W. Richard Bowen and Nidal Hilal 4

5.3 MODIFICATION OF MEMBRANEs 153
protein has a more expanded structure at pH values away from its isoelectric
point (pI) of 4.8 [22]. Thus a steric repulsive interaction between
the interacting surfaces will be higher at pH 3 than at pH 5, leading to
reduced adhesion at pH 3. Finally, PES possesses a slight positive charge
at pH 3 [21], provoking an electrostatic repulsive force.
A linear relationship between adhesion and loading forces for interactions
between BSA-modified probe and initial PVDF membrane at different
pH is shown in Figure 5.8. A similar trend is observed to the initial
PES membrane, with the adhesive force having the highest value at pH 5,
whereas the lowest adhesion force was measured obtained at pH 7.
The PVDF membrane possesses a negative charge in the pH range
studied with its magnitude increasing with increased pH [23]. This
explains the lower adhesion force between the BSA-modified probe
and PVDF surface at pH 7 where both surfaces carry a negative charge.
Increases in steric repulsion at pH 3 and in BSA hydrophobicity at pH 5
resulted in higher adhesion at pH 5.
Effect of Ionic Strength on Adhesion Forces The effect of ionic concentration
on adhesion force is shown in Figure 5.9. Increased NaCl concentration
led to an increase in the adhesion force between the BSA probe and the PES
Adhesion force (mN m –1 )
14
12
10
8
6
4
pH=3
pH=5
pH=7
Loading force (mN m –1 2
40 60 80 100 120 140 160 180 200
)
fIgurE 5.8 Relationship of adhesion and loading forces between initial PVDF and BSA
probe in different pH solutions.