Application of Wide Angle X-ray Diffraction to Hair Conditioners

Consumer products from diapers to laundry detergents to cosmetics to toothpastes are all indispensable for maintaining our health and comfort in daily life. These commodities are consistently expected by consumers to be better than previous products, and therefore relentlessly developed by manufactures towards superior performance. A hightech trend of consumer products requires top-notch methodologies to analyze and evaluate product performance, structure, and composition. The needs for unique and powerful analytical and evaluation tools lead us to the application of synchrotron radiationsourced techniques in order to utilize excellent features of SR's, e.g., super bri l l iance and directionality. These features are favorable for microscopic understanding and elucidation of product efficacies and performance, specifically, of beauty care products based on the interaction of ingredients with the skin and hair at the molecular level. This article reports two examples of the application of an SR-sourced X-ray analytical technique to the development of hair conditioners. X-ray diffraction (XRD) analysis of single hair fibers was carried out to evaluate hair conditioning products, which were supposed to affect the internal structure and mechanical response of hair fibers. The human hair shaft is composed of keratin protein, which has the basic structure of three α-helices wrapped around one another to form a protofibri l , then the assemblance of eleven protofibrils to form a microfibril. Bundles of these microfibrils pass through and around hair cells. XRD data were collected at Hutch A of beamline BL24XU (Hyogo ID), using a monochromatic (15 keV, 0.08266 nm) and collimated incident beam (80 × 80 μm2). An IP Laue camera (camera length at 500 mm) was used with a sample holder, in which a single hair fiber is oriented perpendicular to the incident beam. SR-XRD of single hair fibers shows diffraction patterns of hard α-keratin, which have two major characteristic features in the wide angle X-ray diffraction (WAXD) region [1]: (1) a broad equatorial spot centered at 0.97 nm, corresponding to the mean distance, or spacing, between α-helical axes, and (2) a fine meridian arc at 0.517 nm, which is related to the projection of the α-helix pitch along the coiled-coil axis, above a broader arc around 0.5 nm of less ordered coiled coils. First, changes in the XRD pattern of hair fibers after a chemical treatment indicate the microscopic effect of the treatment on the hair structure. We observed changes in the WAXD pattern of identical single fibers of Asian virgin hair over sequential bleaching and conditioning on site to eliminate variations among fibers. We found that the degree of crystallinity across the hair fiber axis could be an illustrative parameter to describe the mechanism underlying the repair of damaged hair using hair conditioning products. Three key findings follow: (1) The degree of crystallinity across the hair axis was increased by 28±6% at 95% confidence level by the