Franz Wortmann

Thermal analysis of ortho- and para-cortical cells isolated from wool fibers

The nature of the bimodal denaturation endotherm, which has been observed for some α-keratin materials, was investigated in order to evaluate the validity of the two conflicting theories to interpret the phenomenon. Ortho- and para-cortical cells were isolated from Merino wool and subjected to measurements by differential scanning calorimetry in water at elevated temperatures (80–170°C). The results show for the isolated cell fractions denaturation peaks at 138°C (ortho) and 144°C (para) that are with respect to their location and temperature difference in good agreement with the results obtained for the whole fiber material. The denaturation enthalpy of the para- (21 J/g) was found to be higher than of the ortho-cortical cells (17 J/g) in contrast to expectations from electron diffraction studies. The fact that both of these values are higher than for the whole fiber (15 J/g) is attributed to the removal of the cuticle prior and to the enzymatic digestion of some further material during cell separation. The results give unequivocal evidence for the validity of the ortho/para-hypothesis for the interpretation of the endothermic denaturation doublet of keratins.

Investigating hair properties relevant for hair 'handle'. Part I: hair diameter, bending and frictional properties.

The expert working group ‘Hair Care Products’ of the DGK currently conducts a wide study to contribute to the understanding of how single hair fibre and hair collective properties contribute towards hair ‘handle’ and ‘feel’. During the first stage of this study four hair types were selected from a large group of individual European hair braids, according to either similar or widely different panel ratings for handle. Against the background of the panel test and the state of the literature the working group readily identified the bending properties of single fibres interacting in the tress as a fibre collective and fibre friction as being of central relevance for hair ‘handle’ and ‘feel’. Fibre diameters of the hair types were determined by Optical Fibre Diameter Analyzer and by weighing. From these data mean ellipticity and bending stiffness distributions were calculated. Single fibre friction was determined by the capstan method in the root, middle and tip regions. Significant differences were determined between the hair types in diameters, ellipticity, bending stiffness and friction. The results lead to conclude that ‘handle’ is perceived as inferior when the hair is thick and bending stiffness thus high. For such hair differences in handle rating are related to differences in friction, namely in the tip region. For thin and thus ‘soft’ hair fibre friction seems to play only a minor role.

Thermal denaturation and structural changes of α-helical proteins in keratins.

To gain insight into the thermal stability of intermediate filaments and matrix in the biological composite structure of α-keratins, the thermal denaturation performance of human hair fibers was investigated by Modulated Differential Scanning Calorimetry (MDSC) in the dry and the wet state. Denaturation enthalpy ΔH(D) in water was found to be independent of heating rate (11.5J/g) and to be approximately double as high as in the dry state (5.2J/g). The lower enthalpy (dry) and its dependency on heating rate are attributed to effects of pyrolysis. The stepwise change of reversing heat capacity ΔC(p) marks the denaturation process as a classic two-stage transition. The increase of ΔC(p) with heating rate reflects a continuous shift of the nature of the denaturation of the α-helical material, first, into random coil and then towards random β-materials for lower heating rates. Denaturation temperatures follow Arrhenius relationships with heating rate, yielding activation energies of 416kJ/mol (dry) and 263kJ/mol (wet), respectively. A decrease of activation energy (wet) for high heating rates supports the hypothesis of systematic changes of the pathway of denaturation.

Effects of reduction on the denaturation kinetics of human hair

Although human hair as an alpha-keratinous fiber exhibits a complex morphology, it can be considered as a nano-structured filament/matrix composite for the context of thermal analysis. Using differential scanning calorimetry (DSC) in water, the denaturation performance of the alpha-helical protein fraction and the effects of reductive treatments were studied. The results are viewed in the context of a previous study for oxidative treatments. It was found that the course of the denaturation process remains generally unperturbed by the treatment, following an irreversible, one-step, first-order process. Arrhenius activation energies and pre-exponential factors were determined from the DSC-curves by applying the principles of the Friedman-method. Comparing activation energy values between reductive and oxidative processes shows the differences of the effects on the components of the composite. In contrast, the values of the rate constant at the denaturation temperature, though showing differences in their trends with cumulative treatments, are very similar. This further emphasizes the theory that the viscosity of the matrix affects strict kinetic control over the denaturation of the alpha-helical segments. Once the viscosity of the matrix has decreased enough for the denaturation process to occur, this follows a path that is largely independent of the temperature range and of the chemical pre-history.

A morphological interpretation of water chemical exchange and mobility in cellulose materials derived from proton NMR T2 relaxation

Proton T2 relaxation times of water in cellulosic fibres have been interpreted using a 3-term average model. Motional and chemical exchange contributions to relaxation show opposing temperature behaviour, enabling the use of Arrhenius analysis to determine proton exchange rates and water rotational correlation times. Both parameters vary dramatically with extent of hydration, with chemical exchange dominating relaxation at saturated water contents. Interpretations are based on a morphological model with two types of accessible cellulose, at void surfaces and internally within the cellulose phase. In native cellulose fibres, the presence of crystalline fibrils with low internal accessibility leads to rapid proton exchange at low moisture contents. Regenerated cellulose fibres typically have lower crystallinity and higher internal accessibility, which results in slower exchange as result of migration of water between void and internal environments. Exchange behaviour in regenerated fibres is highly dependent on structural organisation, which depends on the manufacturing process.

A somewhat unexpected result from the deconvolution of DSC-curves for human hair: There is no apparent relation between cortical cell fractions and hair curliness.

A deconvolution process has been developed for curves obtained by differential scanning calorimetry in water for Merino wool and the main ethnic hair types. This enables estimation of the fractions of ortho‐ and para‐type cell groups. The results also indicate that hair may contain a further, low‐sulphur subgroup of ortho‐type cells. The sizes of the major cell fractions are in line with expectations from microscopical investigations. The fractions are comparable for hair types, and no consistent association between cell‐type fractions and hair curvature is observed.