Membrane Surface Layer Research Articles

Modification of track membrane structure by plasma etching

An investigation of the properties of poly(ethylene) terephthalate track membranes (PET TMs) treated with a plasma RF-discharge in air has been performed. The influence of the plasma treatment conditions on the basic properties of the membranes, namely pore size and pore shape, porosity and mechanical strength has been studied. It was arranged that the effect of air plasma on the PET TMs results in etching a membrane's surface layer. The membranes' pore size and the form in this case change. It is shown that it is possible to change the structure of TMs directly by gas-discharge etching. Depending on the choice of discharge parameters, it is possible to achieve etching conditions either in a part of the channel or along the whole length of the pore channels. In both cases membranes with asymmetric pore shapes are formed which possess higher porosity and flow rate. The use of the membranes of such a type allows to increase drastically the efficiency of filtration processes.

The morphology characterisation and performance of dense PPO membranes for gas separation

Dense membranes were prepared from poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) using different solvents, that is, carbon disulphide, benzene, 1,1,2-trichloroethylene (TCE), toluene, chlorobenzene and bromobenzene. The membranes were characterised by atomic force microscope (AFM), electron spin resonance (ESR) and Raman scattering. The permeation data for N 2, O 2, CO 2 and CH 4 was taken by using a constant volume system. It was observed that the PPO membrane prepared by using carbon disulphide as a solvent (PPO-CS 2) had a entirely different surface morphology when compared with the surface of the membranes prepared by using other solvents. The PPO-CS 2 membrane's surface layer contained super nodular aggregates of size 1.199 μm in diameter. These super nodular aggregates consisted of nodules of size about 43.0 nm. In contrast no super nodular aggregates were observed in the other membranes. Interestingly, both selectivity and permeability of the PPO-CS 2 membrane were far superior to the other membranes. The physical properties, boiling point (BP), surface tension and viscosity of solvents used for the preparation, were correlated to the data from membrane characterisation as well as permeation experiments. Permeability increased and the selectivity ( O 2 N 2 and CO 2 CH 4 ) decreased with an increase in the BP. A similar trend was observed for other properties. Regarding the roughness parameter of the membranes, it depended also on the physical properties of the solvents used. Generally, it increased with an increase in the BP of the solvents used. A similar trend was observed for other physical properties. The number of spins/g was less in the PPO-CS 2 and PPO-TCE membranes compared with benzene or its derivatives. It was observed that in the presence of CH 4 and N 2 the number of spins/g was always higher than CO 2 and O 2 respectively.

Behavior of tetrahydrolipstatin in biological model membranes and emulsions

Tetrahydrolipstatin (orlistat) (S)-1-[(2S,3S)-3-hexyl-4-oxooxetan-2-yl]methyl]dodecyl N-formyl-L-leucinate, a potent inhibitor of pancreatic lipase, is hydrophobic, amphipathic, and water-insoluble. It binds irreversibly to pancreatic lipases and inhibits fat absorption. The focus of this investigation is on the distribution of orlistat in emulsified fat and vesicular membranes such as might be present in the intestine during fat absorption. The models used were unilamellar vesicles and microemulsion particles. [13C]orlistat was synthesized containing 99% 13C in the leucine carbonyl. Spectrawere collected on a Bruker DMX 500 Spectrometer. The chemical shift of the [13C]leucinate carbon was recorded in solvents with increasing hydrogen bonding capacity. The chemical shift moved downfield as H-bonding increased. [13C]orlistat was incorporated into triolein in the presence or absence of water, into sonocated unilamellar egg yolk phosphotidylcholine (EYPC) vesicles, and into microemulsions approximately 300 A in diameter containing triolein and phospholipid in roughly equal molar proportions. [13C] orlistat was soluble in triolein and had a chemical shift at 20 degrees C of 171.46 ppm. When a small amount of water was added, the chemical shift moved down field to 171.69 ppm. When [13C]orlistat was incorporated into EYPC unilamellar vesicles, the chemical shift increased to approximately 172.0 ppm at 25 degrees C, indicating an orientation of [13C]leucinate in orlistat closer to the aqueous interface of vesicles, i.e., more surface oriented. In all systems there was a modest downfield increase in chemical shift as the temperature was raised from 5 degrees to 46 degrees C. When small amounts of [13C]orlistat (1% relative to the emulsion mass) were incorporated into microemulsions, the chemical shift was identical to that in the unilamellar vesicles indicating a surface-like orientation of [13C]orlistat. However, when 3% was incorporated, two peaks appeared, one related to the surface at about 172 ppm, and one related to the core at about 171.65 ppm. Thus, orlistat first partitions into the surface and then when the surface is saturated, it moves into the more hydrophobic core. The fact that the two pools can be resolved using 13C NMR spectroscopy indicates a modestly slow exchange between the core and surface pools. Thus, the potent lipase inhibitor orlistat is ideally situated in the surface layer of emulsion particles and membranes for interaction with enzymes that superficially bind to such surfaces.

