Chemistry Reference
In-Depth Information
Figure 8.4.
A schematic representation of the possible role of amphiphile mesophases in the
stabilization of foams.
Obviously, the presence of mesophases or other structures such as, for example,
mixed surfactant complexes, can not only increase the stability of the foam from a
surface chemical standpoint but can also significantly enhance the physical strength
of the system. When thinning reaches the point at which bubble rupture becomes
important, the mechanical strength and rigidity of such structures might help the
system withstand the thermal and mechanical agitation that would otherwise result
in film failure and foam collapse.
8.2.3. Effects of Additives on Surfactant Foaming Properties
As we have seen, the foaming properties of a surfactant can be related to its solution
properties through the cmc. It is not surprising, then, that additives in a formulation
can affect foaming properties in much the same way that they affect other solution
properties. The presence of additives can affect the stability of a foam by influen-
cing any of the mechanisms already discussed for foam stabilization. It may, for
example, increase the viscosity of the liquid phase or the interfacial layer, or it
may alter the interfacial interactions related to Gibbs-Marangoni effects or electro-
static repulsions. By the proper choice of additive, a normally low-foaming material
may produce large amounts of foam in the presence of small amounts of another
surface-active material, which itself has few if any useful surfactant properties.
Conversely, a high-foaming surfactant can be transformed into one exhibiting little
or no foam formation by the judicial (or accidental) addition of the right (or wrong)
additive. It is theoretically possible, then, to custom-build a surfactant formulation
to achieve the best desirable combination of surfactant actions to suit the individual
needs of the system. The use of small amounts of such additives has become the
primary way of adjusting the foaming characteristics of a formulation in many prac-
tical surfactant applications.
As mentioned earlier, additives that alter the micellization of a surfactant will
also affect its foaming properties. Such additives can be divided into three main
