Cross-Section Structure of Rodlike Sodium Sulfopropyl Octadecyl Maleate Micelles from Small-Angle Neutron Scattering

Aqueous solutions of the ionic surfactant sodium sulfopropyl octadecyl maleate (SSPOM) have been studied by small-angle neutron scattering (SANS) to characterize the rodlike micelles formed in the presence of 250 mmol/L NaCl at 50 °C. The external scattering contrast is varied by changing the composition of the solvent mixture from D 2 O to H 2 O. Reliable values for the volume of the monomer, the mean aggregation number per unit micellar length and the mean cross-sectional radius of gyration have been obtained by the conventional cross-section Guinier analysis of the data for varied contrast. A more detailed picture of the cross-sectional structure is attaind by application of a model-independent Fourier transformation technique with a subsequent novel deconvolution technique that includes polydispersity. It is shown for the first time that the cross-sectional polydispersity of rodlike micelles can be appreciable. A polydispersity of about 10% of the mean radius of 2.73 nm is obtained for SSPOM, reflecting molecular protrusions of individual surfactant molecules into the surrounding water. The model-independent data analysis further supports the simplified general view of an oil drop-like homogeneous core of the cylindrical micelles formed by alkyl chains with an effective chain length of 13.5 carbon atoms, and a relatively thick and smooth head group shell comprising the remaining atomic groups of the SSPOM molecule.