Chemistry Reference
In-Depth Information
the cloud point is approached. Long-chain carboxylate salts, on the other hand,
which may have limited solubility in water and poor foaming properties at room
temperature, will be more soluble and will foam more as the temperature increases.
Quantitatively, the foaming abilities of some surfactants have been correlated
with their Hildebrand or Hansen solubility parameters (see Bibliography), which
is a semiquantitative, thermodynamically based molecular cohesion parameter
that provides a simple method for predicting and correlating the cohesive and
adhesive properties of materials on the basis of knowledge of their constituent
parts. Although less has been published in relation to foaming in this area than
in the areas of polymer solubility and miscibility or emulsions, it remains a poten-
tially interesting approach for investigating structure-property relationships
between surfactant structures and their activity in foam systems. Since foaming
can be related to the solubility of the surfactant (too high a solubility results in
low adsorption; too low, in insufficient availability of surfactant molecules), it is
reasonable to expect good correlations between surfactant structure and foaming
ability using Hansen parameters or related cohesive energy density approaches.
8.2.2. Amphiphilic Mesophases and Foam Stability
As we have seen, the stability of foams depends on a wide variety of factors invol-
ving several aspects of surface science. The potential importance of mesophase for-
mation to the stability of emulsions and foams was briefly mentioned in Chapter 5.
Although the phenomenon of mesophase stabilization of aqueous foams has been
recognized for some time, although the role of such phases in nonaqueous foaming
systems has been less well documented. However, since nonaqueous systems lack
the advantages of electrostatic interactions in most aspects of their surface and col-
loid chemistry, it is not surprising to find that the presence of mesophase can serve
as a sufficient condition for the production of stable foams in organic systems.
The role of mesophases in stabilizing a foam can be related to their effects on
several mechanisms involved in foam collapse, including film drainage and the
mechanical strength of the liquid film. The effect of mesophases on film drainage
can be considered to be twofold. In the first place, the more extended and ordered
nature of the mesophases impart a higher viscosity to the film than a normal surfac-
tant monolayer. A simple-minded physical picture of the potential extent of meso-
phase penetration into the liquid lamellar phase would intuitively suggest that they
should significantly affect the flow and drainage of liquid from between the two
monolayers making up the bilayer film (Figure 8.4). It might also be expected
that the sheer physical interaction between neighboring mesophase units would
impart mechanical rigidity to the system. In addition, it has been shown that meso-
phases tend to accumulate in the plateau border areas. Their presence there results
in an increase in the size of the areas, a larger radius of curvature, and thus a smaller
Laplace pressure. The second stabilizing function of the mesophases can be related
to the Gibbs-Marangoni effects, in that the presence of a large quantity of surfac-
tant at the plateau borders allows them to act as a reservoir for surfactant molecules
needed to maintain the high surface pressures useful for ensuring foam stability.
