Neutron specular and off-specular reflection from the surface of aerosol-OT solutions above the critical micelle concentration

Neutron reflection has been used to study the air/solution interface of bis(2- ethylhexyl) sulfosuccinate-water at concentrations in the range 1-5% v/v, well above the critical micelle concentration of 0.11%. No features other than the adsorption of a monolayer are observed at a concentration of 1%, but at 2% and above peaks are observed in the specular reflectivity that correspond to a multilayer structure with a repeat distance of 175-180 Angstrom. This is most probably caused by preferential adsorption of the lamellar phase, which in this concentration range coexists with solution and has a spacing of 180 Angstrom. At the same time as the appearance of peaks in the specular reflectivity a strong peak also appears in the off-specular direction and we attribute this to conformal roughness of the multilayer structure. Such a large roughness is not caused by the actual air/water surface, but is probably a consequence of large amplitude undulations (Helfrich waves) in the bilayers constituting the lamellar structure at the interface.