In situ synchrotron small-angle X-ray scattering study of MCM-41 crystallisation using gemini surfactants

The synthesis of ordered mesoporous MCM-41, performed at room temperature with Gemini surfactants as structure directing agents, was followed by in situ time-resolved small-angle X-ray scattering with synchrotron radiation source. This study evidenced that the mesostructure evolution can be described considering two main steps. In the initial stage, upon addition of the silica source to the Gemini solution, small aggregates are formed as indicated by broad and weak features in the X-ray scattering patterns. By performing a Guinier analysis and determining the pair size distribution functions p(r), the presence of cylindrical particles of 20 Å in radius and 40 Å length is evidenced. These particles are non-interacting isolated silicate-encrusted micelles, that are the precursors of the mesostructured silica growth. In the second stage, Bragg diffraction peaks appear, indicating that the cylindrical precursors are joining together to form a structure with higher degree of organization. Initially, only small crystalline domains are visible, which then merge to a long range ordered 2D hexagonal structure. X-ray scattered data allows to follow the gradual polymerisation of silica on the surface of cylindrical micelles, giving the final product in less then 10 min. The evolution of crystallinity fraction with time was used to get information about the kinetics and the growth mechanism involved.