Pore structure, thinning effect, and lateral diffusive dynamics of oriented lipid membranes interacting with antimicrobial peptide protegrin-1: 31P and 2H solid-state NMR study

Membrane pores that are induced in oriented membranes by an antimicrobial peptide (AMP), protegrin-1 (PG-1), are investigated by (31)P and (2)H solid state NMR spectroscopy. We incorporated a well-studied peptide, protegrin-1 (PG-1), a beta-sheet AMP, to investigate AMP-induced dynamic supramolecular lipid assemblies at different peptide concentrations and membrane compositions. Anisotropic NMR line shapes specifying toroidal pores and thinned membranes, which are formed in membrane bilayers by the binding of AMPs, have been analyzed for the first time. Theoretical NMR line shapes of lipids distributed on the surface of toroidal pores and thinned membranes reproduce reasonably well the line shape characteristics of our experimentally measured (31)P and (2)H solid-state NMR spectra of oriented lipids binding with PG-1. The lateral diffusions of lipids are also analyzed from the motionally averaged one- and two-dimensional (31)P and (2)H solid-state NMR spectra of oriented lipids that are binding with AMPs.     

Plasma-induced graft copolymerization of poly(methacrylic acid) on electrospun poly(vinylidene fluoride) nanofiber membrane

Electrospun nanofibrous membranes (ENM) which have a porous structure have a huge potential for various liquid filtration applications. In this paper, we explore the viability of using plasma-induced graft copolymerization to reduce the pore sizes of ENMs. Poly(vinylidene) fluoride (PVDF) was electrospun to produce a nonwoven membrane, comprised of nanofibers with diameters in the range of 200-600 nm. The surface of the ENM was exposed to argon plasma and subsequently graft-copolymerized with methacrylic acid. The effect of plasma exposure time on grafting was studied for both the ENM and a commercial hydrophobic PVDF (HVHP) membrane. The grafting density was quantitatively measured with toluidine blue-O. The degree of grafting increased steeply with an increase in plasma exposure time for the ENM, attaining a maximum of 180 nmol/mg after 120 s of plasma treatment. However, the increase in the grafting density on the surface of the HVHP membrane was not as drastic, reaching a plateau of 65 nmol/mg after 60 s. The liquid entry permeation of water dropped extensively for both membranes, indicating a change in surface properties. Field emission scanning electron microscopy micrographs revealed an alteration in the surface pore structure for both membranes after grafting. Bubble point measurements of the ENM reduced from 3.6 to 0.9 um after grafting. The pore-size distribution obtained using the capillary flow porometer for the grafted ENM revealed that it had a similar profile to that of a commercial hydrophilic commercial PVDF (HVLP) membrane. More significantly, water filtration studies revealed that the grafted ENM had a better flux throughput than the HVLP membrane. This suggests that ENMs can be successfully engineered through surface modification to achieve smaller pores while retaining their high flux performance.     

Characterisation of the electrokinetic properties of plane inorganic membranes using streaming potential measurements

We present and test a device designed to measure the streaming potential of plane inorganic membrane during filtration.Two kinds of microporous membranes (a membrane made of a mixture of alumina-titania and this same type of membrane covered with an additional titania layer) are studied with different pH, ionic strength and electrolyte nature. The modification of the surface acid-basic equilibriums is analysed from the streaming potential measurements. The pores size of the studied membranes is large enough to avoid overlapping of the double layers. Streaming potential measurements are used to determine the zeta potential of the membranes from the Helmholtz-Smoluchowski relationship, corrected for the lowest ionic strengths studied. The shifting of the isoelectric point of the membranes studied with CaCl₂ and Na₂SO₄ solutions shows specific adsorption of calcium and sulfate ions onto the surface. The additional titania layer on the alumina-titania support does not seem to modify the electrokinetic properties of the membrane.The interactions of the alumina-titania membrane with the H⁺ and OH⁻ ions are analysed by studying the variations of pH between permeate and retentate compartments. These variations allow determining the isoelectric point of the membrane with a reasonable precision.     

