by Susannah Molisso, Imperial College London
Susannah Molissoa, Ylang Ylang Lub, Robert V Lawa, Oscar Cesa, Jennifer M Marshc & Daryl R Williamsb*
The fundamental physical processes behind the hysteresis phenomena in keratin water sorption isotherms is still uncertain.
Complex keratins such as hair have a type II isotherm which is typical for microporous materials. Materials which exhibit type II behaviour, often have their hysteresis accounted for by pore filling, where water becomes trapped and is therefore not released during the desorption process.
This explanation cannot be used however to describe hysteresis in hair, as it is non-porous material. The physics of this process is therefore not well understood. In other proteins which also exhibit hysteresis, the cause of hysteresis has been suggested to originate from protein-water interactions at charged and polar amino acid residues , or more vaguely, the sorption kinetics of the protein itself .
For keratins, the cause of hysteresis has been linked to the swelling ability of the fibre. As the fibre swells in the sorption stage, active sites not previously accessible in the dry state become available for binding resulting in high water sorption behaviour. During the desorption phase, as the fibres reduces in swelling with the loss of water, water present at some active sites are retained, resulting in the hysteresis loop .
To examine the role of swelling in this process, hair was damaged via reduction and dying to measure the impact of reduced inter-keratin bonding on hysteresis. Water sorption isotherms were measured using Dynamic Vapour Sorption demonstrating that samples with low disulphide bond content showed maximum hysteresis occurring at lower relative humidity’s (%RH). Diffusional analysis further supported the conclusion that increased swelling ability at higher %RH results in hysteresis becoming more apparent at low %RH. This work concludes that hysteresis is dependent on the swelling ability of keratin, which is strongly governed by disulphide bond content.
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