Molecular Structure of Sodium Lignosulfonate

The molecular structure and properties of four sodium lignosulfonates (LSs) derived from pulping or bioethanol production were evaluated and compared. SXP and SAL were produced by sulfite pulping and sulfonation reaction of lignin from alkali pulping of poplar, respectively. LS-180 and LS-150 were from sulfite pretreatment to overcome recalcitrance of lignocelluloses (SPORL) pretreatment of lodgepole pine for bioethanol production. The molecular weights of the two LSs from SPORL were much smaller than those from pulping.

However, LS-180 and LS-150 had higher sulfur content, especially LS-180. UV–Vis absorption spectra of LS-180 and SXP showed redshift, indicating increased amount of guaiacyl units or conjugated structures. The infrared spectra showed that LS-180 and SXP had more conjugated structures. LS-150 had the highest methoxyl group content, while SAL and LS-180 had lowest.

The 1H-NMR was used to further investigate the difference of molecular linkages within each LS. LS-150 had more β-O-4′ structures than the others, while SXP had more β-β′ structures. When used as dispersant for TiO2 slurry, LS-180, LS-150, and SXP all had excellent dispersibility, but not SAL, suggesting LS from bioethanol production is an excellent source of LS and lignin sulfonation of alkali lignin is not sufficient for dispersion applications.

Graphical abstract