Synthesis of bilayer composite nanomembranes with conductivity asymmetry

The structure and electrochemical properties of polyethylene terephthalate track membranes modified in acetylene plasma are studied. It is found that polymer deposition on the track membrane surface using acetylene polymerization in plasma results in the case of formation of a semipermeable layer covering pores in formation of a composite nanomembrane featuring asymmetry of conductivity in solutions of electrolytes: a rectifying effect similar to that of a p-n junction in semiconductors. It is shown that the observed effect of conductivity asymmetry is caused by a significant decrease in the diameter of pores in the plasma-deposited polymer layer and a change in the pore geometry, same as existence of an interface between the initial membrane and polymer layer that have a different concentration of carboxyl groups in the surface layer. The impedance spectroscopy method allowed obtaining information on ion transfer in the studied membranes.     

Polymeric membranes: Surface modification for minimizing (bio)colloidal fouling

This paper presents an overview on recent developments in surface modification of polymer membranes for reduction of their fouling with biocolloids and organic colloids in pressure driven membrane processes. First, colloidal interactions such as London–van der Waals, electrical, hydration, hydrophobic, steric forces and membrane surface properties such as hydrophilicity, charge and surface roughness, which affect membrane fouling, have been discussed and the main goals of the membrane surface modification for fouling reduction have been outlined. Thereafter the recent studies on reduction of (bio)colloidal of polymer membranes using ultraviolet/redox initiated surface grafting, physical coating/adsorption of a protective layer on the membrane surface, chemical reactions or surface modification of polymer membranes with nanoparticles as well as using of advanced atomic force microscopy to characterize (bio)colloidal fouling have been critically summarized.     

The streaming potential method for the characterization of ultrafiltration organic membranes and the control of cleaning treatments

Streaming potential measurement of ultrafiltration (UF) membranes have been realised a new design. This new design is more convenient to determine the streaming potential on a function of the pressure for all kinds of modules (planar, hollow fiber.h.). The effects of pH, ionic strength and size of pores have been studied. Isoelectric points of different materials (polyethersulfone, celloulse acetate, cellulose triacetate and polysulfone membranes) have been experimentally determined from ν variations with pH at a given ionic are, respectively, 3.1, 4.2, 3.4 and 0.5. The study of the charge organic membranes studied has been shown that adsorbing ions are those of water itself. Then the surface charge of the membrane is a dependent on the pH and at the isoelectric point, the charge density and the streaming potential vanished. The polyethersulfone membrane surface has been modified with TX100 adsorption and the modification observed with our design compared to contact angle and permeabilities measurements. The orientation angle of the surfactant at the membrane surface is obtained: θ=5°, and shows that a flat adsorption occurs. The impact of membranes cleaning procedures have been studied in term of permeability completed by streaming potential measurements. It appeared clearly that streaming potential is a useful tool for the control of cleaning procedures.     

Chemical Cleaning of Microfiltration Membranes Fouled by Whey

Millipore hydrophobic polyvinylidene fluoride (PVDF) microfiltration membranes were used for whey processing. Fouled membranes were cleaned with acid (HCl), alkaline (NaOH) and surfactant (Triton‐X100). The latter resulted in maximum flux recovery and resistance removal. Hydrochloric acid had a moderate effect and sodium hydroxide was the weakest cleaning agent. This is due to the cleaning strength of emulsifiers compared to acid or alkali. However acids are more efficient than alkaline solutions for removal of mineral compounds which remain on the membrane surface. Cleaning efficiency depends on the concentration of cleaning agent being higher for higher surfactant concentration. For acids and alkali, the efficiency increases with increasing the concentration of the reagent reaches a maximum (optimum concentration) and then decreases. This can be explained by changes in permeability of the deposit layer with the concentration of the cleaning agent. Another explanation is the breakage of proteins by acid or alkali which produces more fouling materials and causes less cleaning efficiency. Operating conditions affect the cleaning process. At higher stirring speeds (turbulent flow) or longer cleaning time better removal of deposits and higher cleaning efficiency were observed. The sequential cleaning process may or may not improve the cleaning efficiency. When acidic cleaning was followed by washing with a surfactant an improvement was achieved. This can be attributed to the incomplete removal of deposits by acid. However further cleaning with acid can not improve the cleaning efficiency. During whey processing fouling occurs by deposition of foulants of mostly proteins and macromolecules on the membrane surface or in the membrane matrix. Large substances (compared to the membrane pores) settle on the membrane surface and the small species penetrate and are adsorbed in the membrane pores. Cleaning dissolves and removes the adsorbed foulants from the membrane.     

