18-MEA DEPOSITION ON HAIR 39 DISCUSSION There are many different types of chemicals used for improving the damaged surface of hair, such as cationic surfactants, polymers, and silicones. The aim of the present study was to develop a method to deposit 18-MEA onto alkaline-color-treated weathered hair in order to provide persistent hydrophobicity and low friction. By combining 18-MEA with stearoxypropyldimethylamine (SPDA), we were able to establish a technology that provides persistent hydrophobicity to alkaline-color-treated weathered hair surfaces. The question remains as to why the combination of 18-MEA and SPDA provides persis- tent hydrophobicity to alkaline-color-treated weathered hair surfaces, and additional sur- face studies are needed to determine the answer. Atomic force microscopy (AFM) is a very powerful technique commonly used in nanotechnology. Here, a sharp tip interacts with surfaces to generate high-resolution images or to obtain local physical information such as indentation and adhesion. It is very diffi cult to examine the exact situation of an 18- MEA/SPDA layer adsorbed onto the alkaline-color-treated weathered hair surface, how- ever, since the surface of hair is too rough for the AFM investigation. In order to investigate the thickness and physical properties of the layer formed by 18-MEA/SPDA, an atomically fl at surface is needed for the AFM observation. A mica surface and the alkaline-color-treated weathered hair surface are both hydrophilic, although the compo- nents of these substance are different: mica consists of a hydrous potassium aluminum silicate mineral and the alkaline color-treated weathered hair surface consists of a mixture of modifi ed proteins. In this study, mica was used for AFM investigation as a hydrophilic model surface instead of hair, although it might be open to question that these surfaces would behave differently due to the different compositions of these surfaces. We believe, however, that the attachment and orientation of the 18-MEA/SPDA could be similar in human hair. Figure 9 shows AFM height images of the adsorbed layers on the mica surfaces treated with 18-MEA/SPDA (a), 18-MEA/DAPS (b), or 18-MEA/STAC (c) conditioner solu- tions. These areas were reimaged after 1000 nm × 1000 nm scratching tests by rastering a tip with constant force, and the adsorbed membrane was analyzed by the AFM scratch- ing method, as shown in Figure 10. If the layer is hard to remove, it means that the ad- sorbed layer is strongly bound to the surface. In the image of adsorbed fi lm treated by Figure 8. Relative ion yield of 18-MEA versus the total ion yield of alkaline-color-treated weathered hair treated with 18-MEA complexes and shampooed, measured by TOF-SIMS. The bars represent means for n=3 the whiskers represent the standard deviations. (a) 18-MEA/SPDA (conditioner 1). (b) 18-MEA/DAPS (conditioner 2). (c) 18-MEA/STAC (conditioner 3).

JOURNAL OF COSMETIC SCIENCE 40 18-MEA/DAPS (b) or 18-MEA/STAC (c), a square groove was observed. On the other hand, treatment by 18-MEA/SPDA (a) showed no such groove. These results indicate that the 18-MEA/SPDA conditioner adsorbed homogeneously on the mica surface and had a high wear resistance. Regarding the thickness of the layer, it was estimated that the layer of 18-MEA/SPDA was about 1-nm thick. Molecular orientations of the adsorbed layers investigated by ARXPS are shown in Figure 11. The peak area ratio of COO (carboxyl) from 18-MEA/C (carbon) from conditioner ingredients, such as surfactant, stearyl alcohol and so on, increased with the measurement depth. The peak area ratio of N (nitrogen)/C (carbon) from SPDA increased as a function of the measurement depth in a similar manner. These results suggest that the hydrophilic moiety of the molecules, the carboxyl group of 18-MEA and the amide group of SPDA, attach to the mica surface and orient the alkyl chain (the hydrophobic moiety) to the air interface. Taking into account our results obtained here, the thickness of 18-MEA/SPDA Figure 9. AFM height images of adsorbed layer on mica surface. Dark areas are mica without sorption bright areas are sorbed layer from conditioner. (a) 18-MEA/SPDA. (b) 18-MEA/DAPS. (c) 18-MEA/STAC. Figure 10. 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 1000 nm ×1000 nm scratching tests were done by rastering the tip with constant force. (a) 18-MEA/SPDA. (b) 18-MEA/ DAPS. (c) 18-MEA/STAC.