THE MOLECULAR-STRUCTURE OF AUTOPHOBED MONOLAYERS AND PRECURSING FILMS OF A CATIONIC SURFACTANT ON THE SILICON-OXIDE SILICON SURFACE

We have used X-ray and neutron reflectivity to probe autophobed layers and precursing films of a cationic surfactant on the SiO2/Si surface. Formed on retraction from solution, the autophobed films are monolayers. In the autophobed film, the polar head group of the surfactant is coordinated with its counterion in a region near the oxide surface. The layer thickness is about 11 angstrom and the area per molecule is about 50 angstrom-2. The tails of the surfactants are disordered. When connected to the meniscus of a bulk surfactant solution and equilibrated with saturated vapor, the autophobed monolayer becomes a precursing film. The layer swells with water hydrating the polar SiO2 surface and head groups.