Modification of polysulfone membranes 4. Ammonia plasma treatment

The effect of NH₃ and NH₃/Ar plasma on ultrafiltration polysulfone membranes have been studied. Results of contact angle, FTIR-ATR and X-ray photoelectron spectroscopy experiments clearly showed that both plasmas introduced hydrophilic, nitrogen- and oxygen-containing moieties on the polymer surface and that NH₃/Ar plasma was more efficient. That plasma was also more aggressive--signs of strong etching could be seen on the SEM pictures. Redeposition of etched material seemed to take place inside the pores. On the contrary, ammonia plasma was soft and caused cleaning the surface and pores enlargement. Performance of ammonia plasma modified membranes was greatly improved and independent on solution pH. The last observation proved amphoteric character of the surface. NH₃/Ar plasma treatment gave membranes of acidic surface and filtration indices not so good as for ammonia plasma.     

Relation between membrane characteristics and performance in nanofiltration

The performance of commercial membranes during nanofiltration of aqueous solutions containing dissolved uncharged or charged organic components, was studied on the basis of membrane characteristics by means of multiple linear regression.

The membrane characteristics studied were surface hydrophobicity, surface roughness, surface charge, molecular weight cut-off (MWCO), permeability and porosity of the top layer (expressed as the volume fraction of small and large pores, determined by Positron Annihilation Lifetime Spectroscopy). Filtration and adsorption experiments were performed in the presence of various components, which differ in molecular mass, hydrophobicity and (in the case of charged organic components) in charge.

It was concluded that in order to minimize fouling, the membrane should have a low volume fraction of small pores in the top layer. When the organic components are charged, a membrane with a large surface charge and a high hydrophilicity is also favourable. Not only the membrane, but also the feed characteristics have an influence on fouling: the best results during nanofiltration of dissolved uncharged or charged components were obtained with hydrophilic or negatively charged components, respectively. Dissolved organic components were the best retained by membranes with a low MWCO. In addition, uncharged organic components should be hydrophilic and small to obtain a high retention and minimal flux decline, while the interplay between membrane and component charge is crucial during filtration of dissolved charged organic components.     

SEM研究PET核孔膜的光接枝聚合

以PET核孔膜为基材 ,二苯甲酮为引发剂 ,采用光接枝方法实现了丙烯酸和丙烯酰胺在核孔膜上的接枝 ,用扫描电镜 (SEM)直接观察了接枝前后膜的表面形貌 ,考察了不同因素对于接枝位置和接枝效果的影响 .发现膜材料本身特性和接枝反应条件对接枝位置和接枝效果有较大影响 .通过光接枝能够实现膜孔的封盖、缩小、填堵等不同的效果 .采用正侧涂布法反应 ,标准直孔 ,特别是小孔径膜 (0 4 μm) ,不利于孔内的接枝 ,接枝主要在膜的表面 ,从而产生孔封盖效应 .双锥形的非标准直孔 ,由于孔壁的受光性好 ,容易发生孔壁上的接枝从而被填充 .大孔径的膜 (5 μm) ,需要加入交联剂才能在孔壁上形成厚的接枝层 .提出了一种新的反应方法 背侧吸附法 ,反应液依靠毛细作用由膜的底部吸入膜孔 ,膜的向光侧表面不存在反应液 ,接枝只发生在膜孔内 ,从而得到很好的填孔效果 .     

两亲性梳状聚醚硅氧烷对相转化法聚偏氟乙烯多孔膜的共混改性作用研究

以聚醚链段为侧链的两亲性梳状聚醚硅氧烷(ACPS)为改性剂,研究了相转化法制备聚偏氟乙烯(PVDF)多孔膜的改性效果与机理.采用SEM、XPS、接触角、水通量等考察了ACPS对膜结构与性能的影响.研究发现,ACPS在相转化成膜过程中不流失,随着制膜液中ACPS含量的增加,相分离速度降低,膜中微孔由指状结构向蜂窝状结构发展,膜强度提高,亲水性显著提高.提出了ACPS在膜表面的富集现象和在膜中的稳定性机理和模型.结果表明,两亲性梳状聚醚硅氧烷在原理上是一类适合于相转化法制备聚合物微孔膜表面亲水化改性的有效物质.     

