EFFECT OF ANTEISO-BRANCH MOIETY OF 18-MEA 515 moiety of 18-MEA was responsible for maintaining persistent hydrophobicity to alka- line-color-treated weathered hair. The question still remains as to why the anteiso-branch structure of 18-MEA is essential for providing a persistent hydrophobicity to alkaline-color-treated weathered hair sur- faces. In untreated healthy hair, 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 (22,23). It is gen- erally accepted that a liquid crystalline membrane is very soft and fl uid. This means that the AFM tip cannot trace the surface of the membrane exactly. It is very diffi cult, there- fore, to analyze the thickness of a liquid crystalline membrane. Investigating the thick- ness of an adsorbed layer as a function of temperature is a useful technique for judging the situation of the layer, whether the layer is solid-like or liquid-like. Here, analysis of the effect of the anteiso branch in the adsorbed membrane by using temperature-controlled AFM was attempted. Figure 4 plots the thickness of the adsorbed layers on the mica sur- faces treated with n-HEA/SPDA and 18-MEA/SPDA conditioners as a function of tem- perature. The thickness of the layer on the mica surface treated with 18-MEA/SPDA conditioner was 1.08 nm at 25°C and increased to 1.39 nm as the temperature reached −10°C, while the thickness of the layer on the mica surface treated with n-HEA/SPDA conditioner was 1.19 nm at 25°C and 1.29 nm at −10°C, showing more stability in the temperature range of 25°C to −10°C. This change is considered to be chiefl y due to the anteiso-branched alkyl chains coagulating upon the transition from the liquid crystalline Figure 3. AFM height images of adsorbed layer on mica surface. Dark areas are mica without sorbed com- pound bright areas are sorbed chemicals from conditioner. The white squares indicate where 1-μm × 1-μm scratching tests were performed by rastering the tip at constant force. (a) n-HEA/SPDA. (b) 19-MEA/SPDA. (c) 18-MEA/SPDA. Figure 4. AFM height images of adsorbed layer on mica surface for 18-MEA/SPDA ( ) and n-HEA/SPDA ( ). The data represent means for n = 5 the whiskers represent the standard deviations.

JOURNAL OF COSMETIC SCIENCE 516 state to the solid state. The results suggest that molecular mobility at the upper region of the 18-MEA/SPDA layer, generated by the anteiso-branch moiety of 18-MEA, may be the key for the persistent hydrophobicity of the alkaline-colored-treated weathered hair treated with 18-MEA/SPDA. The dynamic contact angles of alkaline-color-treated weathered hair treated with n-HEA/ SPDA and 18-MEA/SPDA after one instance of shampooing were 76.9° ± 2.5° and 88.0° ± 5.1° (for n = 10), respectively, and the difference was about 11° (Figure 2). Here, we would like to discuss whether the difference of approximately 11° between the n-HEA/ SPDA and 18-MEA/SPDA would be reasonable or not. In order to identify the role of the anteiso-branch moiety of fatty acid on the surface properties of natural healthy hair (nor- mal hair), it is very important to examine the hair in which anteiso-branch fatty acids, such as 18-MEA, are completely substituted with straight-chain fatty acids. The absence of 18-MEA from the hair of patients with maple syrup urine disease (MSUD) provides an interesting route to examine the important role of the anteiso-branch moiety of fatty acid. MSUD is an inherited disease involving failure to metabolize branched-chain amino acids in proteins. Hair from a patient with MSUD in which 18-MEA is replaced by straight-chain fatty acids, such as n-eicosanoic acid (23), is the most suitable for this pur- pose. The dynamic contact angles of normal hair and MSUD hair were already reported by Naito as 91.5° ± 0.7° and 82.5° ± 1.5° (for n = 5), respectively (24). The observation that the dynamic contact angle values of normal hair were higher than that of MSUD hair by about 9° suggested that the anteiso-branch moiety of fatty acids on the surface of hair makes the surface hydrophobic by providing fl uidity at the end of the alkyl chain of fatty acids. The difference in contact angle of approximately 11° between the hair treated with 18-MEA/SPDA and n-HEA/SPDA after one instance of shampooing with a plain shampoo was much closer to the difference in dynamic contact angle of approximately 9° between normal hair and MSUD hair. It was thus concluded that one of the roles of the anteiso-branch moiety of 18-MEA in 18-MEA/SPDA for the persistent hydrophobic- ity of alkaline-color-treated weathered hair was to give higher fl uidity to the 18-MEA/ SPDA layer. Taking into account the results obtained here and in our previous report (14), the follow- ing revised model is suggested (Figure 5). 18-MEA/SPDA forms a layer 1.4 nm in thick- ness, the upper region of which has higher fl uidity due to the anteiso-branch moiety of 18-MEA, with both 18-MEA and SPDA bound tightly to the surface by the carbonyl and amide groups, orienting the hydrophobic part to the air interface at an angle of approxi- mately 35°. Figure 5. Schematic diagram of the cuticle surface of alkaline-color-treated weathered hair treated with 18-MEA/SPDA.