What can we learn from combined SAXS and SANS measurements of the same sample containing surfactants?

We review the information gained when studying the same sample containing surfactants by SANS and SAXS. Specific information can be obtained if intensities I(q) are determined and used on absolute scale as well as over a large q-window, thus extending to the <> asymptotic range. Comparing SAXS and SANS peak positions (when present) allows unambiguous identification of structure factors separated from underlying form factors. Absolute scale refers to scattering cross-section while resolution relates to the qmax/qmin ratio. Taking into account 'external' knowledge of molecular volumes in constrained fitting from explicit models gives better results on aggregation numbers as well as on interfacial thickness than considering only Patterson functions. We assemble in this review results on adsorption isotherms on surfactant films as well as equations of state, quantifying colloidal interactions. Identification of the topological origin of swelling behaviour are made possible by using combined SAXS and SANS on the largest possible q-range. Best results using this general methodology up to now were obtained by considering data separated from background up to qmax= 0.6 A-1 - 0.8 A-1.