TFC polyamide membranes modified by grafting of hydrophilic polymers: an FT-IR/AFM/TEM study

Surface modification using grafting of a hydrophilic polymer onto the membrane surface is a possible route to improving the fouling properties of polyamide thin-film composite membranes. The structure of nanofiltration (NF) and reverse osmosis (RO) membranes modified using graft polymerization of acrylic (AA) monomers was visualized and analyzed using attenuated total reflection–Fourier transform infrared spectroscopy, atomic force microscopy and transmission electron microscopy. The results show that a layer of AA polymer is indeed formed on the polyamide surface, which could be accompanied by a change of the surface morphology. It was observed that for the NF membranes studied polymerization could also take place inside the pores of the support as a result of penetration of the monomer through the active layer, particularly for high degrees of grafting. It suggests that the modification procedures should be optimized so that the latter effect is minimized.     

Chemical polymerization of aniline on a poly(styrene sulfonic acid) membrane: Controlling the polymerization site using different oxidants

Poly(styrene sulfonic acid) membranes (Neosepta CMX, Tokuyama Corp.) have been modified by in situ polymerization of aniline. (NH4)2S2O8, FeCl3, H2O2, and KIO3 were used as oxidizing agents, and two different modification methods (single-step versus two-step) were studied. The composite membranes were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, elemental analysis, electrodialysis, ion-exchange capacity, and conductivity measurements. Our results demonstrate that it is possible to control the polymerization site of aniline which in turn affects the membrane selectivity properties. Hence, composite membranes having a very thin and homogeneous surface polyaniline layer lead to a very low transport of Zn 2+ without increasing significantly the resistance to H+ conductivity. On the other hand, membranes containing about the same quantity of PANI but inside the membrane do not block the transport of Zn 2+.     

Emulsion Filtration Through Surface Modified Ceramic Membranes

In this work we studied the influence of membrane hydrophobicity on the filtration of oil/water (O/W) emulsions with a dispersed phase content of 30% (V/V). The membrane filtration process was realized by using ceramic tubular hydrophilic or hydrophobic membranes with different mean pore size (0.2 pm, 1.2 pm, and 1.4 pm of mean pore radius). Hydrophobic character was obtained by modifying superficially the membrane surface with a very thin polymer layer. The results obtained showed that the emulsion viscosity and droplet size distribution depend on the shearing forces and transmembrane pressure. The operating conditions and the nature of the membrane surface/emulsion interaction are the main parameters which control the type and nature of emulsion changes, such as modification of the mean droplet size, concentration into oleic phase or breaking.     

Pore blocking mechanisms during early stages of membrane fouling by colloids

A method based on a simple linear regression fitting was proposed and used to determine the type, the chronological sequence, and the relative importance of individual fouling mechanisms in experiments on the dead-end filtration of colloidal suspensions with membranes ranging from loose ultrafiltration (UF) to nanofiltration (NF) to non-porous reverse osmosis (RO). For all membranes, flux decline was consistent with one or more pore blocking mechanisms during the earlier stages and with the cake filtration mechanism during the later stages of filtration. For ultrafiltration membranes, pore blocking was identified as the largest contributor to the observed flux decline. The chronological sequence of blocking mechanisms was interpreted to depend on the size distribution and surface density of membrane pores. For salt-rejecting membranes, the flux decline during the earlier stages of filtration was attributed to either intermediate blocking of relatively more permeable areas of the membrane skin, or to the cake filtration in its early transient stages, or a combination of these two mechanisms. The findings emphasize the practical importance of the clear identification of, and differentiation between mechanisms of pore blocking and cake formation as determining the potential for the irreversible fouling of membranes and the efficiency of membrane cleaning.     

Physicochemical characterization of a modified cellulose acetate membrane for the design of oil-in-water emulsion disruption devices

In this paper, a new membrane separation process is presented. This process is based on a multiple chamber cell that allows the separation of oil from water by placing two membranes of different hydrophobicity in contact with the emulsion. Further chemical modifications of the membranes have been performed in order to vary their hydrophobicity in the perspective of optimizing the fractionation process via specific interactions between membrane and emulsion components. For this purpose, methylcellulose was modified with 1,2-epoxydodecane and incorporated in the membrane elaboration process. The modification of membrane surface chemical structure was demonstrated by ATR-FTIR and XPS experiments. Preliminary filtration tests are presented as well as future perspectives to optimize the membrane modification procedure.     

