Chemistry Reference
In-Depth Information
1
0.5
0
0
200
400
t / min
Figure 1 Fraction F of surviving bubbles as a function of time t at different fumed silica
and surfactant concentrations:
'
, 2.2 wt% silica + 1.7
10
6
mol dm
3
DDAB;
n
, 3.0 wt% silica + 10
5
mol dm
3
DDAB;
E
, 3.5 wt% silica + 10
4
mol dm
3
DDAB
250
200
150
100
50
0
0
200
400
t / min
Figure 2 Radius R of average-sized bubble as a function of time t at different fumed silica
and surfactant concentrations:
n
, 3.0 wt% silica + 10
5
mol dm
3
DDAB;
E
,
3.5 wt% silica + 10
4
mol dm
3
DDAB
10
5
and 10
4
mol dm
3
, respectively. However, this does not take into account
the possibility that the bubbles formed in the different systems had different
initial sizes, and therefore different intrinsic stabilities.
In Figure 2, the change in the radius of an average sized bubble is presented.
The average bubble size in the 10
5
mol dm
3
DDAB system was much smaller
compared to that in the 10
4
mol dm
3
system. It is well known
1
that small
bubbles shrink faster than big ones. So, in part, the difference between results in
Figure 1 is due to this difference in average bubble size.
The results described above imply a dependence of stability on the concen-
tration of both the silica particles and the DDAB. That is, it was found that the
stability improved with simultaneous increase in the particle concentration and
the DDAB concentration. But an increase in concentration of one of the
components on its own did not necessarily provide better stability. Contact
angle measurements at different DDAB concentrations showed a significant
increase in
y
from a DDAB concentration of 10
6
to 10
5
mol dm
3
(Figure 3).
Nevertheless, for DDAB concentrations above the cmc,
12
the value of
y
was
found to decrease again. The change in
y
explains the change in stability with
DDAB concentration. At a DDAB concentration of 10
5
mol dm
3
, we found
y
401. The adsorption energy of a particle at the surface of a bubble with such
contact angle can be thousands of kT,
10
making particle detachment from the
interface almost impossible.
E
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