained. Simple buffer systems can be used, and the concentration results constitute outward evidence that the uniformity of buffer ion concentration is maintained throughout the process. Local and spatially averaged concentration are increased by 4 and 2 orders of magnitude, respectively, upon injection with moderate voltages (70-150 V) and concentration times (100 s). The degree of concentration is limited only by the solubility limit of the proteins. The porous polymer membrane can be used repeatedly as long as care is taken to avoid protein precipitation.     

新的载体媒介传递膜

总结了近几年研究和发展的几种载体媒介传递膜。与支撑液膜相比，这些膜具有好的稳定性和长的寿命。对某些物质，如重金属离子、小分子中性碳氢化合物、氨基酸等有高的选择性和通量。它们的传递机理为固定位置跳跃或移动和固定载体2种机理结合。这些研究有望在环境、生物等技术领域中应用。