Mixed surfactant assemblies in the synthesis of mesoporous silicas

MCM-41 type mesoporous silica material is synthesized, under mild conditions, in the presence of cetylpyridinium chloride as the templating species. Seven reflections become observable in the powder X-ray diffraction pattern of this material that can be indexed on the basis of a hexagonal close-packed unit cell with a(0) = 47.62 Angstrom. Synthesis of mesoporous silica materials is also demonstrated using self-assembling micellar aggregates of a mixture of two surfactants: cetylpyridinium chloride (CPCl) and cetyltrimethylammonium chloride (CTACl). As the CPCl:CTACl molar ratio between the two amphiphilic molecules decreases, a gradual increase in the d(100)-spacing is observed starting at ca. 41 Angstrom and in subangstrom increments reaching to that of ca. 43 Angstrom. The materials were also characterized by DTGA, TEM, and Ar adsorption studies. A model is presented that simultaneously accounts for the higher degree of structural order of the mesoporous silica templated with CPCl and the ability to fine-tune d spacings an a subangstrom length scale using CPCl/CTACl mixtures.