Self-Assembly
Living systems provide us many examples of intricately organized composite architectures preserved in silica or calcium carbonate. Most natural composites are formed by complex biomineralization processes; however, recently, self-assembly strategies have emerged as a more efficient means to organize inorganic and organic precursors into precise 2 or 3-D architectures. Self-assembly typically employs asymmetric molecules that are pre-programmed to organize into well-defined supramolecular assemblies. Most common are amphiphilic surfactant molecules or block copolymers composed of hydrophobic and hydrophilic parts. In aqueous solution above the critical micelle concentration (cmc), surfactants assemble into micelles, spherical or cylindrical structures that maintain the hydrophilic parts of the surfactant in contact with water while shielding the hydrophobic parts within the micellar interior. Further increases in surfactant concentration result in the self-organization of micelles into thermodynamically-defined, periodic, hexagonal, cubic, or lamellar mesophases. In addition to surfactant concentration, surfactant shape is also influential in controlling the resulting mesophase due to packing considerations. Surfactant shape may be described by a dimensionless critical packing parameter g = V/aol, where V is the total volume of the surfactant chains plus any co-solvent, ao is the effective head group area at the micelle surface, and l is the kinetic surfactant tail length. Decreasing values of g favor mesophases with progressively increasing curvature: lamellar→cubic(Ia3d)→hexagonal.
Evaporation-Induced Self-Assembly EISA of silica
Two decades ago Mobil researchers demonstrated that surfactant self-assembly conducted in the presence of hydrophilic silicate precursors resulted in the spontaneous formation of surfactant/silica mesophases that upon heating were converted to so-called mesoporous silicas characterized by a precise periodic arrangement of mono-sized pores. However these procedures resulted exclusively in the precipitation of ill-defined, irregular micropowders. In order to form mesoporous silicas as uniform films (or well- defined spherical particles) our laboratory pioneered the development of an evaporation-induced surfactant self-assembly EISA procedure. Beginning with a homogeneous solution of soluble silica and surfactant prepared in ethanol/water solvent with initial surfactant concentration co << cmc, preferential evaporation of ethanol concentrates the depositing film in water and non-volatile surfactant and silica species. The progressively increasing surfactant concentration drives self-assembly of silica-surfactant micelles and their further organization into lyotropic liquid crystalline mesophases. Surfactant removal by washing or pyrolysis creates supported, highly ordered mesoporous films of interest for sensors, membranes, catalyst supports, and low dielectric constant films.
