Synthesis conditions for hexagonal mesoporous silica layers

Hexagonal mesoporous silica layers were prepared by the sol-gel route using silicon alkoxides as silica precursors and alkyltrimethylammonium bromides to form the templating liquid crystal mesophase. The synthesis conditions required to obtain well-ordered crack-free layers were investigated. Thin l ayers exhibiting these properties were deposited from diluted sols. X-Ray d iffraction enabled characterisation of the thermal evolution of their order ed structure and crystalline texture. Their porosity was experimentally mea sured from nitrogen adsorption-desorption analyses carried out directly on the thin layers. Two main synthesis parameters were identified. The first i s the ageing time of the sol before deposition: Si-29 NMR showed that the d isappearance of the ordered structure in the layers is related to the appea rance of the Q(3) species in the sol. The second important synthesis parame ter is the surfactant volume fraction in the medium after the removal of th e volatile components. Using hexadecyltrimethylammonium bromide, well-order ed hexagonal layers were obtained for surfactant volume fractions ranging f rom 0.5 to 0.65, as expected from the corresponding water-surfactant binary diagram. For surfactants with shorter alkyl chains, the domain of existenc e of the hexagonal layers shifts to higher surfactant volume fractions in a greement with the evolution previously observed on the water-surfactant bin ary diagrams.