A Critical Review of the Physicochemical Properties of Lignosulfonates: Chemical Structure and Behavior in Aqueous Solution, at Surfaces and Interfaces

Abstract

Lignosulfonates are bio-based surfactants and specialty chemicals, which are generated by breaking the near-infinite lignin network during sulfite pulping of wood. Due to their amphiphilic nature, lignosulfonates are used in manifold applications such as plasticizer, dispersant, and stabilizer formulations. Function and performance are determined by their behavior in aqueous solution and at surfaces and interfaces, which is in turn imposed by the chemical make-up. This review hence summarizes the efforts made into delineating the physicochemical properties of lignosulfonates, while also relating to their composition and structure. Lignosulfonates are randomly branched polyelectrolytes with abundant sulfonate and carboxylic acid groups to ensure water-solubility. In aqueous solution, their conformation, colloidal state, and adsorption at surfaces or interfaces can be affected by a range of parameters, such as pH, concentration of other electrolytes, temperature, and the presence of organic solvents. These parameters may also affect the adsorption behavior, which reportedly follows Langmuir isotherm and pseudo second-order kinetics. The relative hydrophobicity, as determined by hydrophobic interaction chromatography, is an indicator that can help to relate composition and behavior of lignosulfonates. More hydrophobic materials have been found to exhibit a lower charge density. This may improve dispersion stabilization, but it can also be disadvantageous if an electrokinetic charge needs to be introduced at solid surfaces or if precipitation due to salting out is an issue. In addition, the monolignol composition, molecular weight distribution, and chemical modification may affect the physicochemical behavior of lignosulfonates. In conclusion, the properties of lignosulfonates can be tailored by controlling aspects such as the production parameters, fractionation, and by subsequent modification. Recent developments have spawned a magnitude of products and technologies, which is also reflected in the wide variety of possible application areas.