Fabrication of amphoteric lignin and its hydrophilicity/oleophilicity at oil/water interface

Amphoteric polymers have tremendous applications in colloidal systems. Despite the enormous availability of lignin, the generation of amphoteric lignin polymer has yet to be discovered. In this work, amphoteric lignin was synthesized in an industrially attractive aqueous system via integrating the advantages of grafting quaternary ammonium groups onto lignin and polymerizing lignin with sulfonate containing monomers. The order of the reactions seemed to change the properties of the amphoteric lignin significantly, which was confirmed by ¹H and 2D ¹H COSY NMR analysis. In this research, the behavior of carbon chain length of the reagent containing the cationic group in inducing amphoteric lignin with superior properties was comprehensively discussed. The wettability and adhesion force analyses were performed to understand the impact of amphoteric lignin at the interface of a water/oil mixture. With the help of the contact angle and adhesion force measurements, the superior properties of the amphoteric lignin to those of only anionic or cationic modified lignin were determined. In water, the amphoteric and anionic lignin polymers strongly repelled oil droplets. In oil, both anionic and cationic lignin polymers showed poor water wettability, while the amphoteric lignin was wetted well by water. This paper discusses the fundamental mechanisms associated with the altered behavior of these lignin-based polymers at different interfaces.