Morphology and Transport Properties of Hybrid Materials Based on Perfluorinated Membranes,Polyaniline, and Platinum

The transport properties and morphological characteristics of perfluorinated membranes after deposition of the layer of platinum dispersion on the surface are studied. The significant effect of preliminary modification of perfluorinated membraned with polyaniline on the diffusion permittivity of the composite and the morphology of the layer of platinum dispersion is determined. Testing the composites as proton conductors with a catalytic layer on the surface in an air–hydrogen fuel cell has shown the effect of the asymmetry of the electrochemical characteristics of the membrane–electrode assembly at various orientations of the layer of platinum dispersion towards hydrogen and air flows. A higher catalytic activity of the composite membranes in the oxygen reduction reaction is determined in the case platinum dispersion is deposited onto the membrane preliminarily modified with polyaniline.

     

H2 separation using defect-free, inorganic composite membranes

Defect-free, microporous Al(2)O(3)/SAPO-34 zeolite composite membranes were prepared by coating hydrothermally grown zeolite membranes with microporous alumina using molecular layer deposition. These inorganic composite membranes are highly efficient for H(2) separation: their highest H(2)/N(2) mixture selectivity was 1040, in contrast with selectivities of 8 for SAPO-34 membranes. The composite membranes were selective for H(2) for temperatures up to at least 473 K and feed pressures up to at least 1.5 MPa; at 473 K and 1.5 MPa, the H(2)/N(2) separation selectivity was 750. The H(2)/CO(2) separation selectivity was lower than the H(2)/N(2) selectivity and decreased slightly with increasing pressure; the selectivity was 20 at 473 K and 1.5 MPa. The high H(2) selectivity resulted either because most of the pores in the Al(2)O(3) layer were slightly smaller than 0.36 nm (the kinetic diameter of N(2)) or because the Al(2)O(3) layer slightly narrowed the SAPO-34 pore entrance. These composite membranes may represent a new class of inorganic membranes for gas separation.     

Visualization of distribution of endotoxin trapped in an endotoxin-blocking filtration membrane

Since the finding of β₂-microglobulin as a causal substance in the carpal tunnel syndrome of chronic hemodialysis patients, removal of β₂-microglobulin has been performed using highly permeable dialysis membranes with larger pores. Such large-pore membranes tend to allow endotoxins (Et), harmful substances contained in dialysate, to enter blood. At present, as a countermeasure, Et-blocking filtration membranes are used to remove Et from dialysate. However, Et removal mechanism by these membranes has not been clarified yet. The objective of this study is thus to visualize distribution of fluorescence-labeled Et trapped inside the polyester–polymer alloy (PEPA) membrane, a widely used Et-blocking filtration membrane using a confocal laser scanning fluorescence microscope (CLSFM). Et were observed mainly in the outer skin layer of the hollow fiber, while some in the void and inner skin layers. No Et were present inside the hollow fiber. In conclusion, we succeeded in visualization of Et distribution inside the Et-blocking filtration membrane using CLSFM. This novel visualization technique may allow evaluation of distribution of Et trapped inside various kinds of Et-blocking filtration membranes.     

Modification of the Surfaces of Polymer Films Irradiated by Heavy Ions and Nucleopore Nanofilters by XeF2 Vapors

The effect of xenon difluoride (XeF₂) vapors on the surface properties of the polymer films irradiated by heavy ions and of the nucleopore nanofilters produced from these films is studied. The procedure for modifying nucleopore membranes made of poly(ethylene terephthalate) and polyimide in XeF₂ vapors is developed. The hydrophilicity, electrosurface properties, and selectivity of modified membranes are investigated. It is shown that the water contact angle on the membrane surface decreases as a result of modification, and the extent of its change depends on the duration and temperature of membrane treatment. Electrokinetic measurements did not reveal any changes in the surface charge during the modification of the membranes prepared from poly(ethylene terephthalate), but their ion selectivity increased twofold. It is shown that the hydrodynamic diameter of the pores of modified membranes reversibly decreases with an increase in transmembrane pressure. The dislosed effects are explained by the presence of an elastoplastic gel-layer on the surfaces of the membrane and its pores. The gel-layer accumulates considerable surface and bulk charges and is characterized by the high hydraulic resistance hindering the convective ion transport.