Fragmented structure of adsorbed layer of non-ionic surfactant on colloidal silica

The structure of the adsorbed layer of a non-ionic surfactant on small colloidal silica beads in water at high pH is investigated by small-angle neutron scattering (SANS). Experiments of fluorescence decay on porous silica, neutron reflectivity on wafers and SANS on colloids suggest a fragmented structure in some case, like e. g. at high pH. Here, measurement which unambiguously rule out all continuous layer models will be presented. First, the adsorption isotherm was constructed by centrifugation and subsequent spectroscopic determination of the concentration of non-adsorbed surfactant in the supernatant. Second, the shape of the surfactant-silica complexes was measured by SANS in dilute solutions. External contrast variation was used to enhance scattering from the silica and the surfactant, respectively. Finally, a model of decorated silica beads has been developed. It will be shown to agree quite nicely with the experimentally observed intensities. (C) 2004 Published by Elsevier B.V.