W. Richard Bowen and Nidal Hilal 4

118 4. INvEsTIgATINg MEMbRANEs ANd MEMbRANE PROCEssEs
Imaging force = 4.35 nN
NaCl 10 -1
2
µm
1
%
A
0
2
1.5
µm 1
0.5
0
50
40
30
20
10
0
Imaging force = 33 nN
NaCl 10 -4
0
0.5
1
µm
IF = 4.4 nN
10 -1 M
0
0.05 0.10
Pore size, µm
0.15
2
1.5
1 µm
2
Imaging force = 12.9 nN
NaCl 10 -1
2
µm
A B C
Imaging force = 32.1 nN
NaCl 10 -3
2
1.5
µm 1
0.5
0
0
0.5
1.5
1
µm
D E F
%
50
40
30
20
10
IF = 33.0 nN
10 -4 M
0
0.05 0.10 0.15 0.20
Pore size, µm
%
%
50
40
30
20
10
1
0
0
with numerical double layer calculations, as pore diameters are often less
than the characteristic electrical double layer dimensions.
Fortunately, AFM, in conjunction with the colloid probe technique,
offers an alternative means of membrane surface electrical properties
characterisation. If a colloid probe is approached towards a surface, it
is possible to quantify the force of interaction. Typical data is shown in
Figure 4.13 for a Desal DK membrane, which is one of the least rough
membranes [7].
1
µm
IF = 12.9 nN
10 -1 M
Pore size, µm
B C
IF = 33.1 nN
10 -3 M
0
0.05 0.10 0.15
Pore size, µm
D E F
50
40
30
20
10
0
0.05 0.10 0.15
2
2
%
50
40
30
20
10
Imaging force = 33.8 nN
NaCl 10 -1
2
µm
1
0
Imaging force = 33.2 nN
NaCl 10 -2
2
1.5
µm 1
0.5
0
0
0
IF = 33.8 nN
10 -1 M
0.5
0
0.05 0.10 0.15
Pore size, µm
IF = 33.2 nN
10 -2 M
0
0.05 0.10 0.15
Pore size, µm
1
µm
1
1.5
µm
FIgURE 4.11 AFM images of a Cyclopore membrane in electrolyte solutions. (a)–(c) at
various forces in 10 �1 M NaCl solution; (c), (d)–(f) at approximately constant force at various
ionic strengths. All data at pH 6.5 (IF-imaging force).
%
50
40
30
20
10
2
2