W. Richard Bowen and Nidal Hilal 4

112 4. INvEsTIgATINg MEMbRANEs ANd MEMbRANE PROCEssEs
–3
1000 1200 1400 1600 1800 2000
Piezo displacement [nm]
should be avoided. As an example, Figure 4.5 shows a polystyrene colloid
probe and data for the retraction of such a probe after it has been
pushed into contact with membrane surfaces [3].
The depths of the depressions of the curves in Figure 4.5 give direct
quantification of the adhesion of the probes to the surfaces. The ES404
membrane was an existing commercial membrane and the XP117 membrane
is a development membrane designed to have lower fouling properties
(both membranes are PCI Membranes). The development membrane
had significantly lower adhesion and hence significantly lower fouling
potential than the existing membrane.
Ultrafiltration membranes are used particularly for the processing of
solutions of biological macromolecules. Such molecules are too small to
immobilise singly on a cantilever. However, by adsorbing such molecules
onto colloid probes, it is possible to measure their adhesion to membrane
surfaces. Such a protein (BSA – bovine serum albumin)-modified probe
and the corresponding adhesion data for two membranes is shown in
Figure 4.6 [4].
The data shows that the protein-modified probe has significantly
lower adhesion to the development membrane, XP117, than to the existing
commercial membrane, ES404, and hence the modified membrane
has significantly lower fouling potential in the processing of such protein
solutions.
Membranes are frequently used to process biological cell dispersions.
In order to elucidate such processes, it has been proved possible
to immobilise single cells at tipless AFM cantilevers, creating cell probes,
whilst maintaining the viability of such a cell, Figure 4.7 [5].
Such cell probes allow the direct measurement of the adhesion of biological
cells to membranes. Interpretation of the data for such cell probes
F/R [mN/m]
5
4
3
2
1
0
–1
–2
ES 404
XP 117
FIgURE 4.5 SEM image of a 11 �m polystyrene sphere attached to a tipless AFM cantilever
and force versus piezo displacement plot (retraction) for two membranes in 10 �2 M
NaCl solution at pH 8.0 (ES404, conventional; XP117, modified).
D
C
B
A'
A