Surfactant-Templated Silica Mesophases Formed in Water:Cosolvent Mixtures

Silica/surfactant mesophases have been synthesized in 14 water:cosolvent mixtures by combining tetramethoxysilane with a basic 2 wt % CTAB solution. The effects of the water-to-cosolvent ratio on the formation of supramolecular surfactant templates and ultimately silica/surfactant mesophases is reported for: diethyl ether, ethyl acetate, tetrahydrofuran, tetraglyme, methylene chloride, 2-propanol, acetone, ethanol, methanol, ethylene glycol, acetonitrile, glycerol, formamide, and N -methylformamide. X-ray diffraction (XRD), dynamic and static light scattering (DLS/SLS), scanning and transmission electron microscopies (SEM/TEM), and nitrogen sorption techniques are used to characterize the mesophases. Generally, polar cosolvents decrease the extent of aggregation of CTAB and lead to an evolution from ordered (o-H) hexagonally packed silica (HPS) to disordered (d-H) HPS as the cosolvent concentration is increased. Polar cosolvents allow the unit cell size of the mesophase to be tuned continuously over ∼5 Å: protic solvents decrease the cell size; aprotic solvents increase the cell size. Highly polar protic solvents, such as formamide and ethylene glycol, support substantially nonaqeous synthesis of o-H and d-H mesophases with water:silica ratio less than 4.0. Low dielectric constant cosolvents lead to expanded o-H mesophases at low concentrations, and cubic and lamellar phases at higher concentrations. Cosolvents can be used to synthesize mixed-metal framework structures from homogeneous solutions by premixing molecular inorganic precursors in a compatible nonaqueous solvent and then controllably hydrolyzing the precursors. Cosolvents also influence microstructure, leading to smaller, more curved primary particles than in pure water.