Asymmetry of current–voltage characteristics: a bilayer model of a modified ion-exchange membrane

The asymmetry of the current–voltage characteristics of ion-exchange membranes is explained in terms of the model of a bilayer fine porous membrane with constant charge distributions over the thickness of layers. This model has previously been proposed for determining diffusion permeability of membranes. In the case of one uncharged (neutral) layer, a set of two implicit algebraic equations is derived for determining the total current–voltage characteristics (CVC) of a membrane. For the first time, implicit algebraic equations are obtained for calculating the limiting currents at different orientations of an anisotropic membrane in an electrodialysis cell and explicit expressions are derived for determining specific conductivity of the membrane from the slope of the ohmic region of a CVC under the approximation of “excluded coions.” The model may be successfully used for describing the CVCs of perfluorinated MF-4SC sulfonic cation-exchange membranes, the surface layers of which are modified with polyaniline or halloysite.     

Broadband dielectric spectroscopic studies of molecular motions in a Nafion® membrane vs. annealing time and temperature

The dynamics of macromolecular motions as coupled to proton migration in annealed Nafion® membranes were explored at temperatures above 100 °C using broadband dielectric spectroscopy. Loss permittivity vs. frequency spectra of both β and α relaxations showed increased relaxation times with annealing which was rationalized in terms of water de-sorption and diminished free volume. The α relaxation time increases with increased annealing temperature. A parameter N reflecting connectedness of charge hopping pathways was extracted and indicated that the conductivity network accumulates more charge traps, presumably due to free volume decrease, with increased annealing time. Conductivity increases with increased annealing time at all temperatures despite the decrease in N; this suggests a change in the nature of proton hopping on annealing at high temperatures to be between sulfonic acid groups rather than by hopping across H₂OH⁺OH₂ bonds.     

The stereochemistry of glycolipids. A key for understanding membrane functions?

A Commentary on the paper ”Studies on liquid‐crystalline glycosides" by Volkmar Vill, Thomas Böcker, Joachim Thiem and Fred Fischer. First published in Liquid Crystals, 6, 349‐356 (1989).     

Study on the oscillating phenomena of electrical potential across a liquid membrane

The electrical oscillations across a liquid membrane in water/oil/water system was studied with octanol as oil phase by introducing two opposite charged surfactants in oil and aqueous phase, respectively. The sustained and rhythmic oscillation was observed. To a certain extent, the features of the oscillation (e.g. induction time, frequency, life time and orientation of the pulse pikes) strongly depend on the property of surfactant, dissolved in octanol. The mechanism may be explained by the formation and destruction of dual-ion surfactant membrane accompanying with emulsification at the interface and considering the coupling effect of diffusion and associated reaction in the vicinity of the interface.     

Extraction of cadmium with trilaurylamine–kerosine through a flat-sheet-supported liquid membrane

Cadmium has been extracted as a chloride complex through a flat-sheet-supported liquid membrane (SLM), using the tertiary amine Alamine 304-1 (mainly trilaurylamine or TLA) in kerosine.The typical permeability of the membrane was 1.1×10⁻⁶ m s⁻¹. The rate limiting step is diffusion through the membrane. The cadmium loading of the extractant at the feed–membrane interface is high. Trilaurylammonium chloride crystallizes at the surface of the membrane above 0.2 M TLA. This salt blocks the pores and lowers the extraction rate up to a factor of 3. Apart from this blocking effect, the permeability through the membrane is described well with the presented model, using physically realistic parameter values. When the precipitation can be minimized, the system has good potentials for the extraction of cadmium chloride complexes.     

Evaluating membrane cell e.m.f.ś from different schems of integration of the relevant nernstian equations

Computing the e.m.f. E (and the related membrane potential E_M) of the general concentration cell: electrode J ¦ IJ (m₁) in S ¦ membrane I ¦ IJ (m₂) in S ¦ electrode J (where the membrane perm-selective to counterion I separates two solutions of molalities m₁ and m₂ of the same electrolyte IJ, one mole of which forms n_I moles of I of valency z_I and n_J moles of coion J of valency z_J, with n=n_I+n_J) requires integration between limits m₁ and m₂ of the differential equation: dE=[nRT/Fn_J|z_J|] [τ_Iz_I, - mτ_sM_s]dln(mγ)_IJ, where the transference numbers τ_I of I and τ_s of the solvent S through the membrane are customarily taken as constant. The key point is the integration of the term mdln(mγ), which in the current practice is oversimplified to give [(m₁ + m₂)/2] in (m₂γ₂/m₁γ₁), and may cause large overestimation of the solvent-transfer contribution, with unreliable results for E and E_M. The procedure recommended here is instead that of splitting

mdln(mγ) into the “ideal part”

mdlnm=m₂-m₁ and the “non-ideal part”

mdlnγ, the latter being safely integrable putting lnγ as an explicit function of m in terms of the extended Debye-Hückel equation. Alternative and equally reliable procedures are described.     

