Aggregation and gelation kinetics of fumed silica-ethanol suspensions

The kinetics of aggregation and gelation of fumed silica suspended in ethanol were investigated as a function of volume fraction. At low particle concentrations. gelation is well described by aggregation into a primary minimum arising from hydrogen bonding and dispersion forces. The relation is extremely slow due to an energetic barrier (similar to 25kT) in the interparticle potential associated with solvation forces. The solvation forces also contribute to the formation of a secondary minimum in the interparticle potential. The depth of this minimum (similar to 3kT) is sufficient that, at a critical particle concentration, long-range diffusion is arrested due to the short-range attractions and the cooperative nature of particle interactions, as described by mode coupling theory. The presence of the secondary minimum is also observed in the microstructure of the gels studied using X-ray scattering. These observations reinforce the importance of understanding the role of solvent-particle interactions in manipulating suspension properties. (c) 2006 Elsevier Inc. All rights reserved.