In this paper, surface degradation of hair is reviewed. Surface properties such as hydrophobicity and surface friction change as surface structures of hair fiber, that is, 18-methyleicosanoic acid (18-MEA) and epicuticle, degrade. Comparison of contact angle and amount of 18-MEA from root to tip of the sampled hair fibers clarified the contribution of not only 18-MEA but also epicuticle to surface properties. It was found that chemical treatment by itself, such as bleaching, is not enough to cause complete loss of hydrophobic nature even after 18-MEA is removed. Additional weathering processes, such as repeatedly shampooing, are required. A technology for the deposition of a persistent hydrophobicity to bleached and weathered hair surfaces using 18-MEA is presented. Combination of 18-MEA with specific cationic surfactants (Stearoxypropyldimethylamine: SPDA) made the bleached and weathered hair surface hydrophobic, and its hydrophobicity was 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 effects of the anteiso-branch moiety of 18-MEA to create a persistent hydrophobicity with 18-MEA/SPDA were investigated using controlled AFM. It was revealed that the anteiso-branch moiety of 18-MEA in the 18-MEA/SPDA system produces a persistent hydrophobicity by providing higher fluidity to the upper region of the 18-MEA/SPDA layer. The contribution to hair beauty and sensory feeling as one of the practical functions of the hair surface is described in this paper. The hydrophobic nature of the hair surface reduces surface friction in a wet state, which reduces hair disorder alignment. It is also revealed that the moisturized or dried out feeling strongly depends on the hair shape (meandering and diameter) which depends on hair surface properties in a wet environment.
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