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

University dissertation from Lund University, Faculty of Science, Department of Chemistry, Division of Physical Chemistry

Abstract: This thesis is focused on the detail process of formation of mesoporous silica materials.Mesoporous silica materials are one type of porous materials with a pore size between 2 and 5 nm (1 nm=0.000 000 001 m). The pore walls consist of silicon dioxide – silica (sand also consists of silica). The mesoporous silica materials are formed by amphiphilic molecules, a kind of molecule with one part that loves water and one part that hates water, and a silica source. Within the group of amphiphilic molecules, surfactants are mostly used. Surfactants are molecules that can have positively or negatively charged head groups, which attract water, and long organic tails, that repel water. Examples of surfactants in our daily life are shampoos and detergents. When surfactants are dissolved in water, the organic tails will cluster together and only leave the charged head groups outside in contact with the water. In this case, the aggregated surfactants can form different shapes with the tails inside and the head groups on the surface of the aggregate. The silica components will decorate these aggregates and give rise to an attraction that draws the aggregates together. Accordingly, the “mesopores” of the materials are filled with surfactants during their creation. When the surfactants are removed, the mesopores arise, and the mesoporous materials with silica walls are formed. The networks of mesopores are often well ordered in various structures. When we add acid or salt into the formation mixture of mesoporous silica materials, the structure of the materials can sometimes be changed. The process is like baking, where by adding different ingredients, such as baking powder or salt, the shape or texture of the pastries can be changed.In the work of this thesis, two recipes were used. We call them two synthesis systems. Both systems contain a molecule that eventually gives rise to the creation of the silica wall. We call this molecule the silica source. In system 1, a surfactant with a large positively charged head group was used, while in system 2, a surfactant with a negatively charged head group was used. Moreover, a molecule consisting of a charged part and a silica part was added in the mixture. In this thesis, the latter type of molecule is called CSDA. In system 1, the charged part of the CSDA is negative, and is called the carboxylate group, in vinegar there is a lot of carboxylic groups. In system 2, the charged part is positive, with the chemical name quaternary ammonium group, which is a common component used in softeners. In the mixture, the CSDA will be in contact with the surfactant head group via the charged part and with the silica source via the silica part. After the formation, when the surfactants are removed, the charged groups of the CSDA will remain within the pores, therefore, the materials are functionalized with the charged groups of the CSDA directly in the synthesis. In system 1, addition of acid can change the structures of the materials. In this work we aim to find reasons for the structural change. We found that addition of acid will change the building-up of the silica source. With low acid addition, the silica sources build up walls around the surfactant head groups very fast, whereas with a higher amount of acid, the silica walls build slower. If salt is added in the slower system, a different structure will be formed. This is because the added salt makes the building work even slower.In system 2, we used a special type of instrument, called cryo-EM, to look at the system. This instrument allows us to visualize very small things (hundreds of nm in size) in frozen samples. So we can freeze samples after different formation times, and check what has been built up at these specific times. We found that in this system, the material first forms fibers, and then the fibers grow in width and form ribbons. The ribbons then twist and, with time, grow in width to eventually become helical ribbons that later merge into tubes. We also found that a mathematical model can be used to explain this shapechange process.