**1. Introduction**

Lignosulfonates are biobased specialty chemicals, which are generated as a byproduct during sulfite pulping of wood [1]. They are water-soluble polyelectrolytes, as they contain ionizable moieties such as sulfonate and carboxylic acid groups. Having preserved the polyaromatic backbone of lignin, they are functional amphiphiles with surfactant-like behavior. Due to their rich chemistry, lignosulfonates are found in many industrial applications [2]. This book chapter discusses the fundamentals of one of those applications, that is, emulsion stabilization. By explaining the working principles of interfacial adsorption and emulsion stabilization, the goal is to provide a reference for applied and fundamental research, as well as industry and potential endusers of lignosulfonates.

Next to cellulose, lignin is considered the second most abundant and important polymeric compound in plants [1]. Here, it fulfills functions such as water-proofing the cell-walls and adding to the mechanical cohesion and strength. While wood may contain around 20–30% lignin, its abundance in aquatic and herbaceous angiosperms is generally less. In contrast to α-cellulose, lignin is an amorphous material with a randomly branched structure. This non-uniformity can make technical utilization

challenging, as reactivity and availability of functional groups may vary [3]. Still, some applications may benefit from polydispersity. For specialty chemicals such as surfactants, it is the function or the effect that counts. Salinity changes may induce precipitation of an ionic surfactant. If the surfactant is polydisperse, precipitation may be limited to only a fraction of it. The remaining surfactant can then still perform the desired function. Moreover, mixtures of dissimilar surfactants can form synergies, where the performance may be superior over its individual components [4]. The application of lignin as specialty chemicals can thus benefit from its inherent characteristics, which includes its polydispersity. It is therefore not surprising that dispersants and surfactants are currently the dominant value-added applications of technical lignin.

It is frequently stated 50–70 million tons of technical lignin are produced per year, of which only 1–2% of are utilized for value-added products [5]. The majority is burned for heat and chemicals recovery in Kraft mills [6]. More than 1 million tons of lignosulfonates are produced annually [7], hence dominating the market of technical lignin. In contrast to many other lignin products, lignosulfonates are readily watersoluble. Their most common application is dispersants [8], for example as concrete plasticizers, drilling mud thinners, coal-slurry or dye dispersants [9]. Roughly 50% of the annual production of lignosulfonates were used for concrete admixtures in 1999 [10]. Other applications include dust binders, chelating and complexing agents, soil conditioning agents, floatation agents, and water-in-oil emulsion stabilizers [8, 9, 11, 12]. As the effect on interfacial tension is limited, high shear is usually required during emulsification [13]. Still, lignosulfonates can yield emulsions that are stable over months and longer, finding application for example in agrochemical formulations [8]. Other potential areas with less commercial importance include corrosion and scale inhibitors, CO2 flooding and enhanced oil recovery, as well as polymer precursors and additives [2, 14–20].

Great importance is attributed to biopolymers these days, as they are inherently renewable and largely biodegradable. By substituting fossil-based polymers with biopolymers, the transition to more sustainable technologies is furthered. Expanding the use of lignosulfonates is therefore an important piece in the puzzle. Compared to the global production of surfactants or pulp and paper products, lignosulfonates are currently a niche application. There is hence a potential for growth, in particular for applications that involve liquid–liquid interfaces.