Separation of acetone/water mixtures by a modified γ-alumina membrane via a new method

A new surface deposition method was researched to decrease pore size of ceramic membranes. CaCO₃ was chosen to modify the top layer of γ-alumina membranes prepared by sol–gel processes. Separation of gaseous acetone/water mixtures by vapor permeation was carried out to characterize the membranes. Improvement of membrane separation property after modifications and SEM photographs proved that this new method was effective to reduce the membrane pore size.     

Preparation of γ-Al_2O_3 composite membrane and examination of membrane defects

A boehmite sol with a narrow particle size distribution and most probable diameter of 35 nm was prepared by peptization of boehmite suspension with HNO3 An unsupported γ-Al2O3 membrane made from the above sol possessed a narrow pore size distribution,with an average pore size of 4.8 nm and 320 m2/g specific surface area.A supported y-Al2O3 membrane produced by repeating dipping-drying-calcination procedure twice was proven to be defect-free by gas permeation measurements and SEM.     

Transport of binary water–ethanol mixtures through a multilayer hydrophobic zeolite membrane

Transport of water–ethanol mixtures through a hydrophobic tubular ZSM-5 (Si/Al = 300) zeolite membrane during pervaporation was studied experimentally and theoretically. The zeolite membrane was deposited on a support made of pure titania coated with three intermediate ceramic titania layers. The influence of feed concentration, feed temperature and permeate pressure on permeate fluxes and permeate concentrations was investigated in a wide range. Dusty gas model parameters of the support and all ceramic intermediate layers were calculated on the basis of gas permeation data. Mass transfer resistances and pressure drops in the different membrane layers during pervaporation were calculated for several process conditions. In particular the influence of the undesired but unavoidable pressure drop in the support and the intermediate layers on the effective driving force for pervaporation was evaluated and found to be relevant for predicting the overall process performance. The membrane prepared was found to be suitable for the recovery of highly concentrated ethanol from feed mixtures of relatively low ethanol concentrations at relatively low feed temperatures.     

Synthesis of polypiperazine-amide thin-film membrane on PPESK hollow fiber UF membrane

A new interfacial polymerization (IP) procedure is developed in order to synthesize polypiperazine-amide thin-film membrane on the inner surface of poly(phthalazinone ether sulfone ketone) (PPESK) hollow fiber ultrafiltration (UF) membrane.A hollow fiber composite membrane with good performance was prepared and studied by FT-IR and scanning electron microscopy.     

Separation of binary gas mixtures by means of sol–gel modified ceramic membranes. Prediction of membrane performance

Ceramic membranes based on an alumina support with two successive layers of alumina of decreasing pore size and a sol–gel top layer, were characterized by gas permeability experiments and used for separating binary H₂ and N₂ gas mixtures. A mathematical model based on mass balance calculations was developed to predict the composition of permeated gas as a function of the different experimental parameters. No gas diffusion assumption is made, and it allows, after a previous characterization of the membrane, to find the optimal conditions for gas separation.     

Electro-generation of TGA by Ni-mSA-mCS bipolar membrane

Ni-mSA-mCS bipolar membrane (BM) was prepared by sodium alginate (SA) and chitosan (CS),which were modified by Ca~(2 ) and glutaraldehyde as linking reagents,respectively,mSA-mCS membrane was characterized by FTIR,SEM,TG and used as a separator in the electrolysis cell to produce thioglycolic acid (TGA).The experiment results show that TGA was prepared effectively by electro-reduction of dithiodiglycolic acid (DTDGA) with the mixture of TGA and DTDGA in the cathodic chamber.The current efficiency was up to 66.7% at the room temperature (25℃) during the current density of 10 mA/cm~2.Compared with the traditional metal reduction method,the electro-reduction technology saves the zinc powder and eliminates the pollution to environment. (?)2007 Zhen Chen.Published by Elsevier B.V.on behalf of Chinese Chemical Society.All rights reserved.     

