Colloidal fumed silica and soluble silicates — Interactions and microstructure

Doctoral thesis, 2012

Colloidal dispersions and gels are important material systems with applications in many industrial processes and consumer products. This thesis aims at increasing the understanding of colloidal interactions and the phase transition from a colloidal dispersion to an arrested structure in two systems based on silica. The microstructure is determined and changes in the macroscopic behaviour of these systems are coupled to changes in the microstructure. In the first project, lithium electrolytes based on fumed silica and surface modified fumed silica have been investigated in terms of particle aggregation and gelation as well as their ion conductivity. The surface of the fumed silica has been modified with a lithium propanesulfonate group to enable lithium ion conductivity while restricting the diffusion of the anion by tethering it to the fumed silica surface. The behaviour of the surface modified silica is compared to that of non-modified fumed silica and from these investigations we found that the surface modification fundamentally alters the colloidal interactions. Ionic liquids were also used in part of this project as they are currently being viewed as the successor of organic solvents in battery electrolytes. In this part, gelation properties were examined in fumed silica/ ionic liquid electrolytes. The electrolytes were doped with lithium salt, which changed the behaviour of the electrolytes due to the formation of a stable solvation layer around the silica particles. The second project concerns the gelation of water glass, also known as sodium silicate solutions. Water glass is a widely used chemical that can be produced at low cost. Needless to say, using this inexpensive inorganic silica source in the synthesis of functional materials instead of more costly alternatives, like organo-silicates, would be preferred. The goal of this project has been to better understand the composition and microstructure of these materials to open up for new applications using this material. Sodium silicate solutions at different concentration, SiO2:Na2O ratio and pH were investigated. The results indicate that increased concentration does not lead to condensation of silanol groups and the macroscopic solidification is instead a result of crowding. A decrease in pH on the other hand leads to polymerisation by condensation reactions even though 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. If the ratio is increased, fractal particles are formed in the water glass and by the addition of salt to such a dispersion, a gel is obtained.