Counterion effect in acid synthesis of mesoporous silica materials

Mesoporous silica materials with well-ordered hexagonal structure were synt hesized under acidic conditions by using various acid sources HX. The induc tion time for the formation of mesostructural precipitation increases in th e lyotropic series HNO3 < HBr < HCl < H2SO4 under the same acid concentrati on. The induction rates are analyzed with a micelle-catalyzed reaction sche me. The order in induction time reflects the strength in counterion binding of X-. Nitric acid with the highest binding strength of counterion tends t o form very long micelles, We have determined the counterion association co nstants and found the degree of counterion dissociation under synthetic con ditions in solution. The association is only partial during the early morph ology-setting stage. HNO3 is a suitable acid source for preparing silica ro pes of hierarchical order, consisting of silica fibers formed from parallel nanosized channels. The elongation of the surfactant micelles and shearing flow are the controlling factors in the morphology. Temperature, acid conc entration, surfactant chain length, and the addition of polar solvent also have strong influence on the morphology and nanostructure of the mesoporous materials under acidic conditions.