Characteristics of flocculated silica dispersions

Abstract

Interparticle attractions manifest themselves in the rheology and microstructure of suspensions. One can investigate the effect of particle interactions in a flocculated dispersion by measuring the elasticity mechanically and determining the network dimensionality by static light scattering. For this purpose we synthesized a model system consisting of submicrometer silica spheres coated with octadecyl chains suspended in hexadecane. Lowering the temperature causes the silica-hexadecane system to form a gel network which possesses a measurable yield stress and elastic modulus. Rheological measurements at constant volume fraction reveal that the dispersion changes from fluid to solid within a narrow temperature range, defining a gel transition boundary. To quantify the magnitude of the attraction, we associate the gel transition with the percolation threshold for sticky spheres. At constant temperature, the elastic modulus increases with volume fraction following a power law. The power law exponent increases with increasing temperature, indicating a structural change in the gel network. We have probed the structure of these silica gels with static light scattering. The angular dependence of the scattering intensity differs significantly from that of a stable dispersion. The gel-liquid transition is observed over a 2°C range, substantiating the rheological results. At low temperatures, silica gels assume a fractal geometry with dimensionality of 2.08 ± 0.29. At high temperatures, the scattering intensity profile resembles that of primary particles, suggesting that interparticle attractions are significantly diminished. © 1991.