W. Richard Bowen and Nidal Hilal 4

156 5. AFM AND DEvELOPMENT OF (BIO)FOULINg-REsIsTANT MEMBRANEs
showed a lower adhesion force than PES. This is most likely due to the
PES membranes having a higher degree of hydrophobicity compared
with PVDF. In addition, the PVDF membrane has a higher �-potential
than the PES membrane [21, 23]. Thus a higher repulsive electrostatic
force would be expected between the probe and the unmodified PVDF
membrane than unmodified PES membrane.
Effect of Ionic Strength A comparison of the adhesion force for both
membranes as a function of ionic concentration is shown in Figure 5.12.
In general a higher adhesion force is observed for PES membrane than
PVDF membrane over the studied ionic strength range, except at ionic
strength of 1 mM NaCl where adhesion force was small and no significant
difference in adhesion of both membrane types is observed. At the
highest ionic strength, the difference is the most dramatic. PES membrane
is more hydrophobic than PVDF membrane. As a result a hydrophobic
attractive force leads to higher adhesion force for PES membrane
than PVDF membrane.
Interactions Forces between Membranes Modified with qDMAEMA
and BSA-Functionalised Colloid Probe
Effect of pH The effect of solution pH on adhesion force for PES membranes
modified with qDMAEMA is shown in Figure 5.13. An increase in
adhesion force with increasing pH was observed for modified membranes
with three different DMs and is approximately threefold at a loading force
of 80 mN m �1 .
The dominant contribution in the increase in adhesion force with increasing
pH can be attributed to the electrostatic force between the positively
charged modified membranes and the BSA probe. At pH 3 both surfaces
(membrane and BSA probe) possess a positive charge resulting in greater
repulsion at this pH and hence a lower adhesion than seen for other pH
values. At pH 7 the modified PES membranes remains positively charged,
whereas BSA carries a negative charge. This change in the nature of the
electrostatic interaction leads to a greater observed adhesive force at pH 7.
Figure 5.14 shows the results for the effect of pH on the measured adhesion
force of BSA to PVDF membrane functionalised with qDMAEMA.
An increase in adhesion force with increasing solution pH was again
observed. The experimental results for PVDF grafted with different DMs
reveal the same trend. Taking into consideration the amphoteric behaviour
of BSA, which leads to a change in protein charge with changing
pH, the BSA possesses a positive charge at pH values below the pI and
a negative charge at pH values above the isoelectric point. This explains
the relatively high adhesion at pH 7 where both the BSA and the membrane
surface are oppositely charged. Thus, an attractive electrostatic force
between the interacting surfaces results in higher adhesion.