(179b) Surface Curvature Effects On The Self-Assembly Of Surfactants On Colloidal Silica And In Silica Nanopores

The adsorption of surfactants onto colloidal particles and in narrow pores plays an important role in many technological fields including surfactant-mediated ad-solubilization, or the removal of hydrophobic organics from aqueous media by adsorption and microfiltration processes. Adsorption isotherms of surfactants at hydrophilic surfaces commonly exhibit a sigmoidal shape with an initial low-affinity region, followed by a steep increase at a concentration somewhat below the CMC, indicating surface aggregation. Depending on the relative strength of the polar interactions of the surfactant heads with the surface, and the hydrophobic interactions among the surfactant tails, two limiting types of adsorbate structures can be formed: (a) laterally uniform films, or (b) surface micelles of various shapes. Structural information about adsorbed surfactant layers at flat solid/water interfaces can be obtained by neutron reflectometry and grazing incidence small-angle neutron scattering, and the structure of surfactant aggregates on colloidal silica can been studied by SANS. On the other hand, little is known about the structure of surfactant aggregates in nano-size pores, when confinement and curvature effects become important. We are studying the self-assembly of surfactants in the cylindrical pores of MCM-41 type periodic mesoporous silica by adsorption measurements and small-angle diffraction. The diffraction data can be analyzed in terms of structural models based on the structure factor of a 2D hexagonal lattice and the form factor of the cylindrical pore without or with an adsorbed film. Neutron scattering offers the possibility to choose grossly different contrast scenarios of the surfactant aggregates and water in the pores by using H2O/D2O mixtures of different scattering length densities. In favorable cases the Bragg reflections of the silica matrix can be suppressed entirely by using contrast matching water. In such cases the surfactant aggregates in the pore can be studied against a uniform scattering contrast of silica matrix and water. The potential of these techniques for elucidating the self-assembly of surfactants in the pores will be demonstrated.