Counterion effect in acid synthesis of mesoporous silica materials

Mesoporous silica materials with well-ordered hexagonal structure were synthesized under acidic conditions by using various acid sources HX. The induction time for the formation of mesostructural precipitation increases in the lyotropic series HNO₃ < HBr < HC1 < H₂SO₄ under the same acid concentration. The induction rates are analyzed with a micelle-catalyzed reaction scheme. The order in induction time reflects the strength in counterion binding of X". Nitric acid with the highest binding strength of counterion tends to form very long micelles. We have determined the counterion association constants and found the degree of counterion dissociation under synthetic conditions in solution. The association is only partial during the early morphologysetting stage. HNO₃ is a suitable acid source for preparing silica ropes 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 concentration, 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.