Salt-concentration dependence of the structure and form factors for the wormlike micelle made from a dual surfactant in aqueous solutions

Abstract

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, CD. The SAXS intensity [I(q)] was normalized to CD and the resultant I(q)/CD was extrapolated to CD = 0 to give a form factor P(q) for each [Na+]* [where q = 4πsin(θ/2)/λ is the magnitude of the scattering vector, λ is the wavelength and 2θ 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 (Rc and Rs) did not depend on [Na+]* at all: Rc = 1.2 ± 0.05 and Rs = 3.1 ± 0.05 nm for [Na+]* = 0.42–1.5 mol l−1, indicating that the salt concentration changes did not induce any structural changes and re-assembling 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)/CD by P(q) for each CD and the mean distance (dm) between the micelles was obtained from the first maximum of S(q) versus q plots. The dm value decreased with increasing CD and [Na+]*, which is in good agreement with the theoretical prediction and experimental results for charged wormlike micelle solutions.