Synthesis of silica films at the air/water interface: effect of template chain-length and ionic strength

Full text: We have grown silicated surfactant films at the air-water interface using n-alkyltrimethylammonium bromide and chloride surfactants in an acid synthesis with tetraethylorthosilicate, with and without added salt, and with n-alkyl chain lengths from 12 to 18. They have been monitored by x-ray reflectometry. The surfactant liquid crystal phase behaviour is little modified in these silicated preparations from that in pure surfactant-water mixtures. We can produce glassy, hexagonal and lamellar structures in ways predictable from the pure surfactant-water phase diagrams. The archetypal synthesis appears to proceed initially by growth of a film composed of a glass of silica coated spherical micelles. The shorter chain materials show no further change, but the longer chain materials show a sudden transformation to a hexagonal phase. This occurs when the slowly increasing ionic strength is sufficient to induce a sphere to rod transition in the micellar structure. This, as predicted from the surfactant-water phase diagram, can be accelerated by addition of salts, and is slower for chloride than bromide counter anions. The hexagonal materials change in cell dimension as the chain length is changed in a way consistent with previous micellar models. All the materials have sufficiently flexible silica frameworks that phase interconversion is observed both from glassy to hexagonal, and from hexagonal to lamellar and vice-versa in those surfactant systems where multiple phases are found to exist