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 2 50 mmol/L NaCl at 50 degrees C. The external scattering contrast is va ried by changing the composition of the solvent mixture from D2O to H2 O. Reliable values for the volume of the monomer, the mean aggregation number per unit micellar length and the mean cross-sectional radius o f gyration have been obtained by the conventional cross-section Guinie r analysis of the data for varied contrast. A more detailed picture of the cross-sectional structure is attained by application of a model-i ndependent Fourier transformation technique with a subsequent novel de convolution technique that includes polydispersity. It is shown for th e first time that the cross-sectional polydispersity of rodlike micell es can be appreciable. A polydispersity of about 10% of the mean radiu s of 2.73 nm is obtained for SSPOM, reflecting molecular protrusions o f individual surfactant molecules into the surrounding water. The mode l-independent data analysis further supports the simplified general vi ew of an oil drop-like homogeneous core of the cylindrical micelles fo rmed by alkyl chains with an effective chain length of 13.5 carbon ato ms, and a relatively thick and smooth head group shell comprising the remaining atomic groups of the SSPOM molecule.