Rheological properties of non-modified and n-alkyl surface-modified colloidal silica sols

Abstract

Surface-modified silicas have been prepared from Aerosil 200 and various n-alkylchlorosilanes. The flow behavior of aqueous sols of these silicas was investigated as a function of n-alkyl chain length ( C) and surface coverage (θ) with the chemisorbed species, as well as the polarity of the continuum. Flow behavior and specific viscosity were found to be strongly dependent upon all three factors. Increasing C or θ produced changes from Newtonian to pseudoplastic flow at much lower volume fractions of dispersed phase than seen with nonmodified silica. The dependence of specific viscosity upon volume fraction was directly proportional to θ, although a somewhat different relationship to C was observed. The addition of propan-2-ol in general reduced the specific viscosity and changed the flow pseudoplastic to Newtonian. The sols with the longest chemisorbed chains showed, however, an increase in specific viscosity at low alkanol concentrations. These results were compared with those obtained for sols of nonmodified silica and are considered in terms of the silica/continuum interactions which control the extent to which the highly aggregated silica particles may be broken up during the dispersion process.