Chemistry Reference
In-Depth Information
one at the oil
monolayer interface and a second at the water-monolayer interface.
Since the two tensions would not, except in very unusual circumstances, be equal,
the interfacial layer would be curved, with the direction of curvature determined by
the relative magnitudes of the two tensions. Logically, the film will curve in the
direction of the higher interfacial tension so that the phase associated with that
interface will become the dispersed phase in the system (Figure 9.5).
Aside from the nature of the emulsifier employed, the relative amounts of the
two phases in the system might be expected to affect the type of emulsion obtained.
If one assumes that an emulsion is composed of more or less rigid, spherical dro-
plets of equal size (highly unlikely in reality), simple geometry shows that the max-
imum volume fraction of dispersed phase that can be obtained is approximately
74%. It was suggested that any emulsified system in which that level was exceeded
would produce less stable, deformed droplets and would lead to phase inversion to
an emulsion of the opposite type. Practice has shown, however, that it is possible
to prepare emulsions of dispersed-phase volume fractions far exceeding that theo-
retical limit. Seen with the (sometimes) 20/20 vision of hindsight, there are several
possible ways to explain the failure of such a simple geometric approach.
In the first place, emulsion droplets are not and can likely never be perfectly
monodisperse; as a result, it is possible for smaller droplets to insert themselves
in the void spaces between close-packed, larger droplets (Figure 9.6), increasing
the total potential packing density of the system. In addition, emulsion droplets
are not rigid but highly deformable, spheres; thus they can be easily deformed
from spherical to various oval or polyhedral shapes to fit the demands of the system.
Large excursions from a spherical shape are, of course, generally unfavorable, since
they entail the formation of additional interfacial area for a given dispersed volume
fraction. As mentioned above, such an increase in interfacial area could strain the
ability of the adsorbed emulsifier film to the point of droplet coalescence.
In the past there have been some suggestions that the mechanical process of
emulsification could also play a role in determining the type of emulsion produced.
A number of studies have verified that, in some cases at least, such a mechanical
effect on emulsion characteristics does seem to exist for some specific formulations.
Figure 9.6.
Effects of droplet polydispersity on the potential packing density of emulsions;
interstitial spaces between larger drops may be filled by smaller units: (a) ideal hexagonal
close packing; (b) high-density polydisperse packing.
