Smart foams: switching reversibly between ultrastable and unstable foams.

Fatty acids are surfactants of particular interest in the present context of green chemistry since they can be extracted from agricultural resources. Here we report on the formation of multilayer tubular structures made of ethanolamine salt of 12-hydroxy stearic acid [1-2]. By visual observations, we have demonstrated that these tube solutions produced foams outstandingly stable over months at room temperature (Figure 1.a). We used thin film pressure balance, confocal microscopy, small angle neutron scattering and neutron reflectivity in order to understand the key mechanisms at the origin of the stability. We have shown that those hydroxyl fatty acids quickly go to the interface but also produce solid layers preventing coalescence and coarsening. In addition, the tubes lead to large menisci surrounding the lamella and reduce the drainage flows. Thus, these tubes solution combines the advantages of both solid particles and the low molecular weight surfactants, since this system foams easily and does not coarsen nor collapse because of an optimal arrangement of monomers and tubes within the foam structure [3]. Upon heating, tubes transit to micelles at a given temperature, what yields a very fast foam destabilization. Of particular interest is that the transition of tubes into micelles inside the foam is reversible. This offers us a versatile and simple way to produce temperature tuneable foams. The foam stability can then be readily tuned to weak foam stability by simply changing the polymorphism of the system upon heating (Figure 1.b) [4]. We showed how the macroscopic features as a function of the temperature depend on properties of the supramolecular assemblies in bulk. To our knowledge, such foam lifetimes obtained using low molecular weight amphiphiles are unprecedented and such foams are the first that we can switch reversibly between ultrastable and unstable in only few degrees.