Development of polyelectrolyte complexes of chitosan and phosphotungstic acid as pervaporation membranes for dehydration of isopropanol

Using a solution technique, chitosan-based polyelectrolyte complexes (PECs) were developed as pervaporation membranes by incorporating phosphotungstic acid (PTA). The resulting membranes were characterized by Fourier transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (WAXD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). Membranes were tested for their ability to separate water–isopropanol mixtures by pervaporation in the temperature range of 30–50 °C. The experimental results demonstrated that both flux and selectivity were increased simultaneously with increasing PTA content in the membrane. The permeation flux of pure chitosan membrane was increased dramatically from 4.13 to 11.70 × 10⁻² kg/m² h and correspondingly its separation factor was increased from 4490 to 11,241 and then decreased to 7490 at 30 °C for 10 mass% of water in the feed. The total flux and flux of water were found to be almost overlapping particularly for PECs membranes, suggesting that these could be used effectively to break the azeotropic point of water–isopropanol mixtures. From the temperature dependency of diffusion and permeation values, the Arrhenius activation parameters were estimated and discussed in the context of membranes efficiency. The pure chitosan and a small amount of PTA-incorporated PECs membranes exhibited positive heat of sorption while other PECs membranes exhibited negative heat of sorption, giving exothermic contribution.     

Self-assembled polyelectrolyte multilayer membranes with highly improved pervaporation separation of ethanol/water mixtures

Ethanol/water pervaporation through ultrathin polyelectrolyte multilayer membranes is described. The membranes were prepared by the layer-by-layer technique, i.e. by alternating sequential adsorption of cationic and anionic polyelectrolytes on a porous support. The separation capability was optimized by variation of the chemical structure of the polyelectrolytes, by variation of pH and ionic strength of the polyelectrolyte solutions used for membrane preparation and by annealing of the polyelectrolyte membranes. It was found that the separation is mainly affected by the charge density of the polyelectrolytes which is controlled by the chemical structure and the degree of ionisation of the polar groups. Selectivity for water was highest, if polyelectrolytes of high charge density such as polyethyleneimine (PEI), polyvinylamine (PVA) and polyvinylsulfate (PVS) were used and if the pH of the polyelectrolyte solutions was equal to the mean of the pK_a values of the corresponding cationic and anionic polyelectrolyte. Best results were obtained for PVA/PVS and PEI/PVS membranes which are characterized in detail with regard to their separation behavior.     

Preparation of polyelectrolyte multilayer membranes by dynamic layer-by-layer process for pervaporation separation of alcohol/water mixtures

In the past decades, the layer-by-layer (LBL) adsorption of oppositely charged polyelectrolytes has proven to be a promising method for the preparation of polyelectrolyte multilayer membranes. However, to obtain a good separation capability, LBL adsorption involved relatively long periods because 50–60 bilayers were normally required. The aim of this study was to develop such a new method that would allow simplification of the LBL procedure. LBL adsorption was proposed to proceed under a dynamic condition to prepare polyelectrolyte multilayer membranes. The polyacrylic acid (PAA) and polyethyleneimine (PEI) were alternatively deposited on polyethersulfone (PES) ultrafiltration support membrane under a pressure of 0.1 MPa. The polyelectrolyte multilayer membranes prepared by dynamic LBL process were compared with those prepared by the static LBL process for the pervaporation separation of water–ethanol mixture. The results suggested that a relatively high separation factor could be obtained with only four composite bilayers by using dynamic LBL process. The preparative conditions including bilayer number, filtration time of the first PAA layer, reaction time, ratio between polayanion and polycation concentrations, PAA molecular weight and salt addition were investigated. The pervaporation conditions such as feed temperature and water concentration in the feed were also evaluated. Under the temperature of 40 °C, the separation factor and the permeate flux of the polyelectrolyte multilayer membranes were about 1207 and 140 g/(m² h), respectively.     

Ultrathin cross-linked nanoparticle membranes

The fabrication of functional nanostructured materials for sensing, encapsulation and delivery requires practical approaches to self-assembly on multiple length scales and the synthesis of tough yet permeable structures. Here, the self-assembly of functionalized, photoluminescent nanoparticles at liquid interfaces, followed by cross-linking of the associated ligands, affords robust membranes that maintain their integrity even when they are removed from the interface. These composite membranes, nanometers in thickness, are elastic yet permeable and have potential applications involving controlled permeability and diffusion. Cadmium selenide (CdSe) nanoparticles are used, since their inherent photoluminescence offers a direct way to probe the spatial organization of the particles. Functionalized ligands attached to the nanoparticles provide an effective means to stabilize the interfacial assembly by cross-linking. The concepts shown are adaptable to other type of nanoparticles, ligands, and solvent combinations.     

