Carboxyalkylated Lignin Nanoparticles with Enhanced Functionality for Oil–Water Pickering Emulsion Systems

This paper discusses the creation of lignin nanoparticles (LNPs) from carboxyalkylated kraft lignin with different grafted alkyl side chains (ranging from methyl to pentadaceyl). LNPs could be produced through an aqueous-based acid precipitation of carboxyalkylated lignin. Unmodified LNPs were smaller and more rigid than the carboxyalkylated ones, and by increasing the alkyl chain length, the size and softness of carboxyalkylated LNPs increased. The obtained carboxyalkylated lignin nanoparticles were stable for at least 30 days and exhibited high stability at ionic strengths up to 100 mM, while unmodified LNPs were more sensitive to dispersion salinities. It was also observed that carboxyalkylated LNPs with longer chain lengths had more significant interaction with a solid surface as they adsorbed more abundantly and formed a denser and thinner adsorbed layer. When the LNPs were applied to stabilize xylene-in-water systems, the carboxyalkylated LNPs with more extensive chain lengths exhibited a higher affinity with the oil–water interface and generated a dense adlayer of the nanoparticles on the oil exterior. They also reduced the oil droplet size and formulated more viscous and stable emulsions. On the other hand, unmodified LNPs had more limited adsorption on the oil and formed less stable Pickering emulsions. Overall, the results suggest a novel green process for producing amphiphilic carboxyalkylated lignin nanoparticles with enhanced performance in stabilizing oil–water Pickering emulsions.