Control of mesostructure and morphology of surfactant-templated silica in a mixed surfactant system

Mixed cationic surfactants were used in the synthesis of MCM-41 materials. Synthetic conditions for controlling the nanometer-scale channels size and micrometer-scale morphology in mixed alkyltrimethylammonium halides (CnTMAX) systems (n=8–18; X=Cl or Br) were studied. The d100 spacing of MCM-41 synthesized using the C18TMABr–C14TMABr system decreases linearly with increase in the C14TMABr molar fraction. Such linearity was also reproduced in other systems with some deviation at high molar fractions of short chain length surfactants (n<14). The morphology in mixed CnTMABr system can be changed from particle form to a tubules-within-tubule (TWT) structure by adjusting the average surfactant chain length. It was found that the optimum average chain length for preparing a TWT structure is 15–16 for the mixed CnTMABr system and 16–17 for the mixed CnTMACl system. A high yield TWT morphology also can be prepared by adding the cosolvent acetone to a suitable value of the hydrophobic–lipophilic balance (HLB). The change in morphology of the products with respect to average chain length, counter ions and cosolvents are explained by the principle of hydrophilic–hydrophobic (lipophilic) balance (HLB).