Table IV The Effect of Dialysis on Denaturation Peak Temperature and Enthalpy and Tensile Strength Properties after Multiple Treatments with a Professional Bleach Product Tensile strength measurements Cycle# untreated Post- Plateau load Load@ 25% Break load Peak temp Enthalpy hair treatment dL Gmf/sq.micron (x 103) Gmf/sq.micron (x 103) Gmf/sq.micron (x 103) Td ± s (°C) LlHD ± s 0/g) 0 Untreated 149.7 ± 0.2 13.7 ± 0.4 1 None 46.5 ± 0.8 6.16 ± 0.98 8.71 ± 1.6 19.7 ± 2.9 143.5 ± 1.0 13.0 ± 0.4 1 Dialysed 149.1 ± 0.3 14.6 ± 0.2 3 None 53.8 ± 0.2 5.09 ± 1.69 7.47 ± 2.3 16.5 ± 4.3 137.9 ± 0.2 9.1 ± 0.2 3 Dialysed 145.5 ± 0.7 10.6 ± 0.2 5 None 57.1 ± 0.1 2.79 ± 0.79 3.04 ± 0.8 10.4 ± 4.4 134.5 ± 1.7 5.9 ± 0.03 5 Dialysed 135.1 ± 0.1 3.0 ± 1.4 N 0 0 0-.. ..., � - ;a � z (J tr! ..., 0 z (J 0 z r-rj tr! � tr! z (J tr! VJ N Vl

326 JOURNAL OF COSMETIC SCIENCE R R R R R Bleach Product Dialysis I Washing ➔ ➔ Figure 1. Proposed mechanism: Left is virgin hair fiber: Middle is bleached fiber with compromised covalent cross-linking and residual formulation components. Right is bleached and dialyzed fiber with covalent bonds disrupted but formulation components removed. CONCLUSION We have used High Pressure Differential Scanning Calorimetry to measure changes in the denaturation temperature of the crystalline component of the hair cortex from two commercial hair bleach products which have the same lightening and tensile strength but very different HPDSC peak temperatures and enthalpies. Importantly, we have demonstrated that components of these products, such as the alkalizer, can induce large changes in the denaturation temperature that is not due to oxidative covalent bond cleavage. These changes in the denaturation properties are at least partially reversible on dialysis in deionised water. It is hypothesised that during the bleach process a wide variety of bonds will be broken at different locations in the fibre. Close range electrostatic interactions or salt bridges are readily broken and spontaneously reform as the hair dries. The presence of formulation components such as residual alkalizer and/or salts within the fibre can influence the re-formation of salt bridges and hydrogen bonds. Interference with salt bridges will increase protein flexibility and reduce viscosity in the matrix. This in turn leads to a lowering of the denaturation temperature. By using multiple cycles of a bleach product it can be demonstrated that it is possible to induce both permanent and reversible effects in the fibre. The permanent, irreversible effects are attributed to the oxidative cleavage of covalent bonds such as cystine while the