Liquid crystals and foam stability

Foaming ability of surfactants can also be correlated with the respective solubility parameter, as discussed for emulsions in Chapter 11. In this case, the solubility of the surfactant must be properly balanced—that is, be soluble enough to attain a significant concentration in solution, but not so soluble that significant adsorption does not occur. 

If the solubility of a surfactant is highly temperature-dependent, as is the case for many nonionic polyoxyethylene surfactants and long chain fatty acid soaps such as sodium stearate, it will be found that foaming ability will increase in the same direction as its solubility. Nonionic POE surfactants, for example, exhibit a decrease in foam production as the temperature is increased and the cloud point is approached (solubility decreases). Long-chain carboxylate salts, on the other hand, which may have limited solubility and poor foaming properties in water at room temperature, will be more soluble and will foam more as the temperature increases. 

As we have seen, the stability of foams depends on a wide variety of factors involving several aspects of surface science. The potential importance of liquid crystal (LC) formation to emulsion stability was pointed out in the previous chapter. Not surprisingly, an equally important role for such structures has been identified in foaming applications. Although the phenomenon of LC stabilization of aqueous foams has been recognized for some time, their role in nonaqueous foaming systems has been less well documented. Recently, it has been shown that the presence of a liquid crystalline phase can also serve as a sufficient condition for the production of stable foams in organic systems. 

The role of the liquid crystal in stabilizing a foam can be related to its effect on several mechanisms involved in foam loss, including hydrodynamic drainage, the mechanical strength of the liquid film, and the diffusion rate of 

FIGURE 12.7. The presence of surfactant liquid crystals may add stability to a normal foam (a) by forming a semirigid structure in the plateau border regions (b) and/ or thick lamellar films that provide mechanical as well as colloidal resistance to drainage. 

---