W. Richard Bowen and Nidal Hilal 4

Interaction between colloidal particles and filtration media is of much
interest industrially, due to the propensity of dispersed particles to foul
membrane surfaces and reduce their efficiency [81, 82]. As under conditions
in which membrane filtration most often occurs both the membrane
and dispersed colloids and other particles carry surface charges,
the strength and sign of electrical double layer interactions are of great
importance. The forces involved in the approach of a colloidal particle to
a membrane surface are essentially a balance between electrical double
layer interactions and hydrodynamic forces. Studies have been made of
double layer interactions using model silica colloidal spheres and polymeric
microfiltration membranes [83]. Solutions of NaCl were prepared
of varying ionic strengths at a single pH value (pH 8.0). Representative
force curves for each approach made by the silica probe to membrane surfaces
are shown in Figure 2.6. At all ionic strengths, forces were significantly
repulsive at all separation distances, showing that the electrostatic
double layer repulsion is dominant in all cases. The range that the repulsive
interactions first become detectable increased from approximately
10 nm at 10 �1 M NaCl to approximately 35 nm at 10 �4 M. When measurements
were made in high purity water, this increased to 60 nm. The
change in the magnitude of the forces with decreasing ionic strength is
in accordance with electrical double layer theory, with the decay lengths
of the measured force curves comparable to the Debye charge screening
lengths of the different ionic strength solutions under study. The decrease
F/R [mN/m]
20
15
10
5
2.3 INTERACTION FORCES 57
10�3 10
M NaCl
�2 10
M NaCl
�1 M NaCl
10 �4 M NaCl
0
0 5 10 15 20 25 30 35 40 45
Distance [nm]
FIGuRE 2.6 Plot showing normalised force versus separation distance for the approach
of a silica colloid probe towards a microfiltration membrane surface at different salt concentrations.
As the concentration of NaCl is increased, repulsive forces on approach decrease
owing to charge screening effects.