STRUCTURE ORGANIZATION OF SILICATE POLYANIONS WITH SURFACTANTS - A NEW APPROACH TO THE SYNTHESES, STRUCTURE TRANSFORMATIONS, AND FORMATION MECHANISMS OF MESOSTRUCTURAL MATERIALS

Structure organization of the cubic silicate species, Si8O208-(Si-8), by the cationic surfactant, CH3(CH2)(15)N(CH3)(3)(+), leads to mesopha sic precipitates with layer or rod-based structures depending on the c onditions of the precipitation. Further ordering of the structures of these precipitates has been achieved by acidic vapor treatment leading to ordered materials with cubic (MCM-48), lamellar, and hexagonal (MC M-41) mesostructures. For the first time, step-by-step structural tran sformations have been observed, which follow the-sequence given here, during vapor phase treatment: layered precipitate (L(0)) --> cubic (V- 1) --> lamellar (L(1)) --> hexagonal (H-1). These results strongly sup port a mechanism in which, under typical synthetic conditions, silicat es and surfactants mutually organize into mesoscopic assemblies with g eometries close to those of discs, rods,or spheres as determined by ch arge density matching and then these assemblies further condense into ordered lamellar, hexagonal, or cubic structures depending on the fina l reaction conditions. The present results also demonstrate that it is possible to achieve structure-tailoring and better framework ordering of these mesostructural materials.