Development of novel cosmetic base using sterol surfactant. II. Solubilizing of sparingly soluble ultraviolet ray absorbers.

Previous studies have reported that O/W emulsion prepared using a surfactant with phytosterol as the hydrophobic moiety exhibited unique morphology; a lamellar structure was present on the surface of the emulsified particles. It is suggested that such a unique self-organized structure was due to the large and bulky planar structure of the sterol. On the other hand, sparingly soluble compounds including ultraviolet ray absorbers and medicines (e.g., indomethacine and finasteride) have been used after they are dissolved mainly in polar oils. However, it is very difficult to dissolve them in bases that contain small amounts of oil components such as lotions. Moreover, many of these sparingly soluble compounds have planar structures such as aromatic rings and are easy to crystallize in polar oil. In this study, sterol surfactants were considered suitable for solubilizing sparingly soluble compounds, since they have a bulky planar structure in their molecules. On this basis, the solubilization of ultraviolet ray absorbers using sterol surfactants was investigated. Methods to solubilize ultraviolet ray absorbers stably and effectively by using a surfactant that had a phytosterol structure have been clarified. Further, the following features were also suggested: (1) the microemulsion of phytosterol surfactant is different from that of other surfactants and (2) a rigid core that has solubilized compounds between the hydrophobic moieties was considered; further, the core was surrounded by a polyoxyethylene chain that prevented the self-aggregation. Analysis using NMR measurements suggested that (1) the polyoxyethylene/polyoxypropylene random copolymer dimethyl ether squeezed in a narrow gap between the hydrophobic moieties of the surfactant, and (2) this eventually increased the solubilized amount of an ultraviolet ray absorber.