Insight into the Mechanism of Chemically Stabilizing Collagen
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Abstract
Developing chromium-free tanning agents has attracted much attention due to the potential risks of chromium, however, no alternative can endow leather with the same hydrothermal stability as chromium so far. Indeed, the mechanism of chemically stabilizing collagen still remains unclear. This work used collagen fiber as an adsorbent to explore the source of leather stability by the adsorption thermodynamics of typical tanning agents such as Cr(III), Al(III), glutaraldehyde (GA), and polyphenols (epicatechin, EC). The consistency in optimal conditions among tanning reactions and adsorption processes supported the rationality of the adsorption method. The adsorption for Cr(III) has a slightly lower Gibbs free energy change of -35.82 kJ.mol-1 when compared to other tanning agents, confirming its superiority in leather stability. Significantly, the highest entropy increase of 305.60
and enthalpy change of 61.36 kJmol-1 are observed in adsorbing Cr(III). The adsorption processes of Cr(III), Al(III), and GA are endothermic spontaneous reactions, and the main driving force is entropy, whereas that of EC is an exothermic spontaneous reaction driven by enthalpy and entropy together. Obviously, an increase in entropy is particularly important in the tanning reaction for the stability of products. Thus, the binding water released from collagen fibers was identified to play a crucial role in the source of the stability of tanning agents. This work is not only conducive to deepening the understanding of tanning mechanisms, but also beneficial to developing novel chrome-free tanning agents.