Formation of mesostructured silica based on non-ionic interactions

Doctoral thesis, 2010

Mesostructured silica is a material consisting of a silica matrix with an organization in the meso range of 2-50 nm. The silica matrix is most often amorphous while the organization, induced by a structure directing agent, may generate a long-range mesoorder. The main objective of this project has been to investigate syntheses of mesostructured silica using non-ionic block co-polymers as structure directing agent. The polymers used have a central poly(propylene oxide) segment surrounded on each side by a poly(ethylene oxide) (PEO) segment. Special emphasis has been directed towards the use of inorganic silica sources. The studies included in this thesis therefore focus on topics such as, the silica-PEO interaction, the condensation of silica in aqueous solution, development of a synthesis method of mesostructured silica based on an inorganic silica source, and the formation mechanism of mesostructured silica. These issues have all been important in order to obtain an improved understanding of the formation of mesostructured silica using non-ionic interactions. The focus towards inorganic silica sources resulted in the development of a new synthesis method for mesostructured silica based on a sodium silicate solution. The synthesis gives a stable colloidal suspension and it is shown that the particle size can be controlled by the electrolyte concentration of the solution. This was explained by the electrostatic interactions stabilizing the particles and that this stabilization is size dependent. The results presented in this thesis also demonstrate that the interaction between silica and PEO is dependent on the state of the silica species, referring to their size, molecular weight, and degree of condensation. This is explained by the dependence of the interaction on the possibility to form multiple bonds between the silica and PEO. The silica-PEO interaction is described by hydrogen bonding with an additional strong contribution from entropic effects. It is further shown that the silica source used as starting material in the syntheses of mesostructured silica is very important for the properties of the final material. This is mainly due to differences in the state of the silica in the various silica solutions used.