Molecular design of the surfactant and the co-structure-directing agent (CSDA) toward rational synthesis of targeted anionic surfactant templated mesoporous silica

Abstract

The formation of anionic surfactant templated mesoporous silica (AMS) is investigated from the perspective of the geometrical molecular design of the surfactant and the co-structure-directing agent (CSDA) toward the rational synthesis of the targeted mesostructure. Increasing the geometrical size of two types of head group of the anionic surfactant in the order (i) sulfonate, sulfate and phosphate, and (ii) carboxylate, N-acylalanate/N-acylglycinate and N-acylglutamate, resulted in the mesophase changing from a disordered or lamellar phase to two-dimensional (2D) hexagonal (p6mm) and three-dimensional (3D) cage-type (hexagonal and a modulated cubic phase) with increasing order of the organic/inorganic interface curvature. A change in the chain length of the surfactant also led to different surfactant packing and mesophase interfacial curvatures, and resulted in a change in mesophase from a modulated cage-type mesophase to cubic Fdm, cubic Pmn and a disordered cage-type mesophase. The geometrical change in the CSDA resulted in an increasing micellar curvature in the order: aminopropyl < N-methylaminopropyl < N,N-dimethylaminopropyl trimethoxysilane, which led to a change of mesophase from bicontinuous cubic Pnm to 2D-hexagonal p6mm. An increase in the CSDA/surfactant ratio in the synthesis gave rise to the mesophase change from a disordered cage-type mesophase to cubic Fmm or from lamellar to 2D-hexagonal p6mm, and elemental analysis results showed that the mesoporous silicas have an increased loading of organic moieties using the CSDA method.