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.
A typical formula for shampoo containing cationic polymers and anionic/amphoteric surfactants exhibits liquid–liquid phase separation under certain conditions when the shampoo is diluted with water upon the actual use in bathroom. The lower dense phase is considered to be an insoluble complex formed with the cationic polymer and mixed surfactants. Generally, this associative liquid–liquid phase separation is called ‘coacervation’ and the phase of complex is called ‘coacervate’. Although it is well known that the usage texture of shampoo is influenced by coacervate, there are only a few reports about their relationship. In this work, we studied the properties of coacervate and its effect on the usage texture of shampoo, particularly focusing on the effect of the cationic polymer structure. We prepared five different model shampoos that contain various cationic polymers and studied the effect of the characteristics of the polymers on the condition of the complex formations, the amount of coacervate, the rheological properties and the adhesive behaviour of coacervate on the surface of damaged hair. It was found that both the structure and the degree of substitution of cationic polymer drastically changed the condition of coacervation. Moreover, the cationic polymers showed remarkable effect on the rheological properties and the adhesive behaviour of the coacervate on the damaged hair. These results implied that we could control the usage texture of shampoo by choosing proper cationic polymers to control coacervation.
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