Growth of highly ordered thin silicate films at the air-water interface

The growth of thin silicate-organic films at the air-water interface of surfactant solutions has been studied in situ by X-ray and neutron reflectivity to a resolution of ca. 5 Angstrom. Surfactant in the solution and the air-water interface itself are involved in directing the growth and final structure of the films. Hexadecyltrimethylammonium chloride (C(16)TAC) and bromide (C(16)TAB) have been used as the templating surfactants, and the film structure is independent of the anions under the conditions used. In situ X-ray and neutron reflectivity measurements at an early stage of film growth show a slow development of structure in the top 100 Angstrom of the solution, which is consistent with a monolayer of tilted surfactant molecules at the air-water interface, a layer of partly silicated material, and an interdigitated surfactant bilayer or layer of cylindrical micelles oriented with their long axes parallel to the surface. Following this induction period a rapid crystallization occurs to give a structure with a crystallographic repeat distance of 45 Angstrom perpendicular to the surface and composed of alternating surfactant layers and silicate material. The very narrow observed diffraction peaks indicate that the final silicate film is highly ordered.