Thermal Analysis of Human Hair, Modified by Disulphide Bond Breakage and Variable Alkylation

Thermal analysis of human hair, modified by disulphide bond breakage and variable alkylation Ian Jinks1, Gabriele Wortmann1, Prem Paul2, Franz J. Wortmann11School of Materials, University of Manchester, Manchester M13 9PL, UK.2Unilever Research and Development, Port Sunlight, Wirral, CH63 3JW, UK.Human hair, like other α-keratinous fibres, is a highly complex biomaterial. For the analysis of its mechanical and thermal properties it is, however, well described by a two-phase structure, which contains as morphological components the highly-ordered, crystalline intermediate filaments (IFs) and the less-ordered, amorphous matrix. Unique to α-keratin fibres are disulphide bonds, predominately found within the fibre matrix, which are known to play an important role in the stability and often are responsible for the unique mechanical properties exhibited by α-keratin fibres when compared to similar biomaterials. These bonds are, for instance, the chemical and structural basis for permanent waving and straightening treatments.Breaking of disulphide bonds through reduction and their modification through variable alkylation were carried out on untreated, brown, European human hair using a sequential ‘one-bath’ procedure established by Maclaren and Sweetman [1,2,3]. The subsequent alkylation of the reduced disulphide bonds is designed to prevent re-oxidation to reform the bonds, allowing fibre properties to be analysed. This modification was also intended to further investigate the premise put forward by Hall and Wolfram [4] that hair fibre stability could be restored after disulphide bond reduction through high molecular weight alkylation. This study attempts to determine the effects of disulphide bond reduction and alkylation with groups of different sizes and shapes on the thermal properties of human hair using differential scanning calorimetry (DSC) by, namely, investigating the humidity-dependent glass transition temperature Tg [5] as well as denaturation temperatures TD and enthalpies, ΔHD in water [6].Disulphide reduction and alkylation are shown to have a severe effect on the matrix, where both disulphide bond cleavage and plasticisation by the introduced alkyl chains contribute to the observed decrease in the humidity-dependent glass transition. The effect increases with the alkyl group chain length. Sizeable changes in the IF stability and integrity are shown by substantial reductions in denaturation enthalpies. However, it appears that at high water contents hydrophobic interactions between the alkyl chains are sufficient to kinetically impede the unfolding of the IFs to a similar extent as an untreated matrix. This is shown by a largely unchanged denaturation temperature after modification.[1] B. J. Sweetman and J. A. Maclaren, “Reduction of Wool Keratin by Tertiary Phosphines” Aust. J. Chem., 19 (1966) 2347–2354.[2] J. A. Maclaren, D. J. Kilpatrick, and A. Kirkpatrick, “Reduced Wool Fibres, their Preparation and Alkylation” Aust. J. Biol. Sci., 21 (1968) 805–813.[3] J. A. Maclaren and B. J. Sweetman, “Preparation of Reduced and S-Alkylated Wool Keratins Using Tri-N-Butylphosphine” Aust. J. Chem., 19 (1966) 2355–2360.[4] K. E. Hall and L. J. Wolfram, “Application of Theory of Hydrophobic Bonds to Hair Treatments” J. Soc. Cosmet. Chem., 28 (1977) 231–241. [5] F.J. Wortmann, M. Stapels, R. Elliot, and L. Chandra, “The Effect of Water on the Glass Transition of Human Hair” Biopolymers 81 (2006) 371-375.[6] F.J. Wortmann, C. Popescu, and G. Sendelbach, “Effects of Reduction on the Denaturation Kinetics of Human Hair” Biopolymers 89 (2008) 600-605.