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)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.

Small-angle X-ray scattering (SAXS) from dual-surfactant aqueous solutions made from sodium lauryl ether sulfate and coconut fatty acid amido propyl betaine was systematically measured as a function of the net sodium cation concentration, [Na + ]*, and the surfactant concentration, C D . The SAXS intensity [I(q)] was normalized to C D and the resultant I(q)/C D was extrapolated to C D = 0 to give a form factor P(q) for each [Na + ]* [where q = 4 sin( /2)/ is the magnitude of the scattering vector, is the wavelength and 2 is the scattering angle]. The low-q behaviour of P(q) was consistent with long rigid cylinders. The middle-and high-q profiles fitted well with a core-shell cylinder model for all [Na + ]*. The core and total radii (R c and R s ) did not depend on [Na + ]* at all: R c = 1.2 AE 0.05 and R s = 3.1 AE 0.05 nm for [Na + ]* = 0.42-1.5 mol l À1 , indicating that the salt concentration changes did not induce any structural changes and reassembling of the surfactants comprising the micelles. This fact is in contrast to the rheological behaviour where the relaxation mode strongly depends on [Na + ]*. The structure factor [S(q)] was obtained by dividing I(q)/C D by P(q) for each C D and the mean distance (d m ) between the micelles was obtained from the first maximum of S(q) versus q plots. The d m value decreased with increasing C D and [Na + ]*, which is in good agreement with the theoretical prediction and experimental results for charged wormlike micelle solutions.

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