Aqueous behaviour of characteristic molecules in the synthesis of porous silica materials : - an atomistic description

Abstract: The formation process of porous silica materials is an intricate process, involving a number of components that interact to form a highly organized material. Both zeolites and mesoporous silica materials typically require the presence of organic structure director agents (SDAs) around which the silica network polymerizes. For zeolites the SDAs are molecular and for mesoporous silica the SDAs assemble into aggregates, i.e. micelles. The aim of this thesis is to further the understanding of the initial stages of the formation of porous silica materials. This has been done by investigating aqueous model systems of components critical for the formation of porous silica materials using neutron scattering coupled with empirical potential structure refinement (EPSR) to arrive at atomistic descriptions of the systems. The systems are based on aqueous solutions of single components under different conditions as well as a system in which two components are mixed and allowed to interact. Molecular SDAs, tetramethylammonium bromide and tetrapropylammonium bromide respectively, were found to have different association behavior; small clusters of tetrapropylammonium ions were formed whereas tetramethylammonium only formed pairs. Micellar SDA aggregates of decyltrimethylammonium ions with different counterions reveal a strong dependence on the identity of the counterions. The effect of the counterions appears to originate from a subtle balance between electrostatic interactions and ion-specific effects. A model system for oligomeric silica species was evaluated. The system is based on cubic silsesquioxane. 29Si-NMR was used to identify and quantify the silica species that occur in solution. The silica species reveal a high dependence on additives in solution, which can stabilise the cubic silica molecule. Finally the silica model was probed in the presence of decyltrimethylammonium ions in order to probe interactions that occur in the early stages of the synthesis of MCM-41. The silica species show little affinity for the micelle surface, even though being negatively charged, however their presence in the system greatly decrease the size of the micelles compared to those in a pure aqueous solution.