Nanofiltration of Natural Organic Matter: pH and Ionic Strength Effects

Studies were conducted to examine the effect of solution chemistry, defined here as pH and ionic strength, on the permeability of negatively charged polymeric nanofiltration membranes. Water permeation through the membrane was demonstrated to decrease at conditions of low pH and high ionic strength in the absence of organic macromolecules. The reduction in membrane permeability was attributed to a compaction of the membrane matrix resulting from charge neutralization at the membrane surface and electric double layer compression. An uncharged model organic macromolecule (polyethylene glycol) was used to quantify the effects of solution chemistry on membrane compaction and solute rejection capabilities of the charged membrane. Studies of membrane permeability and rejection were then repeated with solutions containing natural organic matter (NOM), enabling concurrent evaluation of the effects of electric double layer compression as well as changes in both membrane structure and the apparent macromolecular size of charged NOM macromolecules.

Peculiar aspects related to the hydrogen permeation flux through a martensitic steel membrane before and after ion implantation

We have observed peculiar hydrogen transient fluxes through stainless steel membranes which cannot be explained in terms of the linear permeation laws. In particular, before attaining the steady-state regime, the hydrogen flux may show a maximum, depending on the membrane temperature. Moreover, by repeating the measurement without changing the relevant parameters, the steady-state permeation rate may strongly differ from the first measurement. To clarify whether the observed anomalous behaviour is due to surface or bulk effects we have modified the membrane surface layers by implantation of a reactive (N +) or a non-reactive species (Ar +). The subsequent H-permeation experiments show that the implanted surface, when facing the high-pressure side, does not permit the flux peaks (independently of the implanted species). Moreover, permeation processes are reproducible in the sense that there is no difference between the steady rates of the first and second measurements, except at high temperature.

Effect of mutations in the general secretory pathway on outer membrane protein and surface layer assembly in Aeromonas spp.

Mutations in the exeC-N operon of Aeromonas hydrophila, which block extracellular protein secretion, also result in large decreases in the level of the major outer membrane porin, protein II. Immunoblot analysis demonstrated that the porin missing from the outer membrane of the mutant was not accumulating elsewhere in the cell. Pulse-chase and immunoprecipitation analyses showed that the porin was as stable in the mutant as in the wild type, but that far less porin was synthesized in the exe mutants. The relationship between extracellular secretion involving the exe genes and the assembly of other outer membrane and surface proteins was also examined. Both the wild type and exeE mutants of A. hydrophila were capable of assembling the protein I and protein III porins under inducing conditions. Aeromonas sohria As9071, which contains a surface array protein, could secrete A. hydrophila aerolysin and required homologues of the A. hydrophila exe genes to do so; however, an exe− derivative of this bacteria was unaffected in its ability to assemble its surface array. These results demonstrate that the exe genes are not required for general outer membrane protein assembly in these bacteria, but that the synthesis of protein II is specifically downregulated in the exe mutants.Key words: extracellular secretion, outer membrane assembly, surface layer, Aeromonas.

On the effect of organo-silicon block copolymer—solvent—precipitant system composition on asymmetric membrane surface layer thickness

The dependence of dense surface diffusion layer thickness of asymmetric membranes prepared by phase inversion from an organo-silicon block copolymer—solvent—precipitant system on the system composition has been demonstrated.