Suspended self-assembled opal membranes

Suspended self-assembled opal membranes have been prepared from 440- or 170-nm-diameter silica spheres by evaporation of their colloidal solutions over vertically oriented 0.3-mm-thick silicon wafers containing frustum-shaped openings. Robust 0.5 x 0.5 mm(2) membranes with a thickness of ca. 300 mum have been reproducibly prepared using 170 nm silica spheres. The membranes show regular fcc packing with an exposed (111) orientation and are formed in a process that involves drawing silica spheres into the opening with the solvent meniscus.     

Correlation of antithrombogenicity and heat treatment for layer-by-layer self-assembled polyelectrolyte films

Antithrombogenic films with high durability were fabricated in a wet process. Antithrombogenicity was achieved with polyelectrolyte multilayer thin film prepared from poly(vinyl alcohol)-poly(acrylic acid) (PVA-PAA) blends, deposited in alternate layers with poly(allylamine hydrochloride) (PAH). Film durability, assessed by abrasion resistance and water resistance, was enhanced by forming cross-links via amide bonds induced by heat treatment of the film. The film was found to be resistant to protein adsorption, as measured by the amount of fibrinogen adsorbed from an aqueous solution. The antithrombogenic efficacy was assessed in ex vivo experiments by the ability of stainless steel mesh, coated with the polyelectrolyte and inserted into a pig blood vessel, to inhibit thrombus formation. Mesh coated with the polyelectrolyte did not reduce blood flow over a period of 15 min, whereas with uncoated mesh blood flow stopped within 6 min because of blood vessel blockage by thrombus formation.     

Poly(vinyl alcohol)/polyelectrolyte complex blend membrane for pervaporation dehydration of isopropanol

Poly(vinyl alcohol) (PVA) was blended with soluble polyelectrolyte complex (PEC) made from poly(diallyldimethylammonium chloride) (PDDA) and sodium carboxymethyl cellulose (CMCNa). Crystallinity, thermal transition, and thermal stability of the PVA/PEC blends were characterized by using wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and thermal gravity analysis (TGA), respectively. Surface morphology, cross-section and phase structure of the blend membranes were examined by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). Surface hydrophilicity and swelling behavior of the blend membranes were examined by water contact angle (CA) and swelling tests. Blend membranes were subjected to isopropanol dehydration, and effects of blend composition, feed composition and feed temperature on pervaporation performance are discussed in terms of phase structures of blend membranes. A performance of J = 1.35 kg/m² h, α = 1002, was obtained for blend membrane containing 50 wt% PEC in dehydrating 10 wt% water–isopropanol at 70 °C.     

Layer-by-layer self-assembly of polyelectrolyte complexes and their multilayer films for pervaporation dehydration of isopropanol

Two negatively charged polyelectrolyte complex colloidal nanoparticles (PEC⁻) and one positively charged nanoparticle (PEC⁺) were prepared and used as novel layer-by-layer (LbL) building blocks. These PEC nanoparticles include poly(2-methacryloyloxy ethyl trimethylammonium chloride)/sodium carboxymethyl cellulose (PDMC/CMCNa PEC⁻), poly(diallyldimethylammonium chloride)/CMCNa (PDDA/CMCNa PEC⁻) and PDDA/poly(sodium-p-styrenesulfonate) (PDDA/PSS PEC⁺). LbL multilayer films based on (PEC⁺/PEC⁻) were constructed on both quartz slides and modified polyamide (MPA) reverse osmosis support membranes. UV–vis spectroscopy, quartz crystal microbalance (QCM), field emission scanning microscopy (FESEM) and atomic force microscopy (AFM) were utilized to follow the thickness growth and morphology evolution of these multilayer films with increasing bi-layer numbers. LbL multilayer films deposited on MPA support membranes were subjected to pervaporation dehydration of 10 wt% water–isopropanol and effect of bi-layer numbers and feed temperature on pervaporation performance was studied. Generally, PEC⁺/PEC⁻ can be LbL self-assembled successfully on both substrates with a thickness growth rate ca. 200 nm/bi-layer. Moreover, PEC⁺/PEC⁻ multilayer films show high pervaporation performance with film thickness up to several micrometers. For example, performance of the multilayer films in dehydrating 10 wt% water–isopropanol at 50 °C is J = 1.18 kg/m² h, α = 1013 for (PEC⁺/PDMC-CMCNa PEC⁻)₂₄ and J = 1.36 kg/m² h, α = 938 for (PEC⁺/PDMC-CMCNa PEC⁻)₂₅, respectively.     

