W. Richard Bowen and Nidal Hilal 4

5.4 MODIFICATION OF MEMBRANEs wITH sULPHONATED 165
FIR/mN m –1
(a)
FIR/mN m –1
(b)
2
1
0
–1
5
0
–5
–10
–15
–20
–25
–30
0 20 40 60
Distance/nm
–2
0
Approach
Retraction
20
Distance/nm
Approach
Retraction
fIgurE 5.22 Example normalised force versus distance curves against (a) P-P type
membrane and (b) S5-20 membrane.
of the membrane. When the probe is retracted away from the membrane
surface, a large attractive force is again measured. The difference between
the attractive force at its greatest magnitude and the force measured when
the probe has no net forces acting upon it (its free level) is the measured
adhesion force, F off. In contrast, for the S5-20 SPEEK membrane, no snapin
is observed. This is most likely due to the silica probe and membrane
surface carrying charges which are identical in sign. When the particle
is retracted, only a moderate adhesive force was measured. Data for the
40
60