Chemistry Reference
In-Depth Information
overall amount of attention from both academic and industrial research efforts.
Flocculation without coalescence, however, is an important consideration and has
begun to draw closer and more frequent attention, especially concerning the role of
polymeric species that may act as bridging agents between droplets. The physical
characteristics of the interfacial film—its strength and rheological properties—are
beginning to come under closer examination as new techniques for such studies
become available. Several excellent reviews and monographs published more
recently cover in detail the questions raised above. Some of those are listed in
the Bibliography.
9.2.2. Theories of Emulsion Stability
Early ''practical'' theories of emulsion stability recognized the importance of addi-
tives such as surfactants, polymers, and particulates to the processes of emulsion
preparation, the type of emulsion produced, and the overall stability of the final sys-
tem. However, a reasonably sound theoretical picture began to evolve only once an
understanding of the concepts and principles of interfaces and monolayers began
to become clear. Studies of oriented amphiphilic monolayers at interfaces led to
the conclusion that such structures, in which each portion of the adsorbed
molecules showed a strong preference for association with one of the two liquid
phases, offered the best explanation for observed experimental results. As a result,
it became possible to schematically represent the emulsion droplet as shown in
Figure 9.2d.
The concept of an adsorbed monolayer film acting as an emulsion droplet stabi-
lizer found early experimental support in work showing that the equilibrium area
per molecule of surfactant at an oil-water interface, as determined from final dro-
plet diameters, approached a constant value, regardless of initial surfactant concen-
trations; that is, the area occupied by adsorbed amphiphilic molecules, initially
carboxylate soaps, had a lower limit related to the nature of the molecule. It was
found that emulsions in which the level of surfactant at the oil-water interface cor-
responded to an ''expanded'' monomolecular film were much less stable than those
in which sufficient stabilizer was present for the formation of a ''condensed'' film.
For purposes of the present discussion, an expanded monomolecular film may
be roughly defined as one that has a relatively high compressibility compared to
the bulk liquid but exists as a continuous interfacial monolayer (Figure 9.4a);
that is, it has the basic characteristics of a compressible gas, rather than being a
collection of isolated ''islands'' of amphiphilic molecules (Figure 9.4b). The ''con-
densed'' film, on the other hand, will have relatively low compressibility, existing
as a close
packed monomolecular array (Figure 9.4c).
Similar studies explored the role of film tenacity in emulsion stabilization. Such
studies compared the stability of emulsions prepared with mixtures of surfactants
to the ability of monomolecular films of the same compositions at the air-water
interface to resist high surface pressures without film breakdown. The results indi-
cated that the more resistant the film was to high surface pressures, the greater was
the stability of the related emulsion. It was clear to investigators some time ago,