Asymmetric PVDF hollow-fiber membranes for organic/water pervaporation separations

Asymmetric PVDF hollow-fiber pervaporation membranes, with an inner diameter of 0.05–0.06 cm, an outer diameter of 0.07–0.08 cm, and a dense layer (≈ 3 μm in thickness) on the inner fiber wall, have been fabricated and tested for the removal of ppm concentrations of organics from water. Membranes were made by air-drying the outside of the fibers for ca. 20 s and passing a fluid through the fiber bore. The set of casting conditions that produced the best hollow fiber, with a benzene separation factor of 1834 (for a 120 ppm benzene-in-water feed solution at 25°C and a downstream pressure of 0.025 atm) and a tensile strength 26.8 MPa, was a spinning solution of 25 wt% PVDF/30 wt% dimethylacetamide/45 wt% acetone and a bore fluid of 70 vol% water/25 vol% acetone/5 vol% dimethylacetamide. These membranes also effectively separated toluene, chloroform, and styrene from water. A small module containing 6–30 PVDF hollow fibers performed equally well for organic extraction from water with either a bore-side or shell-side feed when the feed-flow rate was sufficiently high to eliminate concentration polarization. Changes in organic flux and separation factor for variations in the organic feed concentration, downstream pressure, and temperature were qualitatively similar to those observed with asymmetric flat sheet PVDF pervaporation membranes.     

Renewable urea sensor based on a self-assembled polyelectrolyte layer

A renewable urea sensor based on a carboxylic poly(vinyl chloride) (PVC-COOH) matrix pH-sensitive membrane has been proposed, in which a positively charged polyelectrolyte layer is first constructed by using a self-assembly technique on the surface of a PVC-COOH membrane, and urease, with negative charges, is then immobilized through electrostatic adsorption onto the PVC-COOH membrane, by controlling the pH of the urease solution below its isoelectric point. The response characteristics of the PVC-COOH pH-sensitive membrane and the effects of experimental conditions have been investigated in detail. Compared with conventional covalent immobilization, the urea sensor made with this self-assembly immobilization shows significant advantage in terms of sensitivity and ease of regeneration. The potential responses of the urea sensor with self-assembly immobilization increase with the urea concentration over the concentration range 10(-5) - 10(-1) mol l(-1), and the detection limit is 0.028 mmol(-1). Moreover, this type of urea sensor can be repeatedly regenerated by using a simple washing treatment with 0.01 mol l(-1) NaOH (containing 0.5 mol l(-1) NaCl) and 0.01 mol l(-1) HCl. The urease layers and the polyelectrolyte layers on the PVC-COOH membrane are removed, the potential response of the sensor to urea solutions of different concentrations returns nearly to zero, and another assembly cycle of urease and polyelectrolyte can then be carried out.     

Optically active polyelectrolyte multilayers as membranes for chiral separations

Ultrathin films of chiral polyelectrolyte complex, prepared by the multilayering process, exhibit selectivity in the membrane separations of optically active compounds, such as l- and d-ascorbic acid. The flux through these polyelectrolyte multilayers, PEMUs, is exceptionally high and may be controlled by the concentration of salt present in the permeating solutions. Both in-situ ATR-FTIR and chiral capillary electrochromatography indicate that flux selectivity is mainly kinetically controlled, stemming from a difference in diffusion rates of various enantiomers through PEMUs, rather than a difference in partitioning.     

Controlled release by polyelectrolyte microcapsule membranes

Polyelectrolyte submicron microcapsules were prepared by interfacial crosslinking of an aqueous salt solution of poly(ethyleneimine) and a toluene solution of brominated poly-(2,6-dimethylphenylene oxide). The two solutions were brought together and mixed by sonication. As a result, a stable emulsion was obtained, which was subsequently cast into a membrane in which the microcapsules were embedded. The salt solution contained in the microcapsules could be released under controlled conditions. The rates of release were measured. They could be controlled by applying osmotic pressures, by additional quaternization of the membrane, or by modification of the structure of the capsule wall by introduction of a surfactant.     

Heterogeneous polyelectrolyte gels as stimuli-responsive membranes

Stimuli-responsive membranes may act as “on–off switches” or “permeability valves”, producing patterns of pulsatile release, where the period and rate of mass transfer can be controlled by external or environmental triggers (e.g. pH, temperature, electric field). In this work, composite-heterogeneous polyelectrolyte gel (composite-HPG) membranes consisting of polymethacrylic acid (PMAA) gel particles dispersed within a polydimethylsiloxane (PDMS) network were developed and evaluated as pH-responsive membranes.The mechanism of permeability control for caffeine and vitamin B₁₂ through composite-HPG membranes was determined to be a synergistic function of membrane hydration and the percolating volume fraction of PMAA gel. Larger changes in permeation as a function of pH were achieved when both hydration and percolation effects occurred together than when either of these effects occurred on their own. Vitamin B₁₂ permeation was observed when the hydrated gel volume fraction was above approximately 0.38, but not below. Furthermore, the percolating fraction of composite-HPG membranes containing 28% (dry basis) PMAA gel particles was manipulated via pH to fall above (pH 7) or below (pH 3) this transition in permeability, resulting in membranes that delivered solutes of high molecular weight (vitamin B₁₂) with large on/off delivery ratios (160).     

