Small angle neutron scattering and model predictions for micelle-decorated colloidal silica beads

The adsorbed layer of surfactant on silica may in some cases have a fragmented structure, as suggested by experiments of fluorescence decay on porous silica, neutron reflectivity on wafers and SANS on colloids. Here we present a structural study of a non-ionic surfactant on small colloidal silica beads in water at high pH by small angle neutron scattering (SANS). The system of Triton X-100 adsorbed onto colloidal nanosilica beads (R;: 80 A) is investigated at pH 9. Using external contrast variation and modeling of the scattered intensity, we have recently characterized the adsorbed layer at maximum coverage. It was shown that it consists of adsorbed micelles of structure similar to the one of bulk micelles, and we have termed this surfactant-silica complex 'micelle-decorated silica'. In this paper we extend our study to a more delicate region of the adsorption isotherm, below maximum surfactant adsorption. Here less and less micelles are expected to adsorb, and we follow the structural changes by SANS. In the second part of the paper, the predictions of the micelle-decorated silica model in the complete parameter space are presented. The scattered intensity is calculated as a function of the number of adsorbed micelles and of their radii.