Unravelling the Complexity of Lignosulfonates─Fractionation and Physicochemical Profiling

Understanding the structural architecture of lignosulfonates is essential for optimizing their performance and enabling targeted modifications for sustainable applications. This study investigates six sodium lignosulfonate fractions (2,200-78,000 g/mol; dispersity 1.7-12.2) obtained via two-step ultrafiltration. Functional group analysis (sulfonation degree, phenolic hydroxyl, and carboxylic acid) using elemental analysis and NMR revealed minimal variation across fractions. Hydrophobic interaction chromatography and intrinsic viscosity measurements showed that increasing molecular weight correlates with reduced charge-to-size ratios and enhanced hydrophobicity. Rheological data showed that high-molecular-weight fractions exhibit greater conformational changes and compaction under high ionic strength. 2D NMR of purified fractions uncovered new structural features, including guaiacylethanol and fully characterized mono- and disulfonated bonding patterns. These findings advance the structural mapping of lignosulfonates and demonstrate that molecular weight is the dominant factor influencing the physical properties. The study highlights the value of combining fractionation, rheology, and NMR techniques to deepen our understanding of lignosulfonate structure and guide future lignin-based materials applications.