W. Richard Bowen and Nidal Hilal 4

152 5. AFM AND DEvELOPMENT OF (BIO)FOULINg-REsIsTANT MEMBRANEs
surfaces are forced into closer contact using higher loading forces [7]. In
addition the deformation of the membrane surface at high loading forces
may increase the area of contact between the probe and the membrane
surfaces. In order to exclude the influence of loading force in comparison
of interactions force, the experimental results are presented by plotting the
adhesion force versus the loading force. For the purpose of comparison,
the adhesion force at loading force equal to 80 mN m �1 was measured,
interpolated or extrapolated from the obtained experimental data.
Interactions Forces between Initial Membranes and Functionalised
Colloid Probes
Effect of pH on Measured Adhesion Figure 5.7 shows the normalised
adhesion force measurements versus the normalised loading force applied
to the cantilever between a BSA-modified probe and initial PES membrane
at different pH values. It can be seen that the adhesive force is highest at
pH 5, followed by moderate values at pH 3 and the lowest at pH 7.
First, at pH 7 both the BSA and the membrane surface are negatively
charged [21], which provoke increases in the repulsive electrostatic interaction,
leading to a reduction in adhesion. BSA is more hydrophobic at
pH 5 than 7, which would be expected to increase adhesion. Secondly, the
Adhesion force (mN m –1 )
12
10
8
6
4
2
pH=3
pH=5
pH=7
Loading force (mN m –1 0
40 60 80 100 120 140
)
fIgurE 5.7 Relationship of adhesion and loading forces between initial PES and BSA
probe in different pH solution. The dotted line is an extrapolated line.