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 Å. Surfactant in the solution and the air-water interface itself are involved in directing the growth and final structure of the films. Hexadecyltrimethylammonium chloride (C₁₆TAC) and bromide (C₁₆TAB) 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 Å 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 Å 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.