Chemistry Reference
In-Depth Information
60
40
20
0
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
[DDAB] / mol dm
-3
Figure 3 Contact angle
y
of droplets of solutions of DDAB of different concentration on
flats of compressed fumed silica particles
At this point, it is important to distance the contact angle measurements
somewhat from the stability data, due to the fact that when the contact angle
was measured the surfactant was always in excess, and so the DDAB surface
coverage on the silica was in equilibrium with bulk concentration of DDAB. In
the bubble stability measurements, however, there could have been a different
fractional surface coverage at different particle and surfactant concentrations,
because the amount of surfactant was a limiting factor, due to the much higher
surface area for adsorption.
24.3.2 Fumed Silica Particles + Protein
In our previous work,
1
it was found that b-LG gave better foam stability
against disproportionation than other proteins. But even b-LG-stabilized
bubbles shrank slowly and relentlessly with time. So, although b-LG forms
highly viscoelastic, gel-like adsorbed layers at the A-W interface,
13
the resulting
interfacial elasticity is not adequate to prevent long-term instability of bubbles.
A question that arises is whether a mixture of hydrophilic silica particles +
b-LG is more effective in providing stability than is b-LG alone. To attempt
to answer this question, experiments were performed under exactly the
same conditions as those used previously by Dickinson et al.
1
for pure
b-LG-stabilized bubbles. The protein concentration was 0.05wt% in pH 7
buffer. Care was taken to generate and study bubbles of the same initial sizes
as those analysed in the previous study.
1
The results in Figure 4 show that, when
a mixture of silica particles + b-LG was used, the shrinkage of the bubbles was
considerably delayed compared to the pure protein system. However, the bub-
bles were not as stable as for the optimum DDAB + silica systems. Figure 5
compares typical shrinkage kinetics of bubbles of similar initial size for the
systems DDAB + silica, b-LG + silica and pure b-LG.
Experiments performed with b-casein, over the same range of protein and
particle concentrations as with b-LG, did not give enough stable bubbles for
any satisfactory measurements to be made. Thus, the ability of b-casein to
stabilize bubbles on its own, while not as great as for pure b-LG, was lost in the
presence of the particles. There are two possible explanations for this. Firstly,
the silica particles may act as foam-breaking agents in this system. Secondly,
the bulk concentration of protein may be reduced by its adsorption to the silica,
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