W. Richard Bowen and Nidal Hilal 4

114 4. INvEsTIgATINg MEMbRANEs ANd MEMbRANE PROCEssEs
membrane. Care is required in the use of MWCO as rejection can depend
on other factors such as charge and molecular shape, but it remains an
extensively used parameter. It is possible to estimate the mean pore diameter
of membranes from rejection data such as MWCO through the use
of an appropriate mathematical expression. As MWCO is a historically
important measure and AFM a relatively new technique, it is pertinent to
investigate the correspondence between the data respectively obtained.
Such an investigation has been carried out for a series of membranes
with MWCO in the range 1000–10 000 (Desal-G, Osmonics) [5]. Images
of membranes obtained in air at the extremes of this range, GE (1000)
and GN (10 000), are shown in Figure 4.8. The higher MWCO membrane
is noticeably rougher. Indeed, there is a significant increase in surface
roughness throughout the range, as shown by the data in Table 4.1.
From such images, it is possible to determine the surface pore diameter
distributions of the membranes. Data is given in Table 4.2, together with
the mean pore diameters from MWCO, using three differing mathematical
Å
268
134
0
3
2
300
µm
Å
3.00
2.00
1.00
0.00
1
200
Å
GE membrane
100
0 0
1
µm
2
3
3
2
µm
1
GN membrane
0 0
GE membrane GN membrane
Å
2.00
0 0
100
Å
200
300
300
200
100
0 0
FIgURE 4.8 AFM images of two Desal G membranes, GE (MWCO 1000) and GN
(MWCO 10 000), at low resolution (above) and high resolution (below).
1.00
0.00
Å
1
100
µm
2
Å
200
3
300