Chemistry Reference
In-Depth Information
each side of the film of 1 mN/m. As in the case of the Marangoni effect, the Gibbs
elasticity will be significantly affected by the surface activity of the adsorbing
species, as indicated by the (
ds
i
/d ln a) term.
The experimental determination of the elasticity of lamellar films has, until rela-
tively recently, been difficult, as have studies of the rates of diffusion of surfactant
molecules to newly formed interface. It is difficult, therefore, to determine the
relative importance of each mechanism in the stabilization of O/W emulsions.
New techniques using photon correlation spectroscopy, which can measure the
duration and amplitude of surface and interfacial waves, promise to provide a
great deal of useful information about the physical properties of such regions as
surfaces, interfaces, and lamellar films.
9.5. MIXED SURFACTANT SYSTEMS AND INTERFACIAL
COMPLEXES
We have seen that the presence of small amounts of surface-active materials can
have a dramatic effect on the surface tension of solutions, even those containing
relatively large amounts of other surface-active materials. The classical example
of such effects is the minimum in the surface tension-concentration curves found
for many anionic sulfate surfactants that contain small amounts of the starting alco-
hol as an impurity. The action of the impurity may be seen as being twofold:
(1) because of its less hydrophilic head group (the hydroxyl), the alcohol will be
more efficiently adsorbed at the surface; and (2) because of the smaller size of
the head group, the impurity can be packed into the adsorbed layer between adja-
cent molecules of the primary surfactant, resulting in a greater surface excess and a
lower surface tension. Once the primary surfactant concentration has reached its
cmc, the less soluble impurity can be solubilized into micelles so that the surface
tension will be determined more directly by the primary surfactant species.
In the discussion of foams and foam stability in Chapter 8 it was shown that
the presence of small amount of a surface-active impurity can contribute greatly
to foam stability as a result of its effect on lamellar film elasticity. Such a dramatic
effect has not been found in the case of emulsion stability against droplet coales-
cence. Although a limited amount of data are available on the interfacial tension of
ternary mixtures, it is generally assumed that surface-active impurities of this type
are probably too soluble in the oil phase to remain at the interface. They will be
extracted into the oil phase, where they are not particularly surface-active and
are not strongly adsorbed back into the interface.
In contrast to the insignificant effect of surface-active impurities on emulsion
stability, it has been found that the presence of two primary surfactant species,
one soluble in water and the other in oil, can greatly enhance the stability of an
emulsion system. The effect has been related to the production of very low inter-
facial tensions and the formation of cooperative surfactant ''complexes'' that impart
greater strength and coalescence resistance to the O/W interface. A broad definition
of the term ''complex'' should be inferred in this context. It is not used, necessarily,
