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 (C(n)TMAX) 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 C(n)TMABr 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 C(n)TMABr system and 16-17 for the mixed C(n)TMACl 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).