W. Richard Bowen and Nidal Hilal 4

120 4. INvEsTIgATINg MEMbRANEs ANd MEMbRANE PROCEssEs
Moreover, if the surface potential or surface charge of the colloid has
been determined and the solution is of defined ionic content, it is possible
to calculate the potential or charge of the surface under investigation
by matching the experimentally obtained curves to theoretical calculations
on the basis of electrical double layer theory. In the example shown,
the best-fit membrane surface charge was �0.00114 C m �2 and the bestfit
membrane surface potential was �64 mV. Furthermore, an important
advantage of the colloid probe technique is that it allows exploration
of variations in surface electrical interactions at different points on the
membrane surface, as the following section shows.
4.5 EFFECTS OF SURFACE ROUghnESS On
InTERACTIOnS WITh PARTICLES
Surfaces are usually imaged using sharp tips in order to produce
images with the highest possible definition. However, from a processing
viewpoint, an important factor is how process stream components such as
solutes and colloidal particles interact with the surface. If a surface has feature
variations that have dimensions comparable with those of the process
stream components, then such interactions may show variations at different
locations on the surface. An effective way of gauging such effects is to
image the surface with an appropriate stream component, e.g. a particle
that is immobilised to form a colloid probe. Figure 4.14 shows results for
a reverse osmosis membrane (AFC99, PCI Membranes) imaged in saline
solution with first a sharp tip and then a 4.2 �m silica sphere [8].
µm
0.4
0.2
4
0
3
P–v = 560 nm
Rms = 66nm
2
µm
1 1
0 0
2
µm
A B
3
4
µm
0.16
0.12
0.2
0.04
4
0
3
2
µm
P–v = 186 nm
Rms = 21nm
1 1
0 0
FIgURE 4.14 AFC99 membrane imaged with a tip (A) and with a 4.2 �m colloid probe
(B) (P–v, peak to valley; Rms, root mean square).
2
µm
3
4