ORIGINAL PAPER An insight into the molecular mechanism of the masking process in titanium tanning Christian S. Pomelli 1 • Cinzia Chiappe 1 • Federica Barontini 2 • Monica Puccini 2 • Maurizia Seggiani 2 • Sandra Vitolo 2 Received: 18 February 2016 / Accepted: 27 April 2016 Ó Springer-Verlag Berlin Heidelberg 2016 Abstract Among the alternative mineral tanning agents investigated for replacing chromium, titanium (IV)-based salts are one of the most promising ones for the develop- ment 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 octa- mer was calculated. Different carboxylic acids, and their corresponding anions, containing at least two polar func- tional 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. Keywords Eco-sustainable process  Leather manufacture  Titanium tanning  Computational modeling  Masking agent Introduction Conventional chrome-tanning process is constantly under threat from increasingly stringent environmental regula- tions on chromium-containing effluents discharge and chromium-containing wastes disposal. In this context, a worldwide interest in the development of novel and cleaner tanning processes has been raised in recent years. Different tanning agents have been proposed, with the aim of totally or partially replacing chromium salts in the tanning operation, e.g., a complex between aluminum sulfate and a hyperbranched polymer (Qiang et al. 2016), co-complexes of aluminum and zirconium with polycar- boxylic acids (Cai et al. 2015), a zirconium–aluminum– titanium complex (Wang et al. 2012), zinc and zinc–tita- nium metal complexes (Cao et al. 2013, 2015), a titanium agent obtained from the wastes of the metal industry (Crudu et al. 2014; Mutlu et al. 2014), complexes of iron with N-hydroxysuccinimide (Crudu et al. 2015), a ferrous- gluconic acid compound (Yang et al. 2013), ferrous sulfate and sulfonated urea–phenol–formaldehyde condensed polymer (Chen et al. 2011), a D-Lysine aldehyde complex (Krishnamoorthy et al. 2013), unnatural D-amino acids– aldehyde (Krishnamoorthy et al. 2012), an amphoteric organic compound synthesized from acrolein and diallyl dimethyl ammonium chloride (Luo and Feng 2015), and an in situ condensate melamine–formaldehyde oligomer (Bai et al. 2013). Vegetable tanning systems, as well as effective alternative cleaner tanning methodologies, e.g., combina- tion tanning systems based on henna and aluminum sulfate & Christian S. Pomelli christian.pomelli@farm.unipi.it & Federica Barontini federica.barontini@unipi.it 1 Department of Pharmacy, University of Pisa, Via Bonanno 33, 56122 Pisa, Italy 2 Department of Civil and Industrial Engineering, University of Pisa, Largo Lucio Lazzarino, 56122 Pisa, Italy 123 Clean Techn Environ Policy DOI 10.1007/s10098-016-1207-y