Chemistry Reference
In-Depth Information
Figure 9.17.
Some possible pathways for the breakdown of multiple emulsions: (a) coal-
escence of secondary emulsion drops; (b) coalescence of primary emulsion drops; (c) loss of
primary emulsion dispersed phase to external phase.
9.8.3. Pathways for Primary Emulsion Breakdown
There are several possible pathways for the breakdown of multiple emulsions. A
few are shown schematically in Figure 9.17. Although all possible mechanisms
for droplet coalescence cannot be conveniently illustrated in a single figure, a con-
sideration of just a few possibilities can help clarify the reasons for instability in a
given system. Even though there may be a number of factors involved, one of the
primary driving forces will be, as always, a reduction in the free energy of the sys-
tem through a decrease in the total interfacial area. As has been noted previously, a
major role of surfactants at any interface is to reduce the interfacial energy through
adsorption. In a typical multiple-emulsion system, the primary mechanism for
short-term instability will usually be droplet coalescence in the primary emulsion.
It will be important, then, to select as the primary emulsifier a surfactant or combi-
nation of surfactants that provides maximum stability for that system, whether W/O
or O/W.
A second important pathway for the loss of ''filled'' emulsion droplets is the loss
of internal drops by the rupture of the oil layer separating the small drops from the
continuous phase. Such an expulsion mechanism would be expected to account for
the loss of larger internal droplets. Unless the two phases are totally immiscible (in
fact, a rare situation), there will always exist the possibility that osmotic pressure
differences between the internal and continuous portions of the system will cause
material transfer to the bulk phase. The high pressures in the smaller droplets would
be expected to provide a driving force for the loss of material from smaller drops in
favor of larger neighbors (Ostwald ripening), as well as to the continuous phase.
Finally, the presence of an oil-soluble surfactant always suggests the possibility
of nonaqueous reversed micelle formation and the subsequent solubilization of
internal aqueous phase in the oil. Such a solubilization process also represents a
convenient mechanism for the transport of material between the two similar phases.
