**5. Summary and conclusion**

This chapter detailed the fundamentals of emulsion stabilization with lignosulfonates. First, basic concepts, the industrial production, as well as chemical make-up and structure of lignosulfonates were discussed. Second, the effect of lignosulfonates on surface and interfacial tension was described. Third, the fundamental mechanisms behind emulsion stabilization with lignosulfonates were explained.

Lignosulfonates are bio-based specialty chemicals and function as surfactants due to their amphiphilic property. Lignosulfonates are readily water-soluble as they comprise ionizable moieties, i.e., sulfonate and carboxylic acid groups at neutral pH. They can hence be utilized in a variety of technical applications, which includes the stabilization of oil-in-water emulsions. Surface and interface adsorption of lignosulfonates is evidenced by a reduction in surface and interfacial tension. Parameters that can enhance the effect on interfacial tension include increased salinity, a reduction in pH, and the presence of co-solvents or co-surfactants. A high reduction of interfacial tension is generally beneficial, as this suggests enhanced interfacial adsorption and reduces the energy required during emulsification. While interfacial adsorption is a prerequisite for an efficient emulsion stabilizer, the stabilization mechanism is related to other phenomena as well. For lignosulfonates, these mechanisms include stearic hindrance, electrostatic repulsion, the Marangoni-Gibbs effect, the formation of viscoelastic interface layers, and particle stabilization. The stabilization mechanisms are furthermore affected by the composition of the lignosulfonates as well as the aqueous and oil phase. Parameters that increase the interfacial activity and yield the formation of more cohesive interface layers are beneficial, as both effects tend to promote emulsion stability.

There has been an increased interest in biopolymers recently, as these are inherently more sustainable than petroleum-derived polymers. Lignosulfonates are no exception to that trend, which is also mirrored by the large variety of lignosulfonate products on the market today. Emulsion stabilization is one of the application areas, which still has the potential for growth. This chapter was hence dedicated to the fundamentals of emulsion stabilization with lignosulfonates, in the hope that this may aid their utilization in new areas and products.
