AGGREGATION OF COLLOIDAL SILICA BY N-ALKYL SULFATES

The coagulation behavior of aqueous colloidal silica (Ludox TM) in the presence of a homologous series of n-alkyl sulfates has been studied. Coagulation concentrations were measured for a simple salt, NaC1, and electrolyte/surfactants NaCnSO4 with chain lengths n = 1, 2, 6, 8, 10 , or 12 carbons. The Cs and shorter homologues had coagulation concent rations equivalent to that-of NaCl, while Cs and higher homologues had coagulation concentrations at lower ionic strengths. Calculations of the Ludox-Ludox interaction potential show that the coagulation concen tration results are consistent with the action of a screened repulsion plus a depletion attraction induced by the presence of surfactant mic elles. Small angle neutron scattering measurements were made of mixtur es with sodium dodecyl sulfate (SDS) under contrast-match conditions t hat isolated the silica-silica interactions. The silica-silica interac tions indeed progressed from repulsive to attractive as more SDS micel les were added, a trend consistent with the observed aggregation. Sili ca in the presence of NaCl with an ionic strength equivalent to that o f 0.40 M SDS showed hard sphere interactions, whereas the sample conta ining SDS micelles showed strong long-range attractive interactions. T hus we show how solvent microstructure influences the stability of a c olloidal dispersion.