W. Richard Bowen and Nidal Hilal 4

158 5. AFM AND DEvELOPMENT OF (BIO)FOULINg-REsIsTANT MEMBRANEs
Adhesion force (mN m –1 )
3.5
3.0
2.5
2.0
1.5
1.0
0.5
Loading force (mN m –1 0.0
60 70 80
)
90
pH=3
pH=5
pH=7
fIgurE 5.14 Relationship of adhesion and loading forces between BSA probe and
PVDF membrane modified with qDMAEMA (DM � 530 �g cm �2 ) in different pH solution.
Steric interaction could be another factor which plays a role in reducing
the adhesion force observed with both membranes at pH 3. The overlap
of the expanded protein layer and the modification of polymer chain
of qDMAEMA, which is fully dissociated at this pH, enhance the repulsive
interactions between the colloid probe and the surface of the modified membrane.
In addition, at pH 5 there is practically no electrostatic interaction
because this pH is close to the isoelectric point of the protein (the net charge
is around zero) and, therefore, the electrostatic repulsion is negligible. As
at pH 5 the protein has no net charge and so the structure of the protein is
more compact and it has hydrophobic properties, such interaction may lead
to high adhesion at pH 5 than at pH 3.
Effect of Ionic Strength Normalised adhesion forces versus normalised
loading forces for interactions involving the modified PES membrane are
shown in Figure 5.15 at different solution ionic strengths. An increase in
adhesion force with increased ionic strength is evident.
The normalised adhesion force measured between BSA versus PVDFmodified
membrane with qDMAEMA was examined as a function of
ionic strength by varying the NaCl concentration (Figure 5.16). The
adhesion force was also found to increase with increasing solution ionic
strength. This is due to the compression of the double layer, which leads
to the increase in adhesion force with increasing ionic strength.