Fundamentals of mesostructuring through evaporation-induced self-assembly

This article gives an overall view of the mechanisms involved in the mesostructuring that takes place during the formation of surfactant-templated inorganic materials by evaporation. Since such a method of preparation is well suited to fabricating thin films by dip coating, spin coating, casting, or spraying, it is of paramount interest to draw a general description of the processes occurring during the formation of self-assembled hybrid organic/inorganic materials, taking into account all critical parameters. The following study is based on very recent works on the meso-organization of thin silica films using tetraethylorthosilicate (TEOS) as the inorganic source and cetyltrimethylammonium bromide (CTAB) as the structuring agent, but we will show that the method can also be extended to other systems based on non-silica oxides and block copolymer surfactants. We demonstrate that the organization depends mainly on the chemical composition of the film when it reaches the modulable steady state (MSS), where the inorganic framework is still flexible and the composition is stable after reaching an equilibrium in the diffusion of volatile species. This MSS state is generally attained seconds after the drying line, and the film's composition depends on various parameters: the relative vapor pressures in the environment, the evaporation conditions, and the chemical conditions in the initial solution. Diagrams of textures, in which the stabilized structures are controlled by local minima, are proposed to explain the complex phenomena associated with mesostructuring induced by evaporation.