J. Cosmet. Sci., 62, 237–249 (March/April 2011) 237 Prevention of hair surface aging ERIK SCHULZE ZUR WIESCHE, ANDREA KÖRNER, KAROLA SCHÄFER, and FRANZ-JOSEF WORTMANN, Henkel AG & Co. KGaA, Hohenzollemring 127-129, 22763 Hamburg, Germany (E.S.z.W.), DWI an der RWTH Aachen e.V., Pauwelsstr. 8, 52056 Aachen, Germany (A.K., K.S.), and School of Materials, The University of Manchester, Manchester M13 9PL, UK (F-J.W.). Synopsis The hydrophobic character of the surface of human hair is particularly attributed to the lipid components of the epicuticle and to a layer of covalently bound fatty acids. This outer f-layer mainly consists of 18-methyl eicosanoic acid (18-MEA), which is covalently bound to the underlying protein matrix, forming the epicuti- cle as composite surface structure. Daily weathering and chemical treatments, specifi cally oxidative bleach- ing, decrease the hydrophobicity of the outer hair surface drastically. Multiple daily stress, simulated by an automatic test device including shampooing, blow drying and sun light exposure, changed the lipid composition of hair signifi cantly. A marked loss of 18-MEA was observed. Decreasing contact angles are the direct consequence. A new method to determine the “pseudo-static” con- tact angle on hair was developed. The results correlate with the corresponding data obtained by dynamic contact angle measurements according to Wilhelmy. Besides that, the resorption time of water droplets by the hair surface provides additional information about the intactness of the outer f-layer. Specifi c proteolipids, which are lipid-modifi ed keratins, are able to reconstruct the surface layer of damaged hair by creating renewed surface hydrophobicity and extending the water resorption time by the hair sur- face. INTRODUCTION Human hair is protected against extrinsic aging stress by a hydrophobic outer layer. The intactness of which is essential for the consumer’s perception of healthy and shiny hair (1,2). This outer layer, generally referred to as epicuticle, is a thin, chemically resistant layer, which is estimated to contain a high proportion of lipids ranging from approx. 22% up to 44% depending upon the method of removal and subsequent treatments (3). In addition to solvent extractable lipids there are furthermore covalently bound lipids at the surface forming the so called f-layer. It mainly consists of anteiso (+)-18-methyleico- sanoic acid (18-MEA), bound as a thioester (5–7) to the cysteine residues in the underly- ing protein layer (4). This arrangement imparts pronounced hydrophobicity to the hair surface, which is in marked contrast to the overall hydrophilicity of the hair bulk. Contact angle measurements of undamaged hair, taken near the scalp, show consistently contact angles with water of 100°–110° (8,9). Due to daily aging and cosmetic infl uences
JOURNAL OF COSMETIC SCIENCE 238 the hydrophobicity of the surface decreases. Hydrophilic regions are induced on the fi ber surface through the oxidation of the lipid end groups and the exposure of protein patches. Thus hydrophilic effects increase along a hair towards the fi ber tip due to removal of lip- ids on the surface, leading to contact angles typically around 70° or 80° at the fi ber tip (9). These effects are mainly caused by UV light exposure, intensive shampooing, me- chanical abrasion and chemical processes, e.g., oxidative bleaching. Oxidative bleaching might decrease the contact angles even further to values around 40° (10,11). In order to investigate effects of daily aging on hair more systematically a recently estab- lished multiple day-by-day stress simulation was applied (12). This automatic test device includes shampooing, blow drying and sun light exposure. The test procedure was carried out by a robot, which offers a high degree of reproducibility with respect to the induced hair damage profi les. The impact of different oxidative bleaching treatments was studied in comparison to the natural aging effects. The contact angle of hair with water is an excellent parameter to indicate the general damage constitution of the hair surface. It is easily measurable by means of the dynamic principle according to Wilhelmy, a broadly published method for the determination of contact angle on single hair fi bers (9–11,13). Determination of the wettablility of a hair by the Wilhelmy balance principle involves the measurement of the vertical force on hair fi ber when contact with the liquid is established (9). The forces F are recorded while the fi ber is immersed into the liquid. The contact angle Θ (Figure 1) is now accessible by knowing the fi ber perimeter L and the surface tension γ of water according to LV F Lcos4 Z J (1) Figure 1. Determination of the dynamic contact angle Θ of single hair fi bers in contact with water (13).
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