Lignosulfonates are described as randomly‑branched polyaromatic polyelectrolytes ,which exhibit water‑solubility and surfactant‑like behavior . Hydrophilicity is imparted by the presence of anionic sulfonate groups, but also by anionic carboxylate groups and (at high pH) phenolic hydroxyl groups .

The counterion is often a remnant from the pulping process, such as sodium, calcium, magnesium, or ammonia, which facilitates dissociation in aqueous solution. Apart from the dissociation equilibrium, the counterion may otherwise determine the physicochemical properties of lignosulfonates, for example by affecting the polymer conformation .

Some of the polar functional groups, that is, ketones, aldehydes, and methoxy groups, are not operative hydrophilic groups . Aliphatic hydroxyl and ether groups can be intrinsically hydrophilic; however, their functionality is determined by the surrounding molecular structure .

Two examples of generic lignosulfonate structures are shown in Figure 1. It should be noted that these are simplifications of a more complicated picture. Lignosulfonates should be considered as statistical entities rather than classical chemical compounds, due to their polydisperse structure and molecular weight .

The molecular weight of lignosulfonates may span from less than 1000 g/mol to more than 400 000 g/mol in molecular weight . Other technical lignin usually exhibits a lower Mw and a less broad distribution, as for example in case of soda lignin (1000 – 15 000 g/mol), Kraft lignin (1500 – 25 000 g/mol) or organosolv lignin (500 – 5000 g/mol) .

Lignosulfonate composition, structure, molecular weight distribution, and abundance of functional groups is dependent on aspects such as the biomass origin, sulfite pulping conditions, and post‑extraction fractionation and chemical modifications.