W. Richard Bowen and Nidal Hilal 4

110 4. INvEsTIgATINg MEMbRANEs ANd MEMbRANE PROCEssEs
Shown alongside the membrane image is the derived pore size distribution.
Such distributions may be readily obtained using commercial
image analysis software, either automatically or manually.
Once successful images of microfiltration membranes have been
obtained, it is a challenge to move downwards in expected pore size or
MWCO. Thus, Figure 4.2 shows a single pore in a PCI Membranes ES625
ultrafiltration membrane, which has a specified MWCO of 25 000 [2].
The derived pore size distribution indicates a mean pore size of 5.1 nm
with a standard deviation of 1.1 nm. This is a pore of dimensions suitable
for separating a protein molecule. It should be noted that it is not always
possible to image such small pores. In particular, it is necessary for the
membrane surface roughness to have characteristic dimensions less than
the pore dimensions for successful imaging.
An important challenge in science is ‘to boldly go’ where no man
(or woman) has been before. Thus, Figure 4.3 shows an image and the
Å
2.00
1.00
0.00
120.0
80.0
Å
40.0
0
0
40.0
80.0
Å
120.0
% of pores
30
20
10
0
2 3 4 5 6 7 8 9
Pore size, nm
FIgURE 4.2 Single pore and pore size distribution of ES625 ultrafiltration membrane.
40
35
30
Å
2.00
1.00
25
0.00
20
16
15
12
10
Å 8
16
5
4
0
0
4
8
12
Å
0
0.0 0.5 1.0 1.5
Pore size, nm
2.0
FIgURE 4.3 Single pore and pore size distribution of XP117 nanofiltration membrane.
% of pores