Stable Foam

Foams often have detergent properties due to their particular texture and the molecules that make up the foam. These molecules, which must be dispersed in water to create foam, are called "surface-active." They are located spontaneously in water and air, so that very thin films of water can stabilize around air bubbles of foam with a special architecture. Due to such properties, various foams have numerous applications in cleaning, decontamination, cosmetics, battling pollution and Scientists have been studying a particular surface-active molecule known as 12-hydroxystearic acid which is produced from castor oil.

This molecule is insoluble in water but it becomes water soluble when a suitable salt is added. This surfactant is very special because even in small quantities, it produces abundant foam and, above all, remains stable for more than six months, in contrast with traditional surfactants that stabilize foams for only several hours.

At temperatures between 20 and 60°C, the surfactant disperses in water in the form of tubes that are several microns in size. The tubes form a structure that is perfectly stable and rigid in very thin films of water located between air bubbles, which explains the foam's resistance.
Above 60°C, the tubes merge into micelles, spherical assemblies that are a thousand times smaller (several nanometers). The previously stable foam then collapses because the rigid structure disappears. The researchers have demonstrated that this transition from an assembly of tubes to an assembly of micelles is reversible. If the foam's temperature is increased, its volume will diminish when micelles start to form, and if the temperature is again reduced to between 20 and 60°C, the tubes will form again and the form will re-stabilize (to regain the initial volume of the foam, air must be re-injected).

Reference
Anne-Laure Fameau, Arnaud Saint-Jalmes, Fabrice Cousin, Bérénice Houinsou Houssou, Bruno Novales, Laurence Navailles, Frédéric Nallet, Cédric Gaillard, François Boué, Jean-Paul Douliez. Smart Foams: Switching Reversibly between Ultrastable and Unstable Foams. Angewandte Chemie, 2011; DOI: 10.1002/ange.201102115

Further Reading
Synthetic Detergents
Soaps and Saponification
Functional Groups
Lipids

Suggested Questions:

1. Give the molecular formula for 12-hydroxystearic acid.
2. On the structural formula of 12-hydroxystearic acid identify the:
   • carboxyl functional group
   • hydroxyl functional group
3. Is 12-hydroxystearic acid a saturated or unsaturated fatty acid. Explain your answer.
4. Draw 2 structural isomers of 12-hydroxystearic acid.
5. Explain why 12-hydroxystearic acid is not very soluble in water.
6. Draw a structural formula for lithium 12-hydroxystearate, the lithium salt of 12-hydroxystearic acid.
7. What properties of lithium 12-hydroxystearate make it a common component in greases used in motor vehicles, aircraft and heavy machinery?
8. Explain how the properties listed above in question 7 relate to the chemical structure of lithium 12-hydroxystearate.
9. Design experiments to test:
   
   • the stability of the foams formed by a range of household detergents
   • the stability of foam at different temperatures
   • the stability of foam in the presence of different salts