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, NaCl, and electrolyte/surfactants NaC_nSO₄ with chain lengths n = 1, 2, 6, 8, 10, or 12 carbons. The C₆ and shorter homologues had coagulation concentrations equivalent to that of NaCl, while C₈ and higher homologues had coagulation concentrations at lower ionic strengths. Calculations of the Ludox-Ludox interaction potential show that the coagulation concentration results are consistent with the action of a screened repulsion plus a depletion attraction induced by the presence of surfactant micelles. Small angle neutron scattering measurements were made of mixtures with sodium dodecyl sulfate (SDS) under contrast-match conditions that isolated the silica-silica interactions. The silica-silica interactions indeed progressed from repulsive to attractive as more SDS micelles were added, a trend consistent with the observed aggregation. Silica in the presence of NaCl with an ionic strength equivalent to that of 0.40 M SDS showed hard sphere interactions, whereas the sample containing SDS micelles showed strong long-range attractive interactions. Thus we show how solvent microstructure influences the stability of a colloidal dispersion.