The Co-Structure Directing Agent (CSDA) Approach to Mesoporous Silica Formation – Exploring the Assembly Characteristics

Sammanfattning: We investigate the formation mechanism responsible for two specific systems of mesoporous silicas formed with the so-called co-structure directing agent (CSDA) route. The synthesis relies on the interactions between silica source (tetraethylorthosilicate, TEOS), surfactants and CSDA. The structures of the mesoporous silica materials were investigated mainly by small angle X-ray diffraction (SAXD), supplemented with transmission electronic microscopy (TEM). The surfactant/water system was investigated mainly by small X-ray scattering (SAXS).In system 1, a cationic surfactant (C18H37N+(CH3)2(CH2)3N+(CH3)3Br2, C18-3-1) and an anionic CSDA (carboxyethylsilanetriol, CES) are used. We have insight into the surfactant aqueous solution, the formation kinetics (reactions of TEOS), and the electrostatic interactions (addition of salt or controlling the concentration of CSDA). Depending on the concentration of HCl in the synthesis, the structure is defined by Fm-3m (at high pH) and by Fd-3m (at low pH), with a gradual transition in the intermediate pH range. When salt is added in the Fd-3m synthesis, the Pm-3n structure is formed. The micellar sizes of C18-3-1 assembled in these three structures are the same. Using SAXD and 13C PT ssNMR, we followed the synthesis process of these three structures, and found that a fast process results in Fm3m, regardless of pH, and a slow process results in Fd-3m. When NaCl is added to the slow system (low pH) the formation is altered resulting in a material with the Pm-3n structure. We suggest that the materials strive to form the densest structure possible which structure that forms depends on the when the condensation of the silica network arrests the formation process. We also investigate the difference of this system with that of a reversed system (i.e. based on an anionic surfactant and a cationic CSDA). We find both similarities and differences between these two “mirroring” systems.In system 2, an anionic surfactant (N-myristoyl-L-alanine, C14-L-Ala) and cationic CSDA (3-aminopropyltriethoxysilane, APES) are used. The cryo-TEM and cryo-SEM images providedetailed information about the formation process of the material. A formation mechanism issuggested. The fibers grow in width. The ribbons twist, and with time grow in width, eventuallyforming helical ribbons. Later on the helical ribbons merge into tubes. This evolutionary progressof the configuration could be explained by the incompatible elastic sheet theory.