W. Richard Bowen and Nidal Hilal 4

96 3. QUANTIFICATION OF PARTICLE–BUBBLE INTERACTIONs
Force, F/R (mN/m)
A
Force, F/R (mN/m)
B
1
0.8
0.6
0.4
0.2
–150 –100 –50
0
0
–0.2
50 100 150 200 250 300
–0.4
–0.6
–0.8
–1
1
0.8
0.6
0.4
0.2
–0.4
–0.6
–0.8
–1
z-Displacement (nm)
–150 –100 –50
0
0
–0.2
50 100 150 200 250 300
z-Displacement (nm)
FIGurE 3.6 Normalised force versus z-displacement curves obtained between aphrons
created using SDS (A) and DTAB (B) in water. Each force curve is an average of several
force–distance cycles (SDS � 10; DTAB � 9). In both cases a jump-in event is observed.
However, when the glass sphere approaches the anionic SDS-stabilised aphron, a repulsive
force is observed prior to contact, most likely due to repulsive (negative) charge interactions.
When an identical glass probe approaches DTAB-stabilised aphrons, only attractive
forces are seen prior to jump-in.
Aphrons created in surfactant solutions which were either anionic
sodium dodecyl sulfate (SDS) or cationic dodecyl trimethylammonium
bromide (DTAB) by mixing at high sheer rates (with rotor speeds
� 5000 rpm) were immobilised on hydrophobic HOPG and rinsed with
an excess of high purity de-ionised water [49] to remove excess surfactant
from solution. Aphrons were made in solutions with surfactant
concentrations of 1 and 2 g l �1 for SDS and DTAB, respectively. Colloid
probes consisting of glass spheres were then allowed to approach the
surface of the aphrons (estimated bubble diameters were � 50 �m) and
the resultant interaction forces were measured. Figure 3.6 shows example
force measurements obtained when approaching aphrons produced