W. Richard Bowen and Nidal Hilal 4

108 4. INvEsTIgATINg MEMbRANEs ANd MEMbRANE PROCEssEs
4.1 InTRODUCTIOn
Membrane processes are one of the most significant developments in
process engineering in recent times. The worldwide annual sales of membranes
and membrane equipment are now worth in excess of 1 billion
Euros. Membranes find widespread application in fields as diverse as water
treatment, pharmaceutical processing, food processing, biotechnology,
sensors and batteries. Membranes are most usually thin polymeric sheets,
having pores in the range from the micrometre to sub-nanometre, that act
as advanced filtration materials. This chapter is particularly concerned
with pressure-driven membrane processes – microfiltration, ultrafiltration,
nanofiltration and reverse osmosis. These are usually classified according
to the size of materials that they separate, with ranges typically given as
10.0 � 0.1 �m for microfiltration (MF), 0.1–5 nm for ultrafiltration, �1 nm for
nanofiltration (NF) and �1 nm for reverse osmosis (RO).
The human imagination, including the scientific imagination, is highly
visual. We are most easily convinced of the existence of phenomena and
processes in the physical world if we can see them. The importance of seeing
is deeply imbedded in language. Thus, when we finally understand
some highly complex and abstract explanation, we may comment, ‘I see
that now’. However, the size range of objects that the unaided human eye
can directly observe is limited. We can use optical microscopes to extend
the lower limit of this range down to objects of sizes comparable to the
wavelength of light. Beyond this limit, we need to use other means to ‘see’.
Atomic force microscopy (AFM) is one means of imaging objects of
dimensions from about the wavelength of light to those below a nanometre.
Thus, in the case of membranes, it is possible to visualise the membrane
surface properties, such as pores and morphology, using AFM.
Fortuitously, the size range of objects that may be visualised by AFM corresponds
closely to the size range of surface features that determine the
separation characteristics of membranes.
However, the separation characteristics of membrane interfaces do not
depend solely on the physical form of surface features. In liquids, surface
electrical properties and the adhesion of solutes to membrane surfaces
may also have profound effects on separation performance. It is thus
exceedingly fortunate that an Atomic Force Microscope may also be used
to determine both of these additional controlling factors. Finally, means
may be devised to quantify all of these controlling factors in liquid environments
that match those of process streams.
The first intention of this chapter is to provide a concise review of the
potential of AFM for the investigation of membranes and membrane
processes, using examples from our own studies. The chapter will begin
with illustrative examples that outline the range of possibilities of AFM