very small, and it can significantly speed up adsorption. During such
processes as foam and emulsion formation, very intensive convection is
deliberately induced and adsorption times as short as a microsecond may
result in some cases (Section 11.3.1).
- ‘‘Consumption’’ of surfactant may occur upon adsorption if it
tends to dissolve in the other phase, which may be a slow process. - Micelles. It is still a matter of debate whether the dissociation of
surfactant molecules from micelles would lead to retardation of adsorption.
It appears very unlikely that the time scale involved would be longer than a
second. On the other hand, some surfactants have a very small solubility in
water (e.g., phospholipids), and they are often present in small solid lumps
or as vesicles. In such a case, it may take a very long time before surfactant
molecules have reached the interface. - Adsorption barrier. A free energy barrier for adsorption of a
surfactant would cause a decrease in adsorption rate, and several kinds of
such barriers have been postulated. Apparently, electrostatic repulsion can
indeed cause a decrease in adsorption rate. This will occur if the adsorbing
species is highly charged (e.g., a protein at a pH far removed from its
isoelectric point), and moreover ionic strength is low (so that 1/kis larger
than the distance between the surfactant molecules at the interface). - Mixtures. Nearly all preparations of surfactants used in practice
are mixtures, and the components vary in surface activity and in the lowestg
they can produce. Often, some components that can produce a very smallg
are present in minor quantities, and these eventually tend to predominate in
the interface after they have finally reached it, displacing other surfactants.
For soluble small-molecule surfactants, the time scale will mostly be short,
as explained above, whereas for a mixture of polymeric surfactants (say,
proteins) it may take a long time before a ‘‘final’’ value ofgis reached. - Change of conformation. Polymeric surfactants, especially pro-
teins, may undergo changes in conformation that lead to a decrease in
interfacial tension. Such changes may take a long time. For flexible proteins,
such asb-casein, time scales up to 10 s may be involved. For globular
proteins, conformational changes upon adsorption may take up to 10^3 s. - Partial desorption. Because a polymer molecule can change its
conformation on adsorption, it may unfold and cover a much larger amount
of interface than it would when equilibrium between dissolved and adsorbed
molecules has been reached. This implies that the interface may be almost
fully covered with surfactant in an early stage of adsorption, which will
greatly reduce the rate of further adsorption: parts of the adsorbed
molecules have to desorb before additional molecules can adsorb, and such
partial desorption will be a slow process. These phenomena are illustrated in
Figure 10.16 for flexible polymers. Something similar will happen with many