J. Cosmet. Sci., 60, 31–44 (January/February 2009) 31 Deposition of 18-MEA onto alkaline-color-treated weathered hair to form a persistent hydrophobicity HIROTO TANAMACHI, SHIGETO INOUE, NORIYUKI TANJI, HISASHI TSUJIMURA, MASASHI OGURI, MIO ISHITA, SHINICHI TOKUNAGA, and FUMIKO SAZANAMI, Beauty Research Center, Kao Corporation, 2-1-3, Bunka, Sumida-ku, Tokyo 131-8501, Japan (H. T., S. T., F. S.) Analytical Science Research Laboratories, Kao Corporation, 1334 Minato, Wakayama-shi, Wakayama, 640-0112, Japan (S. I., N. T.) and Analytical Science Research Laboratories (H. T., M. O.) and Biological Science Research Laboratories (M. I.), Kao Corporation, 2606 Akabane, Ichikai, Haga, Tochigi 321-3497, Japan. Accepted for publication September 29, 2008. Synopsis A technology for the deposition of a persistent hydrophobicity to alkaline-color-treated weathered hair sur- faces using 18-MEA (18-methyleicosanoic acid) is presented. Two approaches were examined in order to make 18-MEA bind tightly to the alkaline-color-treated weathered hair surface. One was to apply 18-MEA as an acid form and the other was to apply 18-MEA as a salt or complex. It was found that the combination of 18-MEA with specifi c cationic surfactants [stearoxypropyldimethylamine (SPDA) and docosyldimethyl- amine (DSDA)] makes the alkaline-color-treated weathered hair surface hydrophobic and that its hydropho- bicity is maintained even after shampooing. Characterization of adsorbed layers of 18-MEA/SPDA on a mica surface, as a possible hydrophilic surface model, was performed using atomic force microscopy (AFM) and angle-resolved X-ray photoelectron spectroscopy (AR-XPS). The results revealed that 18-MEA/SPDA formed a layer with high wear resistance, with an alkyl chain, the hydrophobic moiety, oriented at an angle of around 25° to the air interface. INTRODUCTION 18-MEA has been subjected to study for years in hair research science since the dis- covery of its presence on keratinous fi bers (1–3). 18-MEA is thought to be covalently bound, probably via a thioester or ester linkage, to the outer surface of the cuticle (4–7) and is located specifi cally in the cuticle, not in the cortex (6). It is also known that 18-MEA makes the surface hydrophobic and acts as a boundary lubricant to decrease friction resistance (8–11). The precise role of 18-MEA remains unclear, but the large segmental volume of the anteiso-moiety is expected to provide molecular mobility and exhibit liquid-like behavior compared with a straight-chain fatty acid (12,13).

JOURNAL OF COSMETIC SCIENCE 32 Since 18-MEA is covalently bound to the cuticle surface via thioester linkages, it can be easily removed under alkaline conditions, such as hair coloring or permanent waving, after which the hair surface becomes hydrophilic and friction increases (14,15). The ab- sence of 18-MEA is considered one of the reasons for an increase in friction on the surface of the cuticle, and it may have an infl uence on the sensory perception of hair, such as in- ducing a dried-out feeling and being hard to fi nger/comb (10). Cationic surfactants, polymers, or silicone derivatives are commonly used in hair care products for improving damaged hair surfaces by, for example, moisturizing, reducing friction, and making hair easier to fi nger/comb. Silicones in particular are used. The im- proving effects of these chemicals, however, are not permanent. Although it is expected that a damaged hair surface should be repaired if the 18-MEA layer can be repaired, restoration of 18-MEA on the damaged hair surface has not been reported so far. The objective of this study is to develop a method to bind 18-MEA onto a damaged hair surface to provide persistent hydrophobicity and low friction to the dam- aged hair surface. It is well known that it is impossible to regenerate covalently bound 18-MEA on the damaged hair surface. Therefore, two approaches were examined in this study in order to make 18-MEA bind tightly to the alkaline-color-treated weathered hair surface. One was to apply 18-MEA as an acid form and the other was to apply 18-MEA as a salt or complex. 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 four times, coupled with model weathering 360 times, where model weathering was done 90 times, between every interval of alkaline coloring. This resulted in a process of alkaline coloring combined with daily weathering that simulated a one-year period, assuming the hair was alkaline colored every three months. The model weathering treatment consisted of a series of daily hair care procedures: shampooing, conditioning, drying with a hot drier, and brushing. Hair fi bers were treated with an alkaline-colored lotion (Table I) for 20 min- utes at room temperature at a liquor:fi ber ratio of 1:1. The solution was then rinsed for one minute under running water. A plain shampoo [0.5 ml 15 wt% of sodium polyoxyethyl- ene lauryl ether sulfate (2.5 E.O.) with 2 wt% N,N-bis(2-hydroxyethyl)-dodecanamide solution, adjusted to pH 7 with phosphoric acid] was applied to the wet hair tress (5 g) and was massaged by hand for 30 seconds. The hair tress was then rinsed for 30 seconds under running water. A plain conditioner (0.5 ml formulation No. 6 listed in Table II) was ap- plied to the wet hair tress and distributed manually for 30 seconds, then left on for one minute. The hair tress was rinsed for 30 seconds under running water. The hair tress was towel dried and dried using a hot dryer for three minutes with brushing 20 times. Chemicals. 18-MEA and stearoxypropyldimethylamine (SPDA) were obtained by chemical synthesis (16,17). Other chemicals were commercially available.