W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
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156 5. AFM AND DEvELOPMENT OF (BIO)FOULINg-REsIsTANT MEMBRANEs<br />
showed a lower adhesion force than PES. This is most likely due to the<br />
PES membranes having a higher degree of hydrophobicity compared<br />
with PVDF. In addition, the PVDF membrane has a higher �-potential<br />
than the PES membrane [21, 23]. Thus a higher repulsive electrostatic<br />
force would be expected between the probe <strong>and</strong> the unmodified PVDF<br />
membrane than unmodified PES membrane.<br />
Effect of Ionic Strength A comparison of the adhesion force for both<br />
membranes as a function of ionic concentration is shown in Figure 5.12.<br />
In general a higher adhesion force is observed for PES membrane than<br />
PVDF membrane over the studied ionic strength range, except at ionic<br />
strength of 1 mM NaCl where adhesion force was small <strong>and</strong> no significant<br />
difference in adhesion of both membrane types is observed. At the<br />
highest ionic strength, the difference is the most dramatic. PES membrane<br />
is more hydrophobic than PVDF membrane. As a result a hydrophobic<br />
attractive force leads to higher adhesion force for PES membrane<br />
than PVDF membrane.<br />
Interactions Forces between Membranes Modified with qDMAEMA<br />
<strong>and</strong> BSA-Functionalised Colloid Probe<br />
Effect of pH The effect of solution pH on adhesion force for PES membranes<br />
modified with qDMAEMA is shown in Figure 5.13. An increase in<br />
adhesion force with increasing pH was observed for modified membranes<br />
with three different DMs <strong>and</strong> is approximately threefold at a loading force<br />
of 80 mN m �1 .<br />
The dominant contribution in the increase in adhesion force with increasing<br />
pH can be attributed to the electrostatic force between the positively<br />
charged modified membranes <strong>and</strong> the BSA probe. At pH 3 both surfaces<br />
(membrane <strong>and</strong> BSA probe) possess a positive charge resulting in greater<br />
repulsion at this pH <strong>and</strong> hence a lower adhesion than seen for other pH<br />
values. At pH 7 the modified PES membranes remains positively charged,<br />
whereas BSA carries a negative charge. This change in the nature of the<br />
electrostatic interaction leads to a greater observed adhesive force at pH 7.<br />
Figure 5.14 shows the results for the effect of pH on the measured adhesion<br />
force of BSA to PVDF membrane functionalised with qDMAEMA.<br />
An increase in adhesion force with increasing solution pH was again<br />
observed. The experimental results for PVDF grafted with different DMs<br />
reveal the same trend. Taking into consideration the amphoteric behaviour<br />
of BSA, which leads to a change in protein charge with changing<br />
pH, the BSA possesses a positive charge at pH values below the pI <strong>and</strong><br />
a negative charge at pH values above the isoelectric point. This explains<br />
the relatively high adhesion at pH 7 where both the BSA <strong>and</strong> the membrane<br />
surface are oppositely charged. Thus, an attractive electrostatic force<br />
between the interacting surfaces results in higher adhesion.