Cationic alkyltrimethylammonium
bromides (C
n
TAB, with
n
= 8, 12, 16, 18) and their
mixtures with
n
-octanol as a nonionic surfactant
were chosen as a model system to study the synergistic effect on foamability
(two-phase system) and floatability (three-phase system) of quartz
in the presence of binary mixtures of ionic/nonionic surfactants.
The foam height of one-component solutions and binary mixtures and
floatability of quartz particles were characterized as a function
of the surfactant concentration and the number of carbons (
n
) in the alkyl chain of C
n
TAB.
The experimental results of foamability and floatability measurements
in one-component and mixed solutions revealed the synergistic effect,
causing a significant enhancement in the foam height and recovery
of quartz. In the presence of
n
-octanol, the height
of foam increased remarkably for all C
n
TAB solutions studied, and this effect, whose magnitude depended
on the C
n
TAB hydrophobic tail length,
could not be justified by a simple increase in total surfactant concentration.
A similar picture was obtained in the case of flotation response.
The mechanism of synergistic effect observed in mixed C
n
TAB/
n
-octanol solutions was proposed.
The discussion was supported by molecular dynamics simulations, and
the probable mechanism responsible for synergism was discussed. In
addition, an analysis allowing accurate determination of the concentration
regimes, where the synergistic effect can be expected, was given.
It was shown that for the two-phase system, the
n
-octanol molecule preadsorption at the liquid/gas interface causes
an increase in C
n
TAB adsorption coverage
over the level expected from its equilibrium value in the one-component
solution. In the case of the three-phase system, the synergistic effect
was related to the ionic surfactants serving as an anchor layer for
n
-octanol, which, in water/
n
-octanol solution
(one-component system), do not adsorb on the surface of quartz.