Structure and Dynamics of Silica Dispersions and Gels

Licentiate thesis, 2009

This thesis deal with the sol-gel transition of two quite different systems, both based on silica. The overall aim is to understand what interactions lead to this sol-gel transition and how the structure and dynamics change as a result of this. The first project concern the gelation of water glass, also known as alkaline sodium silicate solutions. Water glass is a widely used chemical and can be produced at low cost. Naturally, using this inexpensive inorganic silica source in the synthesis of functional materials instead of more costly alternatives like organosilicates, would be preferred. To be able to do this more knowledge is needed about the gelation process of these solutions. We have looked at two routes for the gelation of alkaline sodium silicate solutions, namely concentration or pH-adjustment by adding hydrochloric acid to the solution. The results indicate that a concentration does not lead to any condensation of silanol groups and the macroscopic solidification is a result of crowding. A decrease in pH on the other hand lead to a polymerization by condensation reactions although the pH is above 10.5 which is generally thought to be the high limit for any substantial amount of stable colloidal silica to be present. In the second project a lithium electrolyte based on surface modified fumed silica has been investigated in terms of aggregation and gelation as well as lithium ion conductivity. The surface of the fumed silica have been modified with a lithium propanesulfonate group to enable lithium ion conductivity while restricting the diffusion of the anion by tether it to the colloidal particle. The behaviour of the surface modified silica is compared to that of non-modified fumed silica. From these investigations we found that the surface modification fundamentally alters the interactions between the particles. Conductivity measurements reveal that it is indeed possible to immobilize the anion while reaching high lithium ion conductivities in these systems. As the main technique, Photon correlation spectroscopy was used in both projects to probe the relaxation processes and investigate how these systems change with concentration, pH or time. Conductivity measurements, infrared spectroscopy, Nuclear magnetic resonance spectroscopy and viscosity measurements was used as complementary methods to further increase the understanding of these systems.