An insight into the molecular mechanism of the masking process in titanium tanning

Among the alternative mineral tanning agents investigated for replacing chromium, titanium (IV)-based salts are one of the most promising ones for the development of novel and cleaner tanning processes in the leather industry. The molecular mechanism of the solvation of titanyl salts and their interaction with masking agents was investigated in the present work. A computational study based on the quantum mechanical density functional theory was carried out, focusing on the behavior of complexes between titanyl oligomers of suitable length and different masking agents. The optimized structure of a titanyl octamer was calculated. Different carboxylic acids, and their corresponding anions, containing at least two polar functional groups suitable for the complexation of the titanyl, were considered as masking agents: glycolic, lactic, malic, citric, and phthalic acid. The most significant geometric and energetic parameters of the different titanyl-masking agent complexes were calculated. The results obtained indicate that the lactic acid system shows the best binding energies and the best flexibility. The OH-CRR'-COOH structure appears to be well fitted to complex the titanyl chain. The effect of the R and R' sidechains on complex stabilization seems to be related to electrostatic factors (presence of charge) rather than to steric hindrance.