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Membrane Separation: Basics and Applications 289
chemical stability and selectivity of membranes for reverse osmosis, pervaporation and gas
separation applications (41). Mosqueda-Jimenez and coworkers (45) studied the addition
of surface-modifying macromolecules and the use of the additive polyvinylpyrrolidone
(PVP). Their results showed that membranes prepared with PVP had pure water permeation
rates significantly higher than membranes prepared without the PVP additive. Pyrolysis method
can lead to significantly improved separation performance of synthetic membranes, especially
molecular sieve membranes made from pyrolyzed polyacrylonitrile and polyimide (46).
4. MEMBRANE CHARACTERIZATION
Membranes need to be characterized to ascertain which may be used for a certain separation
or class of separations (13). Membrane characterization is to measure structural membrane
properties, such as pore size, pore size distribution, free volume and crystallinity to
membrane separation properties. It helps gather information for predicting membrane performance
for a given application.
Membrane characterization means the determination of structural and morphological
properties of a given membrane. As membranes range from porous to non-porous depending
on the type of separation problem involved, different characterization techniques are required
in each case. For example, in MF or UF membranes, fixed pores are present. MF membranes
have macropores (pore diameter > 50 nm) while UF membranes have mesopores (2 nm <
pore diameter < 50 nm). The pore size (and size distribution) mainly determines which
particles or molecules are retained or pass through. On the other hand, for dense or nonporous
membranes, no fixed pores are present and the material chemistry itself mainly
determines the performance.
4.1. Porous Membrane
Two different types of characterization method for porous membranes can be distinguished
(13):
1. Structure-related parameters: determination of pore size, pore size distribution, top layer thickness
and surface porosity
2. Permeation-related parameters: determination of the actual separation parameters using solutes
that are more or less retained by the membrane
There are a number of characterization techniques available for porous membranes, the
following methods are usually used:
l Electron microscopy
l Atomic force microscopy (AFM)
l Bubble point method
l Permeation measurement
The first three methods involve the measurement of structural-related parameters while the
last one is a typical permeation-related technique. Both electron microscopy and AFM can