Study of ion transport across an amphoteric ion exchange membrane — general transport properties of a simple electrolyte

Basic electrochemical properties of an amphoteric ion-exchange membrane (1.0—PA—29), in which almost equal numbers of cationic and anionic sites are distributed homogeneously over the functionalized polymer networks, were studied for a variety of concentration-cell systems. Unlike the usual uniform ion-exchange membrane, potentiometric permselectivities were demonstrated even for ions of the same charge, as shown by the result that distinctive potential responses were observed for a series of alkali metal chloride, sodium salt, and tetra-alkylammonium chloride systems. The sequence of membrane permeabilities to cations and anions calculated from the electrochemical data could well be correlated to the sequence of the hydrated radii of the respective ions. So the potentiometric responses to any electrolyte system can be predicted from the permeability data or hydrated radius values of individual cations and anions. The present results suggest that the homogeneous amphoteric ion-exchange membrane functions effectively as an ion sieve, sifting out the sizes of permeating hydrated cations and anions.     

Characteristics and retention properties of a mesoporous γ-Al2O3 membrane for nanofiltration

A mesoporous γ-Al₂O₃ membrane was produced by the sol gel dipping technique, followed by a thermal treatment (calcination and sintering). Different sintering temperatures were applied, which led to membranes with an average pore diameter ranging from 8.7 to 3.4 nm, the latter one corresponding to a MWCO of 900 Da.Salt retention was very much dependent on the pH of the solution as such membranes have an amphoteric character. Minimal salt retention was found at the isoelectric point (pH 7.5). Experiments were carried out with NaCl, MgCl₂ and LaCl₃ at different concentrations and in both single salt solutions and mixtures. The results are interpreted in terms of Donnan exclusion and in terms of the formation of an electrical double layer in the pores.Dynamic corrosion tests showed that some corrosion occurs at a pH of 2 or lower.     

Interfacial enzyme activation, non-lamellar phase formation and membrane fusion. Is there a conducting thread?

Previous studies from this laboratory have shown that the enzymic generation of diacylglycerol in bilayers by phospholipase C may lead to membrane fusion through the formation of transient non-lamellar lipidic intermediates. The present paper intends to explore the correlations existing among the three main processes involved, namely (a) the induction (or inhibition) of lamellar-to-non-lamellar phase transitions in lipid mixtures through the addition of small (< 5 mol%) proportions of other lipids, (b) the promotion, by the latter lipids, of fusion in otherwise stable phospholipid vesicles (large unilamellar liposomes) under conditions leading to inverted hexagonal/inverted cubic phase formation in bulk lipid systems, and (c) the modulation, by the same small proportions of lipids, of phospholipase C hydrolysis of phosphatidylcholine in liposome bilayers. It is concluded that phospholipase C may give rise to non-lamellar lipidic structures that in turn permit liposomal fusion to occur, but neither enzyme activity is directly modulated by non-lamellar phase formation, nor will whatever kind of enzyme-induced non-lamellar structure give rise to fusion. Moreover, only under certain kinetic conditions will the enzyme give rise to the organization of non-lamellar structures that are conducive to the fusion event.     

N-glycosides. 9. Reaction of 3,5-O-isopropylidenexylofuranosylamine with β-isothiocyanatoaldehydes. Synthesis of new cyclonucleosides with a hexahydropyrimidine aglycone

The reaction of 3,5-O-isopropylidenexylofuranosylamine p-toluenesulfonate with -isothiocyanatoaldehydes in the presence of triethylamine gives 4,2-anhydro-4-hydroxy-3-(3,5-O-isopropylidene--D-xylofuranosyl)hexahydropyrimidine-2-thiones. The structure of these compounds and their deblocking products was studied by IR, UV, PMR, and ORD spectroscopy and mass spectrometry.For Communication 8, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1252–1258, September, 1993.     

Determination of oil–water partition coefficients of polar compounds: silicone membrane equilibrator vs. SPME passive sampler

Experimental determination of oil-water partition coefficients often poses difficulties associated with emulsion formation. The aim of this work was to find an appropriate technique for determination of oil–water partition coefficients of polar, nonvolatile compounds. Two different methods were tested. The first method used a “silicone membrane equilibrator.” For the second method, solid-phase microextraction (SPME) fibers with a polyacrylate (PA) coating were used as a passive sampler. With both methods, oil–water partition coefficients for 14 compounds with polar functional groups were determined at 37 °C with good repeatability (standard deviation 0.11 log units or lower). The partition coefficients determined with the silicone membrane equilibrator method ranged from 0.50 to 3.49 log units. The oil–water partition coefficients obtained with the PA-SPME passive sampling approach were significantly higher than those obtained with the silicone membrane equilibrator method for nine of 14 compounds. The differences were up to 0.39 log units (i.e., a factor of 2.5). Additional experiments suggested that this difference occurred because the sorption properties of the PA fibers used were influenced by the surrounding phase, e.g., through swelling of the polymer phase. Therefore, the SPME passive sampling method using PA fibers seems to be less reliable, whereas the silicone membrane equilibrator method was found to be a convenient technique for the determination of oil–water partitioning.     

