J. Cosmet. Sci., 61, 1–12 (January/February 2010) 1 Reduction-induced surface modifi cation of human hair YASH K. KAMATH and SIGRID B. RUETSCH, TRI/Princeton, PO Box 625, Princeton, NJ 08542. Accepted for publication July 1, 2009. Synopsis A microfl uorometric method has been developed to characterize lipid removal or “delipidation” of the human hair cuticula during light exposure and chemical grooming processes such as oxidation (bleaching) and reduction. In the case of photochemical and chemical oxidation, lipid removal (“delipidation” of the F-layer or lipid-layer) from the outer β-layer of the exposed scale faces and generation of cysteic acid groups occurs. This “delipidation,” which ultimately results in “acidifi cation” of the scale faces, leading to a change in surface chemistry from hydrophobic to hydrophilic, can be detected and quantifi ed by microfl uorometry by tagging, e.g., with the cationic fl uorochrome Rhodamine B. In the case of reduction, similar tagging of the acid sites on the scale faces is possible, but this time, Rhodamine B reacts with the mixed disulfi de containing a carboxyl group that will be ionized above a pH of about 4. In addition to this, we have shown by microfl uorometric scanning that the negative charges generated in the cuticle surface can be used to bind low-molecular-weight quaternary conditioners. This process can be considered as “relipidation” or “refat- ting” of the scale faces. We have shown in earlier studies (1) that this entire process of oxidation-induced “delipidation” and subsequent “relipidation” of the acidic scale faces with a cationic conditioning molecule can also be reliably quantifi ed by X-ray photoelectron spectroscopy (XPS). Furthermore, single-fi ber wetta- bility scanning using the Wilhelmy technique, which is highly sensitive to any changes in surface chemis- try, is well-suited to detect and characterize treatment-induced changes in the chemical nature of the hair surface from hydrophobic to hydrophilic. INTRODUCTION In earlier publications (1,2), we discussed studies concerned with the characterization and quantifi cation of damage done to the hair fi ber by photochemical and cosmetic chemical oxidative processes. We hypothesized that the initial attack of these oxidative processes is on the outer (or upper) β-layer of the exposed surface cuticle cell. The outer β-layer consists of covalently bound lipids (also called the F-layer)—more specifi cally, the fatty acid 18-MEA (18-methyl eicosanoic acid) and cysteine residues, where the 18-MEA is presumably bound to the protein through thioester linkages. This cova- lently bound lipid layer, rich in 18-MEA gives a hydrophobic character to the hair fi ber surface. The lipid layer is a rather chemically damage-resistant layer, except under al- kaline conditions. On the other hand, removal or scission of the surface lipid layer The current affi liation of Yash K. Kamath is Kamath Consulting, Inc.
JOURNAL OF COSMETIC SCIENCE 2 (delipidation), as well as formation of acid groups (acidifi cation) of the exposed scale faces, changes the originally hydrophobic nature of the scale faces and gives them hydrophilic properties. Most of the information we have on the nature of lipids on the surface of hair origi- nated from studies on wool. On the basis of persistence of hydrophobicity in wool after solvent extraction and mild scouring, Leeder and Rippon (3) concluded that the lipids left on the surface were covalently bound to the epicuticle. Evans et al. (4) and Kalkbrenner et al. (5) found that the covalently bound lipids can be released from the surface by treatment with potassium tert-Butoxide in tert-butanol. Negri et al. (6) found that chlorination released over 50% of the bound fatty acids, and these were mainly bound by thioester linkages. However, fatty acids bound by ester or amide linkages were cleaved only by hot aqueous treatments. The observation that acidic chlorine water is capable of releasing covalently bound fatty acids from the surface of hair is relevant to human hair, since the hair often encounters chlorine in the water of swimming pools. Wertz and Downing (7) approached the problem of lipids on the surface of human hair from the studies of lipids of mammalian epidermis. Their detailed analytical study of human hair showed that 18 MEA (C-21 ante-iso) forms only 40% of the covalently bound lipids. In the current study we attempt to characterize and quantify changes in the surface chemistry of hair as a function of progressive reduction. Since surface chemistry is important from the point of view of the spreading of hair care products and the friction that affects the feel of hair, it would be important to characterize the surface of such chemically treated hair. As in earlier work (1,2), we again use the same microfl uoromet- ric technique with the help of the cationic fl uorochrome Rhodamine B to detect the change in the surface chemistry of reduced hair. More specifi cally, we attempt to quan- tify and compare the level of photochemically and cosmetic-chemically (oxidation and reduction) induced breakdown of the thioester linkages, removal of the surface lipids (irrespective of their chemical composition), and formation of acid functionalities on the scale faces. In addition to microfl uorometry, we attempt to measure this change in sur- face chemistry by XPS analysis and wettability scanning (8). The effect of delipidation of the hair surface on friction and the positive effect of depositing conditioners on such damaged hair have been presented in an earlier publication (9) and will not be pursued in this communication. The concept of the microfl uorometric approach used in this research is based on the fact that the intact lipid layer resists rapid adsorption of the cationic fl uorochrome and is indicated by the low fl uorescence intensity of the scale faces. On the other hand, removal of the lipid layer, and formation of a large number of acid groups on the exposed scale faces, enhances rapid adsorption of large amounts of cationic fl uorochrome, resulting in the high fl uorescence intensity of the scale faces. Alternate methods, which are highly sensitive to changes in the surface chemistry of the scale faces, are single-fi ber wettability scanning and XPS analysis. Wettability scanning, which is a measure of surface wettability, detects and measures changes in surface chem- istry from hydrophobic to hydrophilic as a result of oxidation or reduction processes. XPS can establish treatment-induced changes in the concentration and chemical state of all detectable elements at a surface depth as shallow as 25Å. We use these techniques to confi rm and support the results obtained by the microfl uorometric approach.
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