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 degrees C. The external scattering contrast is varied by changing the composition of the solvent mixture from D2O to H2O. 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 attained 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.