Potentiometric discrimination of organic amines by liquid membrane electrodes based on a long alkyl chain derivative of β-cyclodextrin

A long alkyl chain derivative of -cyclodextrin (6^A,6^B,6^C,6^D,6^E,6^F,6^G-hepta-S-dodecyl-6^A, 6^B,6^C,6^D,6^E,6^F,6^G-heptathio--cyclodextrin tetradecaacetate) was examined as a new type of sensory element of an ion-selective electrode for organic amines. Poly(vinyl chloride) (PVC) matrix liquid membrane electrodes based on this host were prepared by using several membrane solvents, among which 2-fluoro-2-nitrodiphenyl ether (FNDPE) was found to be by far most effective for potentiometric responses by the cyclodextrin host. Guest-induced potential changes were examined at pH 5.0 for several groups of protonated organic amine guests having different types of nonpolar moieties. The magnitude of the guest-induced potential change was in the order of 2-phenylethylamine > 1-adamantanamine > benzylamine > cyclohexanamine > dopamine. Potentiometric discrimination was also observed for the positional isomers of di- and trimethoxybenzylamines, though the discrimination between the isomers of monomethoxybenzylamines was negligible. These potentiometric selectivities were quite different from those for a liquid membrane electrode containing no particular sensory element. No appreciable effect was observed by addition of potassium tetrakis(p-chlorophenyl)borate as a fixed anionic site. Such a characteristic feature of this cyclodextrin-based liquid membrane electrode was explained in terms of the availability of an inclusion complex between the cyclodextrin host and each guest in a geometry that would be stable at the membrane surface.Presented in part at the IUPAC 2nd International Symposium on Bioorganic Chemistry, Fukuoka, Japan, June 6–10, 1993. Abstracts, p. 239     

Is use of the hydrophobic moment a sound basis for predicting the structure-function relationships of membrane interactive alpha-helices?

Amphiphilic alpha-helices play a fundamental role in protein membrane association and show a segregation of polar and apolar amino acid residues. Based on correlations between amphiphilic properties and biological function, a number of theoretical approaches have been developed, which quantify alpha-helix amphiphilicity and then attempt to assign function. The most commonly used measure of amphiphilicity is the hydrophobic moment, < microH >, which, when used in conjunction with an alpha-helix's mean hydrophobicity, < H >, has been used to classify membrane interactive amphiphilic alpha-helices as either surface active or transmembrane. Here, the predictive efficacy of plot methodology is reviewed by examining published data, which compare the function of known membrane interactive amphiphilic alpha-helices to that assigned by this methodology. The results of this review are discussed in relation to the reliability of < microH > as a quantifier of alpha-helical amphiphilicity, and the ability of < microH > and < H > to describe alpha-helical structure / function relationships. It is concluded that hydrophobic moment plot methodology is not a generally reliable predictor of alpha-helical structure / function relationships. It appears that the inefficacy of plot methodology is primarily due to the inability of the plot diagram to accommodate the heterogeneity of the alpha-helical classes it attempts to define. However, the predictive efficacy of the methodology appears to be improved if other alpha-helical parameters are also considered when assigning alpha-helical function. It is suggested that the conventional methodology should be seen only as an indicator for the assignation of structure / function relationships, providing a guide to future experimental investigations.     

846 — Microtubule-dependent membrane interactions studied in two types of double bilayer membrane systems

The microtubules consisting of tubulin subunits are ubiquitous cytoskeletal elements implicated in a variety of cellular functions. It is probable that they play an important role in the integration of the membrane processes and in the structural-functional organization in the cells by mediating long-range interactions between membranes, and by functioning as an intermembrane linking system. This suggestion was tested by using two different techniques for studying the interrelationships between two bilayer lipid membranes (BLMs). In both systems evidence for microtubule-assisted interconnections between the membranes was obtained. When electric pulses with amplitude 60–70 mV and duration 30 ms were applied across one of the membranes a displacement in the potential across the other membrane was observed even when the membranes were separated apart. This effect was found only in the presence of microtubules. It was not observed in MES-buffer or in the presence of depolymerized microtubule protein, colchicine-treated tubulin or albumin. On the basis of the data, the suggestion is made that defined configurations of the microtubule fiber networks and bundles may form structural bridges and coupling between membranes. These findings may be related to the role of the microtubules in the processes of information transfer in neurons and other cells.