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.