There's nothing special about foaming soap solutions; however, a soap foam that lasts several months, even at 60°C, is unusual. Especially if the foam is made from a natural substance and can quickly be destroyed or restored only by changing ambient temperature. This sums up the work of the teams of INRA, CEA and CNRS, which makes possible new applications that will be of interest to manufacturers of cosmetics and detergents.
The results of this research have been published in the August 29th issue of Angewandte Chemie.
Due to their particular texture and the molecules that compose them, foams often have detergent properties. In physical chemistry, such 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 fire, agribusiness and mining.
In this case, the researchers of INRA, CEA and CNRS have studied a particular surface-active molecule, the 12-hydroxystearic fatty acid, produced from castor oil. In order to disperse the molecule which is initially insoluble in water, they added a salt. They then demonstrated the special advantages of the surfactant: 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. The researchers observed and explained this phenomenon using microscopy and neutron scattering, so as to monitor structural change on a nanometric scale in situ.
They thus demonstrated that within a range of average temperatures between 20 and 60°C, 12-hydroxystearic acid, mixed with the "right" salt, 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 spherical assemblies that are a thousand times smaller (several nanometers), which the researchers call "micelles." 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).
This is the first time where such a stable foam has been created with such a simple, natural surface-active molecule. The transition temperature between the state where the foam contains tubes and the "micelle" state depends on the salt chosen to disperse the molecule in water, which increases its potential uses.
This "green" chemistry, since it is produced from an organic molecule, creates new possibilities because foams have many industrial uses. For example, it should be possible to produce detergents and shampoos in which the quantity of foam can be controlled simply by adjusting temperature, thus facilitating drainage. Some cosmetic products require numerous chemical ingredients to produce a stable foam; use of 12-hydroxystearic acid would limit the quantity of synthetic ingredients while retaining "foaming" properties over a longer period.
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