2006 TRI/PRINCETON CONFERENCE 315 processes. In this context European human hair was investigated that had undergone either multiple oxidative, bleaching or reductive, perm-waving treatments (13). For both types of treatments, enthalpy decreases by following apparent first-order ki- netics with respect to the number of treatments and treatment times. Figure 4 compares the dependencies between denaturation temperature and enthalpy for the two types of processes. The linear relationship for bleached hair in Figure 4A indicates that the oxidative treatment leads to largely homogeneous damage in IFs and IFAPs. For the reductive treatment, enthalpy drops much faster than denaturation temperature, giving evidence that the reductive damage, in comparison, is more pronounced in the helical segments of the IFs compared to the surrounding, highly sulfur cross-linked matrix. KINETIC ANALYSIS For the oxidatively treated hair the course of the denaturation was further investigated by a kinetic analysis of the DSC-curves. Oxidation was chosen, since it represents the comparatively straightforward case, where the treatment affects both morphological components to very similar extents. The kinetic analysis, assuming a priori a first-order denaturation process and using the Friedman-method, is based on (14): daldt= (l - a)· A· exp(-E A IRT) (1) where a is the degree of conversion, determined from the DSC-curves. A and EA are the pre-exponential factor and the activation energy in the Arrhenius law, respectively, describing the reaction rate constant. R is the gas constant. The analysis shows ( 15) that the curves can be well described by this model for a wide range of the degree of conversion. Activation energy and pre-exponential factor (as lnA) each show only a comparatively small and highly correlated decrease with the number of treatments. Since the correlation between the two parameters cannot be explained by mathematical compensation effects, it is concluded that keratins may well show a genuine enthalpy-entropy compensation effect (16) for the denaturation of the a-helix in the IF/IFAP composite. This is in pronounced contrast to the change of the denaturation 22 18 � .? 16 """) 20 y = -33.6 + 0.33 X • ·= """) r::: 0.96 .: 14 18 , 12 ia -$ .i::. 10 C: 16 w C: 0 14 :8 6 E • l! .a � 4 (1l 12 A C: C: Cl) 2 ii) Q Cl 10 0 135 140 145 150 155 160 110 115 120 125 130 135 140 145 150 155 160 Denaturation Temperature in •c Denaturation Temperature in 'C Figure 4. Denaturation enthalpy plotted vs TD for the oxidized (A) and reductively treated samples (B). In Fig. 4A the regression line is given, while the solid line in Fig. 4B is a guide for the eye (Adapted from Ref. 13).

316 JOURNAL OF COSMETIC SCIENCE 3.4 '":'".,, ., 3.2 0 � 3.0 -..... 2.8 C 2.6 8 s 2.4 t! C 2.2 2.0 I 1.8 135 140 145 150 155 160 Denaturation Temperature, 0 c Figure 5. Reaction rate constants at the peak temperatures, k(T 0), for hair samples, repeatedly treated by oxidation. The whiskers denote the standard deviations for fivefold determinations (15). temperature from l 58 ° C for untreated hair to l 38 ° C after the 7 th treatment and a concurrent decrease of the relative amount of native, denaturable a-helix by 40% (13). In view of this compensation, a more comprehensive parameter to assess the changes of the denaturation process is the rate constant at the respective peak temperatures. These were determined from the individual experimental curves ( 15) and are graphically summarized vs. T v in Figure 5. A slight increase of k(T 0) is observed with decreasing temperatures, that is with increasing oxidative changes. This reflects the fall of the activation energy, which overrides the decrease of the pre-exponential factor, which is linked to a decrease of the activation entropy (15 ). CONCLUSIONS The results from the various facets of the investigation show that the kinetic hindrance of the unfolding of the a-helix by the matrix in the IF/IFAP-composite is in fact the primary controlling mechanism of the onset of the denaturation process. Once the temperature rise in combination with the natural composition and/or the chemical change has induced a suitable drop of the viscosity of the matrix around the IFs, their denaturation occurs along a process pathway that is largely independent of temperature and of the previous treatment. REFERENCES (1) A. Schwan-Jonczyk, G. Lang, T. Clausen, J. Koehler, W. Schuh, and K. D. Liebscher, "Hair Prepara- tions," in Ullmann's Encyd. Ind. Chern., 3.Ed. (Wiley-VCH, Weinheim, D, 1998). (2) R. D. B. Fraser, T. P. MacRae, and G. E. Rogers, Keratins: Their Composition, Structure, and Biosynthesis (C. C. Thomas, Springfield, 11, 1982). (3) M. Feughelman, Mechanical Properties and Structure of Alpha-Keratin Fibres (UNSW Press, Sydney, Australia, 1997).