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J. Cosmet. Sci., 53, 219-228 (July/August 2002) Investioations of cosmetically treated human hair by differential scannino calorimetry in water F.-J. WORTMANN, C. SPRINGOB, and G. SENDELBACH, Deutsches Wollforschz•ngsinstitut e.V., Veltmanplatz 8, D-52062 Aachen (F.-J.W.), and Wella AG, Berliner Allee 65, D-64274 Darmstadt (C.S., G.S.), Germany. Accepted for publication March 15, 2002. Presented at the 21st IFSCC International Congress, Berlin, 2000. Synopsis By applying differential scanning calorimetry (DSC) on human hair in water, the thermal stability of hair's major morphological components is determined. Against the background of the two-phase model for o•-keratins, these components are identified as the partially helical, fibrous intermediate filaments (IF) and the intermediate filament associated-proteins (IFAP) as a cross-linked, amorphous matrix. DSC yields the denaturation enthalpy AH• which depends on the amount and structural integrity of the o•-helical material, and the temperature To, which is kinetically controlled by the cross-link density of the matrix. To assess the effects of cosmetic treatments, hairs were investigated that had undergone either multiple bleaching or perm-waving treatments. The respective dependencies between denaturation temperature and enthalpy show that both morphological components are similarly affected by bleaching, while reductive damage, in comparison, is more pronounced in the IFs. For both types of treatments, changes in enthalpy follow apparent first-order kinetics with respect to the number of treatments as well as treatment time (perm-waving), yielding characteristic reaction rate constants. It appears that DSC in water is an especially suitable method to determine the kinetics of damage formation in human hair resulting from cosmetic treatments. INTRODUCTION Materials made from hard o•-keratin, including human hair, exhibit a complex, mor- phologically fine structure (1). The study of the mechanical properties (2) has led to the two-phase filament-matrix model for o•-keratins, originally proposed by Feughelman (3). In this model, axially oriented, crystalline filaments, traditionally called microfibrils are embedded in an amorphous matrix. One main component of the model can be identified as being comprised of the partly g-helical intermediate _filaments (IF), with their helical fraction as the crystalline fila- mentous phase. The matrix in consequence comprises summarily the rest of the mor- Dedicated to Prof. Dr. H. Zahn on the occasion of his 85 •h birthday. 219