Question
A fairly dilute O–W emulsion is made with a protein as the sole surfactant, and a
polysaccharide is added to increase viscosity. The emulsions show undesirable
aggregation of the droplets. How can you establish the probable cause of the
aggregation by simple means?
Answer
The first question may be whether the polysaccharide is involved. This is readily
checked by omitting it in the formulation. If that makes no difference, the cause must
be that the protein does not give sufficient repulsion. This can in turn have various
causes. The pH can be too close to the isoelectric point, or the ionic strength too high
to provide sufficient electrostatic repulsion; the checks needed to find this out are
obvious. Bridging can result from the protein concentration being too small, and this
can be remedied by adding more protein before emulsification; the bridging may be
as depicted in Figure 12.8a or c. In the former case, the protein must have an
unfolded conformation; in the latter case, it probably implies that the protein has
formed fairly large aggregates, and measures that cause better solution would also
prevent aggregation. Bridging can be due to cross-linking, e.g., by 22 S 22 S 22 bonds or
by 22 Ca 22 linkages. This can be established by adding reagents that specifically break
such bonds, e.g., dithiothreitol and ethylene diamine tetra-acetate, respectively.
If the polysaccharide is involved in the aggregation, the most likely cause is
depletion flocculation, but it may also be caused by adsorption of the polymer onto
the protein-covered droplets in such a way that bridging occurs. It may be difficult to
find out which mechanism is responsible. An unequivocal criterion is whether the
polysaccharide concentration in the serum phase of the emulsion is increased or
decreased as compared to the original solution, but determining this cannot be called
a simple method. Dilution with the solvent may give a clue, since depletion
flocculation tends to depend more strongly on polymer concentration than does
bridging. The latter will generally involve electrostatic interaction, say between
positive groups on the protein and negative ones on the polysaccharide. This can be
established by varying the pH.
12.5 RECAPITULATION
Several colloidal interaction forces can act between particles dispersed in a
liquid. If these particles attract each other, they will aggregate, which means
that the dispersion is unstable. The interaction forces can also affect the
stability of a thin film (e.g., between air bubbles) and the rheological
properties of particle gels.
One generally tries to calculate theinteraction free energyVbetween
two particles as a function of the distance between their surfacesh. Van der