A Neutron Reflectivity Study of Drainage and Stratification of AOT Foam Films

Structural properties of Aerosol-OT (AOT) foam films were studied by neutron reflectometry. The drainage of large (10 cm2), horizontally oriented AOT films under zero applied external pressure is slow and proceeds from thick colored or gray films to black films of approximately 200 Å thickness over many hours, resulting in quasi-static conditions over the time scales required for the acquisition of reflectivity profiles throughout the whole drainage process. After formation of the foam filmwhile still showing colored interference fringesand during the early stages of thinning, the appearance of Bragg diffraction peaks demonstrates that the film contains multilamellar structure, in agreement with studies at free air/water interfaces. Upon further reduction of the film thickness, below approximately 250 Å, the reflectivity profile is well fitted by a three-layer slab model with two surfactant layers and an aqueous core with a high surfactant content. The composition of the aqueous core and the manner in which the reflectivity during the latter stages of draining evolves from Bragg peaks to a Kiessig fringe structure indicate stratification within the thin foam film.