Abstract

Though the tanning process of both chromium(III) and aluminum(III) are mainly the reactions of coordination with the collagen side chain carboxyl groups, the stability toward water and denaturing temperature of Cr(III)-tanned leather is much higher than that of Al(III)-tanned leather. In this paper, the differences between Cr(III) and Al(III) tanning mechanisms are explored from the species distribution of tanning liquors and the thermodynamics of coordination reaction. First, there are more multinuclear metal species in Cr(III) tanning liquor than in Al(III) tanning liquor through Visual Minteq. For example, at pH4 Cr3(OH)4(H2O)10 5+ accounted for 39% in 0.05mol/L Cr2(SO4)3, and Al3(OH)4(H2O)5 10+ only 0.04% in 0.05mol/L Al2(SO4)3. In addition, a large amount of negative species Al(SO4)2(H2O)2 – (always about 30% in Al2(SO4)3) appeared in Al(III) tanning liquor, which would show low coordinating effect because of the electrostatic repulsion with the ionised collagen sidechain carboxyl groups. The density functional method (DFT) was used to study the kinetics (the electrophilic indexes ω) and thermodynamics (the Gibbs energy change ΔrG) of tanning reaction with Lanl2dz for metal atoms and 6-31g(d) for non-metal atoms. The results showed that the reactivity of multinuclear species is higher and the complexes formed by collagen and multinuclear species are more stable for both Cr(III) and Al(III), and the differences of ΔrG between corresponding Cr(III) and Al(III) tanning reactions are minor. Then the main reason for the lower shrinkage temperature of Al2(SO4)3 may be the lower contents of multinuclear aluminum species and the higher content of the negative specie Al(SO4)2(H2O)2 – in Al(III) tanning liquor. These results suggested that appropriate masking agents which could increase the multinuclear aluminum species content and decrease the negative species content should improve the Al(III) tanning process.