Casey D. Stokes

Evaluating dispersant stabilization of colloidal suspensions from the scaling behavior of gel rheology and adsorption measurements

Maintaining suspension stability by effective particle dispersion in systems with attractive interactions can be accomplished by the addition of dispersants that modify the interparticle potential to provide steric or electrostatic barriers against aggregation. The efficacy of such dispersants is typically considered simply by the modification of suspension rheological properties as a function of the overall added dispersant concentration. However, such considerations do little to reveal the molecular origin of differences in dispersant efficacy because they do not consider differences in surface activity. We combine measured adsorption isotherms with the rheological characterization of the elasticity of colloidal gels formed by particle aggregation to provide a more meaningful assessment of dispersant efficacy. The rheological data show that the dispersants are effective at reducing particle aggregation, whereas, from the adsorption isotherms, they differ considerably in their surface coverage at constant overall concentration. When compared at constant dispersant particle surface coverage, the gel rheology shows marked differences across the different dispersants, as opposed to comparisons at constant overall dispersant concentration in the suspensions. In particular, the power-law volume fraction scaling of gel elasticity at constant coverage reveals clear differences in the critical volume fraction for gel formation for the different dispersants. The most efficacious dispersant is that associated with the largest critical volume fraction for gel formation at a given surface coverage. This work demonstrates the utility of rheological investigations coupled with accurate determinations of surface coverage to better differentiate dispersant performance, which may improve efforts to engineer new dispersant molecules.