very fast (say, in 1ms), and it involves a temporary increase of the surface
area of the drop, maybe by a factor of 2, which would mean halving the
value ofG. Hence at the moment of disruption the value ofg can be
significantly higher than the equilibrium value.
Comparing surfactant types, small-molecule amphiphiles tend to give
a lowerg-value than proteins: see Figure 10.13. Hence amphiphiles give a
smaller average droplet size, as illustrated in Figure 11.11.
Naturally, the concentration of surfactantcbecomes smaller during
emulsification due todepletion: the total interfacial area markedly increases
due to the decrease in droplet size, the more so for a higherjvalue. Thus
more surfactant becomes adsorbed. This also happens during foam
formation, but the increase in area is far smaller. A decreasedcvalue gives
a longer adsorption time. Consequently, the effectivegvalue will increase,
often strongly, during the emulsification process.
Presence of surfactant at the interface will also directly affect
deformation. The surfactant allows formation of ag-gradient. This would
affect the deformation mode of a drop, which has indeed been observed.
Moreover, enlarging the interfacial area causesgto increase, as mentioned.
This implies that the interfacial free energy increase includes two terms:
g?dAþA?dg. The first term is due to the deformation of the drop being
counteracted by its Laplace pressure; the second is due to surface
enlargement being counteracted by the surface dilational modulusESD.
Making use of Eq. (10.20) forESD, we obtain
gdAþAdg¼gdAþAESDdlnA¼ðgþESDÞdA ð 11 : 15 ÞModel experiments have indeed shown some additional resistance to
deformation (over that caused by the Laplace pressure). To what extent
these phenomena affect the droplet size obtained, for instance by
homogenization, is not yet clear.
11.4.2 Prevention of Recoalescence
An essential role of the surfactant is to prevent the newly formed drops from
coalescing again. Drops frequently encounter each other during the
emulsification process. Recoalescence has been shown to occur in the
following type of experiment. Two O–W emulsions are made that have
identical properties, except that two oils are used that differ, e.g., in
refractive index. These emulsions are then mixed and the mixture is
rehomogenized. By comparing the refractive index of the droplets so
obtained with that of the original ones, it follows that droplets of mixed oil
composition have indeed been formed.