J. Cosmet. Sci., 60, 509–518 (September/October 2009) 509 A role of the anteiso branch of 18-MEA in 18-MEA/SPDA to form a persistent hydrophobicity to alkaline-color-treated weathered hair HIROTO TANAMACHI, SHIGETO INOUE, SHINICHI TOKUNAGA, HISASHI TSUJIMURA, NORIYUKI TANJI, MASASHI OGURI, and TAICHI HABE, Beauty Research Center, Kao Corporation, 2-1-3, Bunka, Sumida-ku, Tokyo 131-8501, Japan (H.T., S.T.) Analytical Science Research Laboratories, Kao Corporation, 1334 Minato, Wakayama-shi, Wakayama, 640-0112, Japan (S.I., N.T, T.H.) and Analytical Science Research Laboratories, Kao Corporation, 2606 Akabane, Ichikai, Haga, Tochigi 321-3497, Japan (H.T., M.O.). Accepted for publication May 18, 2009. Synopsis The effect of the anteiso-branch moiety of 18-MEA (18-methyleicosanic acid) to create a persistent hydro- phobicity of alkaline-color-treated weathered hair treated with 18-MEA/SPDA (stearoxypropyldimethyl- amine) was investigated by comparing a straight-chain fatty acid (n-heneicosanoic acid, n-HEA) and an iso-branch fatty acid (19-methyleicosanic acid, 19-MEA) with the anteiso-branch fatty acid (18-MEA), using dynamic contact angle measurements, quantifi cation of 18-MEA by LC/MS, and temperature controlled atomic force microscopy (AFM). The dynamic contact angle measurements indicated that the anteiso-branch moiety of 18-MEA is critical for the creation of a persistent hydrophobicity to alkaline-color-treated weath- ered hair. The temperature-controlled AFM investigations revealed that the anteiso-branch moiety of 18-MEA in the 18-MEA/SPDA system produces a persistent hydrophobicity to alkaline-color-treated weathered hair by providing higher fl uidity to the upper region of the 18-MEA/SPDA layer. INTRODUCTION Since healthy and beautiful hair is of interest to many females who have had their hair damaged by chemical treatments, heat styling, and environmental factors, various hair care products have been developed for improving the beauty of hair. Detailed knowledge of the surface structure and properties of hair is essential for developing unique hair care technologies. The cuticle is the outer surface of the hair fi ber, protecting the cortex. The outermost surface of the cuticle cells has been suggested to be covered by a monolayer of covalently bound fatty acids, a major component of which is 18-MEA (18-methyleico- sanic acid). 18-MEA has been subjected to study for years in hair research science, since the discovery of its presence on keratinous fi bers (1–3). 18-MEA is thought to be covalently

JOURNAL OF COSMETIC SCIENCE 510 bound, probably via a thioester or ester linkage, to the outer surface of the cuticle (4–7) and locates specifi cally in the cuticle, not the cortex (6). It is also known that 18-MEA makes the surface hydrophobic and acts as a boundary lubricant to decrease friction resis- tance (8–13). Although it is expected that a damaged hair surface could be repaired if the 18-MEA layer could be restored, few studies have been made regarding the restoration of 18-MEA on damaged hair surfaces. In our previous study, however, we reported that treatment of alkaline-color-treated weathered hair with 18-MEA, combined with a specifi c tertiary amine (SPDA), formed a persistent hydrophobicity on alkaline-color-treated weathered hair surfaces, and its hydrophobicity was maintained even after shampooing. The mecha- nism for the persistent hydrophobicity by 18-MEA/SPDA was analyzed by various analy- sis techniques, including AFM (atomic force microscopy), ARXPS (angle-resolved X-ray photoelectron spectroscopy), and surface pressure-area isotherms, and it was suggested that the sustainable hydrophobicity could be achieved by forming a durable hydrophobic layer of 18-MEA/SPDA on the hair surface (14,15). In this study, we have investigated whether the anteiso-branch moiety of 18-MEA in 18-MEA/SPDA is essential for provid- ing persistent hydrophobicity to alkaline-color-treated weathered hair treated with 18-MEA/SPDA, by comparing a straight-chain fatty acid (n-heneicosanoic acid, n-HEA) and an iso-branch fatty acid (19-methyleicosanic acid, 19-MEA) with the anteiso-branch fatty acid (18-MEA). The chemical structures of these fatty acids and SPDA are shown in Figure 1. EXPERIMENTAL MATERIALS Hair samples. Hair fi bers were kindly provided by a Japanese female aged 30. The fi bers were cut at a distance of approximately 20 cm from the root end on the back of the head. The hair had never been treated with any chemical agents, such as bleaching, coloring, or permanent waving. Preparation of alkaline-color-treated weathered hair. The hair was exposed to alkaline- coloring treatment four times, coupled with model weathering 360 times, where model weathering Figure 1. Chemical structures of fatty acids and SPDA.