W. Richard Bowen and Nidal Hilal 4

94 3. QUANTIFICATION OF PARTICLE–BUBBLE INTERACTIONs
3.5.2 Effect of Surfactant on Particle–Bubble Interactions
During the froth flotation process, surfactant compounds are commonly
added to the flotation chamber to modify bubble–particle
attachment and increase mineral recovery. A number of AFM-based
measurements have been carried out to assess the effect of surfactants in
solution on particle–bubble interactions and hence, presumably, on flotation
efficiency.
Preuss and Butt [36, 48] studied the effects of adding two surfactants,
sodium dodecyl sulphate (SDS) and dodecyltrimethylammonium bromide
(DTAB), to solution when carrying out measurements between
both hydrophobic and hydrophilic particles with air bubbles. When
hydrophilic silica particles were allowed to approach the bubble in aqueous
solution without surfactant present, repulsive forces were measured
prior to contact, as reported previously for hydrophilic silica particles [37,
39]. As the concentration of SDS in solution was increased, this repulsive
force also increased and the decay length became decreased. As SDS has
a negative charge in solution, as do the silica–water and air–water interfaces,
the most likely explanation was that the repulsion was electrostatic
in origin and increased with increasing quantities of SDS at the interfaces.
When the effects of DTAB were investigated long-range forces between
the particles and the bubbles occurred, and adhesion was observed when
retracting the particle, most probably due to DTAB coating the silica surface
and increasing its hydrophobicity.
When silanized hydrophobic silica particles were used, capillary forces
were dominant, with large adhesions on the order of 70 mN m �1 detected
upon particle retraction [36, 48]. In the absence of SDS, small repulsive
forces were measured on approach, probably due to electrostatic repulsion.
When SDS was introduced, adhesion appeared to be dependent
upon the maximum force used to press the particle into the bubble, reaching
a threshold value of approximately 6 mN m �1 for an SDS concentration
of 5.6 mM. Below this threshold value no adhesion was seen, with no
hysteresis between the approach and the retract parts of the force curves,
suggesting that the SDS was reducing the interaction between the particle
and the bubble. As the loading force increased above the threshold, a
jump-in event occurred and high adhesion was observed. This threshold
force was increased, as the SDS concentration was increased as illustrated
in Figure 3.5. When the effects of DTAB were investigated, they were
found to follow a similar trend to that seen with SDS. At concentrations
�6 or 12 mM, for SDS or DTAB, respectively, formation of the TPC was no
longer observed for the load forces that were reached [36, 48]. The most
likely explanation for this behaviour is that a surfactant film was built up
between particle and bubble, preventing TPC formation. As the surfactant
concentration was increased, this film would most likely be thicker and