Surface fluorination of composite membranes. Part II. Characterization of the fluorinated layer

The fluorinated surface layer of membranes exposed to a dilute stream of fluorine gas has been characterized. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy attenuated total reflection (FTIR-ATR) were used to determine the concentration and profile of reacted fluorine in poly (4-methyl-1-pentene) membranes after exposure to fluorine. The thickness of the fluorinated layer as a function of reaction time was calculated using the results of bulk elemental analysis and of XPS. The permeability of this layer was determined using the calculated thickness and the ( P f/ l f) values reported previously. It is shown that the permeability of the fluorinated material is two orders of magnitude lower than that of the original poly(4-methyl-1-pentene). A simple diffusion limited reaction model is able to give a good estimate of the thickness of the fluorinated layer as a function of fluorine reaction time.

Membrane potential and Donnan potential

The Nernst-Planck-Poisson equations for the potential profile across a membrane are exactly solved without recourse to the assumption of constant field within the membrane. It is assumed that the membrane core of thickness dc is covered by a surface layer of thickness ds in which the membrane-fixed charges are distributed at a uniform density N. The membrane boundary potentials as well as the diffusion potentials contribute to the membrane potential. It is shown that for ds greater or similar 1/k, k being the Debye-Hückel parameter, the potential in the membrane surface layer except in the region very near the membrane/solution boundary is effectively equal to the Donnan potential and that its contribution to the membrane potential becomes dominant as N increases. For low N, on the other hand, the membrane potential arises mostly from the diffusion potential.

Ultrastructural aspects of the histogenesis of diffuse and localized mesothelioma.

During an ultrastructural review of 30 diffuse and 10 localized mesotheliomas, it was apparent that some micrographs showed various stages in the developmental processes involved in the formation of histological patterns in diffuse mesotheliomas and a histogenetic link between diffuse and localized mesotheliomas. Cells in the stromal or sarcomatous regions of diffuse mesothelioma often show varying degrees of mesothelial differentiation and a gradual transition to cells with typical mesothelial characteristics that organize into structures recapitulating the surface layer of serosal membranes. Tumor cells in localized mesotheliomas had many similarities to the "stromal" cells in the diffuse counterpart including intercellular junctions, rare microvilli and occasional foci of basal lamina. It is postulated that diffuse and localized mesotheliomas share a common histogenetic origin as a result of neoplastic induction of specialized submesothelial cells. In this concept, tumor cells in diffuse mesotheliomas reflect stages in the differentiation and organization of normal serosal membranes and localized mesotheliomas mirror the earliest phases of this process.

Lipid and salt effects on carbocyanine dye-induced photo-voltages in bilayer membranes

Voltage transients induced by flash illumination of bilayer membranes with sorbed dye, 3,3′-dimethyl-2,2′-oxacarbocyanine chloride (diO-C 1-3-Cl), vary with membrane lipid composition and aqueous solution salt concentration. The voltage transients are probably induced by physical movements of sorbed dye molecules following photo-isomerization. Increased salt (NaCl and NaBr) concentrations in the aqueous solutions reduced the photo-voltage amplitudes by reducing the amount of dye sorbed to the membranes, and by decreasing the effective membrane thickness. The photo-voltage risetimes and falltimes varied systematically with salt concentration and membrane lipid composition, reflecting structural changes in the membrane's surface layer.

PERMEABILITIES OF OXYGEN AND CARBON DIOXIDE GAS THROUGH POLYPEPTIDE MEMBRANES

In order to obtain the polypeptide membrane having high permeability ratio of oxygen to carbon dioxide gas, the effect of chemical structure of the side chain on permeation and sorption behaviors was studied. Poly (L-methione) (PLM) was selected as the reference material, and its oxidation by hydrogen peroxide aqueous solution was made. The oxidized membranes obtained under severe oxidation condition and under rather mild condition are named as PLMO-I and PLMO-II, respectively. Each membrane has -SO- group in the side chain. Though the PLM and PLMO-II membranes have α-helical structure throughout the membrane, PLMO-I in the surface layer of the membrane may have somewhat disordered helical structure. The ratios of the permeability of oxygen to that of carbon dioxide gas are 0.37 for PLM, 0.53 for PLMO-II and 1.7 for PLMO-I. All of these values are higher than those, 0.1_??_0.2, of usual polymers. The high ratio for PLMO-I is due to the high affinity of oxygen for polymer and low diffusibility of carbon dioxide gas in the partially random coiled structure of the membrane surface layer.

