Bi-liquid foams

Concentrated emulsions have structures similar to foams; for this reason they are also called 'bi-liquid foams'. For oil in water emulsions, they are made of polyhedral oil cells separated by aqueous surfactant films. The limited stability of these Systems is a major nuisance in their applications. In this work, we tried to understand and to control the mechanisms through which bi-liquid foams can loose their stability. In a first stage, we characterized the states of surfactant films in bi-liquid foams submitted to different pressures. We determined their hydration, the surfactant density at interfaces as well as their thicknesses. The bi-liquid foams were made by concentrating hexadecane-in-water emulsions through centrifugation. The initial emulsions contained submicron oil droplets that were completely covered with surfactant. We measured the resistance of the films to dehydration, and we represented it by pressure-film thickness curves or pressure-film hydration curves. We also obtained evidence that the interfacial surfactant density increases when the film thickness is decreased (SDS case). The Newton Black Film state is the most dehydrated metastable state that can be reached. In this state, the films can be described as surfactant bilayers that only contain the hydration water of the surfactant polar heads. Two different processes are involved the destabilization of bi-liquid foams: Ostwald ripening (oil transfer from small cells to large cells) and coalescence (films rupture). The first mechanism can be controlled by choosing oils that are very insoluble in water, avoiding ethoxylated nonionic surfactants of low molecular weight, and making emulsions that are not too fine. The second mechanism is responsible for the catastrophic destabilization of bi-liquid foams made of droplets above one micron or with a low coverage in surfactant. In these cases, destabilization occurs in the early stages of concentration, when the films are still thick. It is caused by fluctuations in film thickness and in interfacial surfactant density. We have found that submicron emulsions with interfaces that are saturated with surfactant can form very thin films with a very high metastability. The rupture of these films is controlled by the nucleation and growth of defects in the films (holes, curvature inversions). The barriers that oppose this process depend on the interfacial parameters of the films: cohesion parameters and curvature parameters. Surfactants with polar heads that are very hydrophilic, as well as sufficiently long apolar tails, will be well anchored at the interface and will keep a curvature towards oil. As a consequence, films made with these surfactants have better chances to maintain a good metastability against dehydration. These results can be used to make dry emulsions. For highly cohesive films, dehydration often causes a crystallization of the surfactant that destabilizes the films. Therefore the stability of the films can be improved by using surfactants that make organized dehydrated phases or by small molecules or ions that plasticize the surfactant. (author)