Adsorption of Macromolecules at Oil
Water Interfaces during Emulsification - Food Colloids: Self-Assembly and Material Science - page 438

equilibrium concentrations these two samples show an apparent increase in the
surface load. The adsorption isotherm for sample B39-22 shows an even
stronger dependence on the apparent equilibrium concentration than the two
other samples and the surface load decreases strongly with increasing equilib-
rium concentration. At low apparent equilibrium concentrations the surface
load can become very high (reaching levels of 16 mg m 2 ) and the variation
between replicates was found to be large. These lower bulk concentrations are
also where the emulsion surface area is smaller, i.e., the droplet size larger
(Figure 2).
The non-Langmuir shape of the adsorption isotherms suggests that the
adsorption of the different OSA-starch samples, and in particular B39-22, is
a nonequilibrium process. (Hence, we prefer the use of the term 'apparent
equilibrium concentration'.) A possibility is that the surface load may depend
on polymer availability in relation to the specific surface area during emulsi-
fication rather than on the equilibrium concentration. The dependence of the
surface load on the bulk concentration of polymer versus the specific emulsion
surface area has been suggested earlier by Walstra, 4 and experimental results
from other authors 20, 21 also suggest a correlation. This relation is here termed
the 'dynamic surface load', where we are assuming that the adsorption rate
controls the adsorbed amount. If the polymers adsorb rapidly, there will not be
sufficient time for rearrangement and a thick adsorbed layer will be obtained.
In contrast, if the polymers adsorb slowly, full rearrangement is possible and a
thin layer will be obtained. The surface load versus the dynamic surface load is
shown in Figure 4. The results indicate that the surface load increases linearly
with dynamic surface load for sample B39-22, while samples B27-21 and
B86-10 behave more ideally, i.e., more surface area is created, rather than a
greater amount of OSA-starch at the interface.

20

15

10

5

0

0 5 10 15 20

Γ Dyn (mg/m 2 )

Figure 4 Emulsion surface load G in emulsions with different levels of dynamic surface
load ( G dyn ), i.e., the amount of OSA-starch available per specific emulsion
surface area: ' , B39-22; J , B27-21; n , B86-10
(Redrawn from Ref. 9).