Effect of 1,3:2,4-dibenzylidene-D-sorbitol (DBS) additive on reverse osmosis properties of cellulose acetate membrane

The performance of the modified membranes prepared by adding aprotic solvent and DBS to Manjikian-named Batch 47-type casting solution was investigated. The addition of 1.5 wt-%, DBS to the casting solution made by mixing cellulose acetate, acetone, formamide, and N,N-dimethylformamide in the proportion of 5:9:2:4 by weight brought about the optimum performance of the resulting membrane. From the results obtained, it is suggested that an addition of proper amount of DBS does not affect the separation mechanism of the cellulose acetate membrane but DBS has the effect of forming a number of homogeneous pores in the membrane surface layer.

Effect of trypsin, phospholipases, and membrane-impermeable reagents on the uptake of palmitic acid by isolated rat liver cells

Rat liver cells isolated by the collagenase-hyaluronidase perfusion method were treated with membrane-impermeable protein reagents (7-diazonium, 1–3-naphthalene disulfonate, diazotized sulfanilic acid, 8-anilino-naphthalene disulfonate), trypsin, phospholipase A, phospholipase C, and phospholipase D. The treated cells were incubated with [1- 14C]palmitate and the 14CO 2 produced was taken as a measure of fatty acid uptake by the cells. 14CO 2 production by the cells was not inhibited after treatments with the membrane-impermeable protein reagents or phospholipase D. Treatments with small amounts of trypsin or phospholipases A or C caused inhibition of CO 2 production from tracer amounts of palmitate. The inhibition by trypsin was partially, and that by phospholipase A was fully, reversed by increasing the amount of palmitic acid in the incubation medium. The oxidation of shorter-chain fatty acids such as octanoic acid was not decreased but increased after treating the cells with trypsin or phospholipase A. The membrane-impermeable reagents inhibited the oxidation of palmitate to CO 2 by liver cells isolated by mechanical dispersion. These reagents also inhibited the long-chain acyl CoA ligase activity of liver microsomes. From these results it is suggested that the inhibition of CO 2 production by intact liver cells from palmitate after enzyme treatments, is due to partial removal or modification of a normal transport component for long-chain fatty acids on the plasma membrane. The possibility of proteins (or lipoproteins) buried below the surface layer of plasma membrane in fatty acid uptake by liver cells is indicated.

Cathodic hydrogen-permeation paths and segregation sites in iron

Microscope examination was used to study the sites at which cathodic hydrogen diffusing through an armco-iron membrane emerged from the metal. Changes taking place at these sites and the formation of discontinuities in the metal structure during protracted hydrogen-permeation experiments were also studied. Electrical resistance measurements were used to investigate the structural damage in armco iron subjected to cyclic hydrogen charging. It was established that the gradual reduction in hydrogen permeability of iron, the preferential accumulation of hydrogen in the metal membrane surface layer, and the limited hydrogen-saturation level in this layer are all due to the formation in the metal of hydrogen collectors (pores) connected with the surface being charged.

Studies on Alkaline Milk Phosphatase III: A Theory for the Mechanism of Reactivation of Alkaline Milk Phosphatase in a Model System

AbstractA hypothesis on the mechanism of the reactivation of alkaline phosphatase from cream and milk is formulated. By adding pasteurized cream to raw cream the isozyme pattern of the alkaline phosphatases was found to be characteristically changed, leading to the formation of a strong zone of β‐isozyme. This so called „β‐effect”︁ principally resembles the reactivation process and may be considered as a simple model system for this process. During pasteurization of cream a part of the outer surface layer of the fat globule membrane particularly rich in the β‐isozyme of alkaline phosphatase is dispersed into the aqueous phase. By this mechanism denatured β‐isozyme migrates into the aqueous phase of cream or milk after pasteurization. When the pasteurized extract containing the denatured β‐AP is admixed to raw cream, a renaturation of the denatured dß‐AP to renatured rß‐AP is effected under influence of the conformation of the native β‐AP and associated lipoproteins present in the membrane.