Change in surface chemistry of the cuticle of human hair by chemical and photochemical oxidation
This is a multifaceted study on the characterization and quantification of damage to the hair fiber surface caused by photochemical and cosmetic chemical oxidative processes. Unique techniques were used, including a microfluorometric method that had been adapted to characterize and quantify the delipidation and acidification of the human hair surface during light exposure and cosmetic chemical grooming processes such as bleaching. During photochemical and chemical oxidation, breakdown of the lipid domains (also called the F-layer) of the outer b-layer occurs on the exposed scale faces and cysteic acid groups are generated on the cuticle cell surface. The newly formed acid functionalities can be tagged with the cationic fluorochrome Rhodamine B, allowing not only quantification of the level of progressive acidification but also localization of the newly formed acid functionalities. On the other hand, the negative charges generated on the hair surface by oxidation can also bind low molecular weight quaternary cationic conditioning compounds such as cetyltrimethylammonium bromide. This process can be considered a relipidation by adsorption. We have shown that the entire process of delipidation/acid formation and subsequent relipidation by adsorption on the scale faces can be quantified by X-ray photoelectron spectroscopy. Since X-ray photoelectron spectroscopy analysis is highly sensitive and able to detect atomic species at the very surface of the hair fibers, receiving signals from an escape depth as shallow as 25 Å, it appeared ideal for the characterization of treatment-induced changes in the hair surface. However, X-ray photoelectron spectroscopy provides an average elemental composition of the hair surface including scale faces and scale edges. The microfluorometric technique, on the other hand, can distinguish progressive delipidation of the scale faces from changes occurring at the broken scale edges. This distinction was shown and characterized in detail by slow speed microfluorometric scanning of the hair surface. Chemical and photochemical oxidative processes at the hair surface result in certain collateral effects. Particularly changes in surface wettability and fiber friction are of significance to the cosmetic chemist because they affect the spreading and wicking of products in hair as well as the managability and the body of hair assemblies. Methods of characterizing these effects are discussed briefly.
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