Modeling in Situ Small-Angle X-ray Scattering Measurements Following the Formation of Mesostructured Silica

Abstract

A new model for analyzing in situ small-angle X-ray scattering measurements of the formation of hexagonal ordered mesoporous silica has been developed. It employs a factorization of the scattering intensity into contributions from the form factor and the structure factor. The model can therefore be used for the entire synthesis from the first aggregation between the silica and surfactant to the formation of the final well-ordered material. It is shown that the model describes very well the experimental data from a synthesis of a material similar to SBA-15 where a nonionic surfactant is used as a structural directing agent for making mesoporous silica. The actual synthesis has been developed recently, and the main difference, as compared to the SBA-15 synthesis, is the synthesis pH of 2−3. This gives a comparably slow reaction rate and therefore better temporal resolution of the changes during the synthesis. The combination of the advanced model and the relatively slow synthesis means that the synthesis can be followed in greater detail than reported before. The results illustrate that the silica and the poly(ethylene oxide) (PEO) of the Pluronic interact at an early stage, forming cylindrical micelles. The increase in concentration of silica in the PEO region of the micelles can be followed, and it is determined how this affects the radii of both the core and the corona. The interaction then leads to the formation of larger aggregates of cylinders, and at this stage the hexagonal order also appears. The degree of order in the material can be followed in detail with the model, and it is shown that it increases rather slowly with time.