W. Richard Bowen and Nidal Hilal 4

Adhesion force (mN m –1 )
10
8
6
4
2
5.3 MODIFICATION OF MEMBRANEs 159
1 mM NaCl
10 mM NaCl
100 mM NaCl
Loading force (mN m –1 0
0 20 40 60 80 100 120
)
fIgurE 5.15 Relationship of adhesion and loading forces between BSA probe and
modified membrane with qDMAEMA (DM � 510 �g cm �2 ) in different ionic strength solutions
at pH � 5.8. The dashed line is an extrapolated line.
Comparison of (Bio)Adhesion Force Measurements Between
Unmodified Membranes and Membranes Modified with qDMAEMA
Effect of pH The normalised adhesion forces at the same loading force
(loading force � 80 mN m �1 ) for initial and modified PES membranes
with qDMAEMA are shown in Figure 5.17. It is clear that the unmodified
membrane has a higher adhesion force than the modified membranes at
pH 3 and 5, whereas the lowest adhesion was observed at pH 7. In addition,
at pH 3 modified membrane with the lowest DM exhibits the lowest
adhesion. At pH 5 modified membrane with DM of 510 �g cm �2 showed
the lowest adhesion force whereas both membranes show almost the
same adhesion at pH 7.
The BSA-functionalised probe and both modified and unmodified
membranes all possess a positive surface charge at pH 3. However, the
qDMAEMA-functionalised membrane possesses a markedly greater
charge than the unmodified PES membrane. The resulting greater electrostatic
repulsion accounts for the greater adhesive forces observed with
the initial unmodified PES membrane than with the modified membrane.
When the surrounding solution is at pH 7, both the initial PES membrane
and the BSA possess negative surface charges, with the qDMAEMA
membrane still carrying a positive charge. This accounts for the modified
membrane having a greater adhesive force with the BSA-coated probe
than the unmodified membrane.