W. Richard Bowen and Nidal Hilal 4

4.5 EFFECTs OF sURFACE ROUgHNEss ON INTERACTIONs wITH PARTICLEs 121
The membrane surface features are still apparent but less well defined
in the colloid probe image. This is a rather rough membrane showing
clear peaks and valleys. Following imaging, it is possible to position
the colloid probe at any point on the surface to quantify colloid–surface
interactions. Figure 4.15 shows long-range electrostatic interactions quantified
at peaks and valleys respectively, and some analysis of the data is
presented in Table 4.3.
Both the magnitude and range of the electrical double layer interactions
on the peaks are greatly reduced compared to that in the valleys. In
both cases, the range is very different from that at a planar surface, both
experimental – data for mica is shown for comparison – or theoretical,
the Debye length. It is also possible to quantify the adhesive interaction
between colloid probe and membrane surface at different locations, as
shown in Figure 4.16 and Table 4.4.
F/R/mN/m
10
1
AFC99, peaks
10 –1 M
10 –2 M
10 –3 M
0.1
0 10 20
Distance/nm
30 40
F/R/mN/m
10
1
AFC99, valleys
10 –1 M
10 –2 M
10 –3 M
0.1
0 10 20
Distance/nm
30 40
FIgURE 4.15 Long-range electrostatic interactions at peaks and valleys for an AFC99
membrane at various ionic strengths in NaCl solutions (F, force; R, sphere radius).
TAbLE 4.3 Effective Experimental decay Lengths for Approach Curves between
silica Colloid Probe and AFC99 Membrane and Mica, and debye Length.
Effective decay lengths (nm)
[NaCl] (M) Membrane peak Membrane valley Mica Debye length (nm)
10 �3 7.7�1.4 12.1�1.5 9.2�0.5 9.6
10 �2 3.2�0.6 6.8�1.2 3.5�0.3 3.1