Membranes GEORG WAHL, Technische Universit€at Braunschweig, Braunschweig, Germany PAUL B. DAVIES, University of Cambridge, Cambridge, United Kingdom ROINTAN F. BUNSHAH, University of California, Los Angeles, United States BRUCE A. JOYCE, University of London, London, United Kingdom COLIN D. BAIN, University of Oxford, Oxford, United Kingdom GERHARD WEGNER, Max-Planck-Institut f€ur Polymerforschung, Mainz, Germany MARKUS REMMERS, Max-Planck-Institut f€ur Polymerforschung, Mainz, Germany FRANCIS C. WALSH, University of Portsmouth, Portsmouth, United Kingdom KONRAD HIEBER, Fraunhofer-Institut f€ur Festk€orpertechnologie, M€ unchen, Germany JAN-ERIC SUNDGREN, Link€oping University, Link€oping, Sweden PETER K. BACHMANN, Philips GmbH, Research Laboratories, Aachen, Germany SHINTARO MIYAZAWA, NTT LSI Laboratories, Kanagawa, Japan ALFRED THELEN, Ingenieurb€uro Thelen, Schmitten-Seelenberg, Germany HEINER STRATHMANN, University of Twente, Enschede, The Netherlands KAREN J. EDLER, University of Bath, Bath, United Kingdom 1. 2. Fundamentals . . . . . . . . . . . . . . . . . . . Membrane Structures and their Preparation . . . . . . . . . . . . . . . . . . . . 1 3. Applications . . . . . . . . . . . . . . . . . . . . 2 2 Membranes consist of thin solid or liquid films that act as barriers for the transport of molecular components. Synthetic membranes are widely used in applications such as water desalination and gas separation or the concentration and purification of food and chemical products, and notably in medical devices, such as artificial kidneys or drug delivery systems. In energy conversion and storage systems, e.g., batteries, fuel cells, and electrolyzers, membranes also play an important role. 1. Fundamentals Membranes used in various applications differ in their functions, structures, and materials. In general terms, a membrane is defined as a solid or liquid, homogeneous or heterogeneous barrier that separates two phases and restricts the transport of different components # 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 10.1002/14356007 from one phase to the other in a specific way. To separate molecular mixtures, membranes must have different permeabilities for different components. For high mass transport rates per unit area, membranes should be as thin as possible; they are usually thin films or hollow fibers. To obtain a transport through a membrane, driving forces are required, i.e., differences in the electrochemical potentials of the various components such as differences in concentration, hydrostatic pressure, or electrical potential between the two phases separated by the membrane. Transport of a component is determined by its concentration and mobility in the membrane. Mobility is determined by the hydrodynamic resistance in pressure-driven processes, and by diffusivity of the component in the membrane matrix in concentration gradient- or electrical potential-driven processes. Concentration in the membrane is determined by solubility in homogeneous 2 Membranes Table 1. Synthetic membranes Structure Material Function Porous films, hollow fibers Homogeneous films, hollow fibers Homogeneous or porous films with fixed ions Asymmetric porous or homogeneous films Thin film composite membranes Liquid films with selective carriers polymers, ceramics, metals polymers, metals polymers, ceramics polymers, ceramics polymers, ceramics organic liquids gas separation micro- and ultrafiltration, dialysis reverse osmosis, gas separation, pervaporation electrolysis, electrodialysis, dialysis pressure-driven membrane processes pressure-driven membrane processes selective removal of ions, gases, etc. membranes, by exclusion due to size in porous membranes, or due to electrical charge in ionexchange membranes. Concentration and mobility of certain components in a membrane may be facilitated by functional groups incorporated in the membrane matrix (facilitated transport). 2. Membrane Structures and their Preparation Classification. Although synthetic membranes vary widely in structure, function, and material, they can be classified into typical groups (Table 1). This classification is rather arbitrary and many structures used today may fit none or more than one of these classes. Preparation. The techniques used for manufacturing the various membrane structures are very different. For the preparation of microporous membranes from ceramics or polymers, pressing and sintering of a fine powder is used. Irradiation of a thin film followed by acid or alkaline etching or the precipitation of polymer from a solution by a non-solvent are also used to prepare porous membranes. Homogeneous membranes are prepared by extrusion of a metal or a polymer into a thin film or by casting polymer solution as a thin film and evaporating the solvent. Asymmetric structures are generally made by phase inversion techniques and composite membranes by dip-coating or interfacial polymerization. Techniques for making liquid membranes include the formation of emulsions for the preparation of unsupported liquid membranes and the filling of porous structures with liquid for the preparation of supported liquid membranes. Processes. Membrane separation processes can differ significantly with regard to the membranes, the applied driving forces, the areas of application, and the industrial relevance (Table 2). 3. Applications Synthetic membranes are used not only in separation processes, but also in medical Table 2. Technical relevant membrane separation processes Separation process Membrane type Driving force Application Microfiltration symmetric porous membranes sterile filtration, clarification Ultrafiltration asymmetric porous membranes Reverse osmosis asymmetric homogeneous membranes symmetric porous membranes hydrostatic pressure 10–500 kPa hydrostatic pressure 0.1–1 MPa hydrostatic pressure 2–10 MPa concentration gradient Dialysis Electrodialysis Electrolysis Gas and vapor separation Pervaporation ion-exchange membranes ion-exchange membranes asymmetric homogeneous membranes asymmetric homogeneous membranes filtration of macromolecular solutions desalination of saline solutions electrical potential gradient electrical potential gradient vapor pressure gradient separation of microsolutes from macromolecular mixtures desalination of ion-containing solutions chlor-alkaline production separation of gases and vapors vapor pressure gradient separation of azeotropic mixtures Membranes 3 Table 3. Technically relevant applications of synthetic membranes Separation process Controlled-release systems Membrane reactors, immunoisolation Energy conversion and storage Microfiltration Ultrafiltration Reverse osmosis Dialysis Electrodialysis Gas Separation Pervaporation therapeutic drug delivery devices artificial organs sustained-release of pesticides enzyme and catalytic reactors biosensors immunoprotection of cell implants battery separators fuel cell and electrolyzer separators solid electrolytes devices as barriers for the controlled release of drugs and in energy-conversion systems as ion-permeable separators (Table 3). For large-scale applications in the chemical and food industry or in environmental protection, separation processes, such as micro- and ultrafiltration or gas separation are most important, but commercially the medical uses of membranes, e.g., in artificial kidneys, are equally interesting.
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