Enhancement of Stability of Bubbles to Disproportionation Using Hydrophilic Silica Particles Mixed with Surfactants or Proteins - Food Colloids: Self-Assembly and Material Science - page 363

1

0.5

0

0 50 100 150 200

t / min

Figure 6 The fraction F of surviving bubbles as a function of time t for 1 wt% fumed silica
+ 1.3 10 3 mol dm 3 lecithin

or three hours, with lecithin F decreased at approximately the same rate
throughout. The relatively poor performance of lecithin compared to DDAB
can probably be attributed to its lower adsorption efficiency on silica. The
overall net charge on lecithin will be approximately zero under the conditions
used. The charge could be made more net positive by decreasing the pH, and so
increasing the tendency for adsorption on the negatively charged silica, but this
was not tested. Higher lecithin concentrations could also have been tried, but,
at the highest concentration used, the solutions were starting to become cloudy,
indicating that the limit of solubility had been reached. But this is not to say
that variations in temperature or other solution conditions (e.g., ionic strength,
concentrations of specific ions) could not be used to enhance the adsorption
efficiency of lecithin on the silica.

24.3.4 Colloidal Silica Particles + DDAB
The DDAB concentration was varied from 10 7 to 10 2 mol dm 3 and the
colloidal silica particle concentration from 0.5 to 3 wt%. It was found that, for
all the particle concentrations in this range, only when the concentration of
DDAB was above 10 4 mol dm 3 was there a significant amount of foam
created. Also, for silica contents above 0.5 wt%, the foamability or foam
stability did not markedly increase. The effect of DDAB concentration was
therefore only investigated in more detail for the case of the particle concen-
tration of 0.5 wt%. The optimum DDAB concentration for maximum stability
was found to be 8 10 4 mol dm 3 . In Figure 7 the F values after 1 h for all the
five DDAB concentrations are plotted. As with the fumed silica, the value of F
was found to decrease when the DDAB concentration was too high, illustrating
the importance of the ratio of surfactant to the particle concentration. Exper-
iments were also carried out to record the evolution of the radius of individual
bubbles in the same systems, as shown in Figure 8. Only at the highest
concentration of DDAB used (4 10 3 mol dm 3 ) did the bubbles shrink
and collapse completely within 60-80 min. This is in agreement with the trend
in F in Figure 7: for all the other DDAB concentrations, bubbles were formed
that remained stable for many hours.