Abstract
A novel “pearl” luster develops in low-concentration aqueous solutions composed of cetanol as a higher alcohol and polyoxyethylene (20) polyoxypropylene (4) cetyl ether as a surfactant. To elucidate the mechanism by which this pearl luster manifests, we simultaneously measured the small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) on the aqueous solutions at a synchrotron radiation facility (SPring-8). The SAXS profile of a 15 wt% mixed-sample solution at 40°C showed multiple sharp peaks. The higher order peaks were observed at the scattering vector (q) positions, which were integer multiples of the primary peak position, suggesting a lamellar structure formed. On the other hand, the WAXS profile showed a sharp diffraction peak near q=15 nm-1 at 40°C, which indicates the formation of an association structure derived from α-gels. The scattering profile exhibited no peaks attributable to a long-period structure for a 2.5 wt% mixed-sample solution. However, as there was a diffraction peak of the α-gel-derived structure in the WAXS profile, we believe that this structure is involved in the pearl luster formation. The aforementioned results suggested that the pearl luster solution is a dispersion possessing a bilayer structure, and we surmised that the pearl luster is the result of multiple light reflections by the dispersion. Our findings were applied to cosmetic lotions, and the pearl luster lotion with an α-gel structure was beautiful in appearance. It was suggested that the confined water present in the α-gel structure contributes to the moisturizing effect of the lotion.
