Chemistry Reference
In-Depth Information
its maximum extent. In this slow creaming experiment, even the surfactant-
stabilized emulsion displays the same type of feature [Figure 5(b)], but it is only
some 8 mm in depth at its greatest extent. Finally, the phase volume at the top
of the cream layer at the end of the experiment is quite significantly different for
the two systems, i.e., higher by 0.09 in the surfactant-stabilized system.
One possible explanation for the lower dispersed phase volume of the cream
layer in the protein systems is that there was less rearrangement in the layer due
to its elastic behaviour. The possible presence of voids or unfilled interstitial
spaces in the cream layer was investigated by confocal microscopy. These
observations showed that the WPI emulsions exhibited evidence of voids,
whereas the surfactant emulsions appeared to show few voids. Image analysis
of the phase volumes was in agreement with the ultrasonic analysis. However,
one of the most telling differences was not obvious from the static images. In the
surfactant-stabilized systems that were investigated, the voids contained small
droplets that were undergoing Brownian diffusion. However, in the protein-
stabilized systems there was no sign of movement of smaller droplets,
30
dem-
onstrating a clear difference in the close-range interaction for the two systems.
Aliquots taken from the base and the top of the two samples were measured in
order to determine the particle-size distribution. There were found to be only
minor differences between the samples, the main difference being between the
top and base of the cream layers (up to a factor of 100 in mean droplet size).
26.4 Discussion
One of the most striking differences between the two systems is in the interfacial
rheology (Table 1), particularly the shear rheology. The high moduli values
attained by the WPI are typical for interfaces occupied by globular proteins.
While the interdroplet interaction range of the two systems was designed to be
comparable in terms of the measured zeta potentials, this does not necessarily
mean that the short-range interdroplet interaction was similar. Calculation of
the local pair potential is difficult to perform accurately for the protein system
because of its polyelectrolyte nature. Nevertheless, we can expect substantial
differences from the surfactant case.
The objective of the overall project is to determine the importance of the
interfacial characteristics of proteins in controlling the bulk rheology of high
dispersed phase volume emulsion systems. The systems are chosen to have
particular interfacial properties such as high surface elasticity or specific net
charge. The two main samples investigated here have similar long-range
interaction potentials while having very contrasting interfacial rheological
properties. The ionic strength was kept constant, and the two systems had a
double-layer thickness of the order of 3 nm. In contrast to previous work using
pure b-lactoglobulin as emulsifier,
26
the double-layer thickness was large
enough to prevent flocculation in the creaming emulsion. However, once the
cream layer becomes compressed, the droplets are forced closer together, and
Search WWH ::
Custom Search