Flexibility and cross-sectional structure of an anionic dual-surfactant wormlike micelle explored with small-angle X-ray scattering coupled with contrast variation technique.

The contrast variation technique by adding sucrose to aqueous solvents as an electron density adjusting reagent was applied to small-angle X-ray scattering (SAXS) from a dual surfactant wormlike micelle consisting of sodium lauryl ether sulfate and coconut fatty acid amido propyl betaine in order to evaluate the flexibility and the cross-sectional structure of the micelle. The salt concentration dependence of the zero-shear and dynamic viscosities were found not to be affected by the cation species nor the presence of sucrose in the solution. SAXS showed that the scattering profile [I(q)] can be fitted a double layer cylinder model. By systematically changing the sucrose concentration (CS), a "matching composition" was found in which the shell layer can become invisible in SAXS for each sodium cation concentration ([Na+]). From this composition, the electron densities of the shell (rhoS) and core (rhoC) layers were evaluated to be 370 and 216 e/nm(-3), respectively. These values were consistent with the chemical structure of the surfactant. At these matching compositions, I(q) was measured at low q range (0.03 nm(-1)<q<4 nm(-1)) and the data points were perfectly fitted with a single layer rigid cylinder model, except for [Na+]>1.02 M. In these larger [Na+], the I(q) showed an up-turn deviation from the rigid rod limit (i.e., I(q) proportional, variant q(-1)) at low q, indicating appearance of the flexibility of the cylinder. By applying a wormlike cylinder theory, the Kuhn statistical segment length was evaluated as a function of [Na+]. At the infinite limit of [Na+], the intrinsic Kuhn length (excluding of the electrostatic interactions) can be evaluated to be about 20-40 nm. This value was comparable with that of the wormlike micelles made of nonionic surfactants.