Pickering or Non-Pickering Dilemma: A Complicated System of Anionic Lignin-Incorporated Oil-Water Emulsions

Lignin (L)-stabilized emulsions have gained interest as sustainable systems. Despite their advantages, the interaction of lignin derivatives with oil and water in emulsion systems remains unclear. In this work, we verified a hypothesis that different modification strategies would generate lignin derivatives with different emulsifying performances, even if lignin is anionically charged to a similar degree. To verify this hypothesis, we generated sulfoethylated lignin (SL) and carboxyethylated lignin (CL) softwood kraft lignin (L) as functional emulsifiers for soybean water emulsion systems. It was observed that lignin derivatives with a more negative zeta potential (ζ-potential) and smaller oil particles resulted in more stable emulsions at alkaline pH due to enhanced electrostatic repulsion. Due to well-dispersed oil droplets and a strong electrostatic system, the viscosity of emulsions was lower at alkaline conditions. It was also noted that SL and CL generated Pickering emulsions via depositing on oil droplets and developing steric hindrance with oil droplet sizes of 436 and 452 nm at acidic pH. However, such systems had shorter lifespans under acidic environments, indirectly implying that steric hindrance was insufficient to generate emulsions with long-term stability. These findings verified the involvement of different mechanisms for stabilizing oil emulsions at various pH levels.