Chemistry Reference
In-Depth Information
the metastable foams must eventually collapse, since the diffusion of gas from
smaller to larger bubbles (Ostwald ripening) will occur, regardless of the strength
of the interfacial film.
Neither of the first two foam types can be considered to be thermodynamically
stable. The third class, the solid foams, could be so considered since they possess a
mechanically rigid structure formed as a result of a (presumably) irreversible che-
mical process during or just after foam formation. Although formulations for the
production of solid foams contain additives such as surfactants and blowing agents
to produce the foam matrix, their action in sustaining the foam structure is negli-
gible. Such foams therefore are not discussed further here.
Foams and emulsions have a great deal in common with regard to the basic phy-
sical principles controlling their stability. Some of the equations and concepts pre-
sented in this chapter, therefore, will be referred to in the following chapter,
although the exact form may change due to the circumstances. The major differ-
ences lie in the natures of the dispersed phases (liquid vs. gas) and in the fact
that foams will generally involve a much higher volume fraction of dispersed
phase than normally is encountered in emulsions. For example, a typical foam
(say, angelfood cake or ice cream) may have several factors of 10 more dispersed
phase volume than would that of the continuous phase. The ratio of dispersed to
continuous phase in an emulsion is unlikely to exceed 3 : 1. The theoretical limit
for a monodisperse emulsion of spheres is about 76% dispersed phase, although that
can be greater in polydisperse systems. If the dispersed phase is present as
deformed spheres or polygons, its content can also exceed the theoretical limit.
When considering the physical and chemical factors involved in the formation
and stabilization of foams, it is necessary to consider differences between foaming
and nonfoaming systems in general. A foam is produced by the introduction of air
or other gas into a liquid phase, during which time the bubbles become encapsu-
lated in a film of the liquid. The thin liquid film separating two or more gas bubbles
is referred to as a lamellar film, indicating that its nature is related to a layered
(laminated) structure that possesses two essentially identical interfaces in close
proximity. In the case of a foam of small bubble size, each interface will possess
a significant degree of curvature, concave toward the gas phase. The Laplace
equation, in the form
l
r
1
þ
l
r
2
p
¼
s
ð
8
:
2
Þ
states that there will exist a pressure difference across each interface related to the
major radii of curvature of the system, r
1
and r
2
, and the interfacial tension s.
When three or more bubbles are in contact, especially when the foam has reached
a generally stable honeycombed structure, a region will be developed in which the
curvature of the lamellae is much greater than that in the main body of the system.
These regions, referred to as ''plateau borders'' (Figure 8.1), possess a greater pres-
sure difference than exists elsewhere in the foam. Since the gas pressure within the
bubble must be the same throughout, the liquid pressure within the plateau borders