Homogeneous polyimide/cyclodextrin composite membranes for pervaporation dehydration of isopropanol

Composite membranes with a sub-nanoscale homogeneous distribution of CD toroids in the Matrimid matrix were developed for dehydration of aqueous isopropanol. The composite membranes demonstrated separation factor far surpassing that of the neat Matrimid dense membrane. The heart of this innovation is the utilization of a CD derivative, ethylenediamine-β-cyclodextrin (EDA-β-CD), where the amine of CD could react with the imide of Matrimid and efficiently immobilize the CD rings during membrane formation. The superior separation properties for membranes embedded with 2–5% EDA-β-CD were attributed to the additional water channels created by the hydrophilic outer surface of CD and its interactions with the polymer matrix. FT-IR, density measurements and XRD have confirmed these hypotheses. Nevertheless, the separation factor exhibited an increasing then decreasing trend as a function of CD content and the opposite trend was observed with permeation flux. Investigation on the effect of feed water concentration showed that the neat Matrimid membrane possessed almost constant performance, but the Matrimid/EDA-β-CD (0.05) composite membrane exhibited an obvious increase in permeability and a decrease in selectivity at high water content. Even though the composite membrane swelled more at higher water content due to the intensified hydrophilicity ascribed to the introduction of CD structure, it always had much better separation factor. In addition, the Matrimid mixed matrix membranes embedded with 2–5% EDA-β-CD held reasonably tensile strength and modulus. The newly developed mixed matrix membrane approach may open up a new way to prepare next-generation high-performance asymmetric pervaporation membranes for isopropanol separation.     

Long-term pervaporation performance of microporous methylated silica membranes

The incorporation of methyl groups in microporous silica membranes was proven to enhance the service time in the dehydration of a butanol-water mixture at 95 degrees C from a few weeks to more than 18 months with a water flux of about 4 kg m(-2) h(-1) and a selectivity between 500 and 20 000.     

On the behaviour of asymmetric membranes in pervaporation

Symmetric and asymmetric membranes of the Loeb type are compared with respect to their performance in pervaporation. The experiments are carried out with water—isopropanol mixtures, employing cellulose acetate membranes of different structure, but of the same total thickness. These results are compared with calculations based on a 2-layer model for asymmetric membranes. Design criteria for optimal asymmetric membranes for pervaporation, as well as the performance characteristics for the two possible modes of installation — active layer facing the feed or facing the permeate — are discussed. Contrary to reverse osmosis, the installation of the membrane with the active layer facing the permeate proves to be superior — at least for low permeabilities of the membrane material. The interdependencies between thickness and permeability of the active layer, and porosity and thickness of the support layer are much stronger than in reverse osmosis.     

Molecular sieving MFI-type zeolite membranes for pervaporation separation of xylene isomers

Molecular sieving MFI-type zeolite membranes were prepared by a secondary growth method without using an organic template. Silicalite membranes with intercrystalline pores minimized or eliminated were obtained by this synthesis method which avoids the template removal step. The silicalite membrane exhibits molecular sieving characteristics with pervaporation separation factor for p-xylene to o-xylene or m-xylene of as high as about 70, the highest ever reported for a pervaporation membrane.     

Layer by layer self-assembled polyelectrolyte multilayers with embedded phospholipid vesicles

We describe a method to embed phospholipid vesicles into polyelectrolyte multilayers built up by the alternate deposition of polyanions and polycations. Before deposition, the vesicles are rigidified by polycation adsorption onto their surface avoiding their fusion once deposited on the multilayer surface. The vesicles adsorb to form a compact and "hard" monolayer as imaged by atomic force microscopy. The thickness of the adsorbed vesicle layer, of the order of 250 nm, is very close to the diameter of the vesicles in solution. This work should open the route to the buildup of multilayer films containing phospholipid vesicles that could act as "reservoirs" for drugs or enzymatic nanoreactors.     

Separation of liquids by pervaporation through polymeric membranes

Fundamental and applied aspects of liquid separation by means of pervaporation through polymeric membranes are considered. The review gives the state of the art as well as prospects of development of this branch of membrane science and technology.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 208–219, February, 1994.