(Section 10.3.3), the stability may be considerably impaired: the amphiphile
gives a lower interfacial tension and may cause weaker repulsion than a
protein.
Results on coalescence of emulsions made withpeptidesare shown in
Figure 13.17. The amphiphilic peptides were derived fromb-casein, and the
molar mass was about half that of the casein. (Comparable emulsions made
with the unmodified protein showed no coalescence in 10 days.) It is seen
that coalescence occurred, especially at the lower peptide concentrations,
which corresponded to a low value ofG. It is also seen that the coalescence
rate decreased with time, most likely because coalescence will lead to a
higherGvalue, hence greater stability. Finally, the coalescence rate was
faster for a higher ionic strength, indicating that the stabilizing action of
these peptides is largely due to electrostatic repulsion.
The method employed to obtain the results of Figure 13.17, i.e.,
determination of average droplet sizeas a function of time, is quite suitable to
establish the rate of coalescence in emulsions. It may even be better to
FIGURE13.17 Average droplet diameter as a function of time after making of
O–W emulsions with various concentrations (numbers near the curves in mg per ml)
of amphiphilic peptides. Volume fraction of oil 0.2, pH 6.7, ionic strength 75 mmolar
or 150 mmolar(*). (Results of P. Smulders et al. Roy. Soc. Chem. Special Public.
227 (1999) 61.)