W. Richard Bowen and Nidal Hilal 4

4.2 THE RANgE OF POssIbILITIEs FOR INvEsTIgATINg MEMbRANEs 111
corresponding pore size distribution of a single pore in a nanofiltration
membrane, XP117 from PCI Membranes.
Caution is needed here. It is only in exceptional instances that it is possible
to obtain images of such small pores. Further, some scepticism is in
order. The existence of pores in microfiltration membranes may be confirmed
by optical images. It does not seem unreasonable to push the limit
of belief in pores down to the ultrafiltration range. But in the nanofiltration
range, we need to be especially aware of the possibility of artefacts that
look like pores and of seeing what we wish to visualise.
It was mentioned in Section 4.1 that an Atomic Force Microscope can
also quantify the other key properties controlling the membrane performance.
The crucial innovation in the determination of such properties is
the development of colloid probes. Such probes are formed by attaching
particles of dimensions of the order of �1 �m to the end of tipless cantilevers.
Such attachment may be carried out using the manipulation properties
of an Atomic Force Microscope, but greater success is achieved with
the use of specially designed micromanipulation equipment. An example
of a colloid probe is shown in the electron microscopy image of Figure 4.4.
The silica colloid probe shown is at about the lower size limit for successful
micromanipulation and subsequent measurement.
If such probes are manipulated to approach a single point on a membrane
surface in a controlled manner in an electrolyte solution, it is possible
to directly quantify the electrical double layer interactions between probe
and membrane, also shown in Figure 4.4, for two solutions of differing ionic
strengths. Such electrical interactions are very important in determining the
rejection of colloids and biological macromolecules during ultrafiltration.
The adhesion of process stream components to membranes also has an
important influence on membrane performance. Ideally, such adhesion
(F/R) / (mN/m)
20
15
10
5
10-4 10
M NaCl, pH8
-1 M NaCl, pH8
0
0 5 10 15 20 25 30 35 40
Distance / nm
FIgURE 4.4 SEM image of a 0.75 �m silica sphere attached to a tipless AFM cantilever
and force distance curves for the approach of the probe to a Cyclopore microfiltration membrane
in NaCl solutions at pH 8. (F is force and R is sphere radius.)