Surfactant Self-Assembly in Cylindrical Silica Nanopores

The aggregative adsorption of a nonionic surfactant (C 12 E 5 ) in the cylindrical pores of SBA-15 ordered mesoporous silica (pore diameter 8 nm) was studied by small-angle neutron scattering (SANS). Scattering profiles obtained at contrast-matching conditions between the aqueous solvent and the silica matrix can be represented quantitatively by an analytical scattering function for the diffuse small-angle scattering superimposed with Bragg reflections from the pore lattice. These two contributions provide complementary information about the self-assembly of the surfactant in the pores: diffuse scattering indicates the formation of surfactant aggregates at preferred distances from each other, and analysis of the Bragg peaks shows that a layer of surfactant is formed at the pore walls. These findings suggest that adsorption of the surfactant starts by formation of discrete surface aggregates, which increase in number and later merge to interconnected patches as the plateau of the adsorption isotherm is approached.