Comparison of molecular weight and molecular weight distributions of softwood and hardwood lignosulfonates

Lignosulfonates obtained from spruce (Picea abies), aspen (Populus sp.) and two species of Eucalyptus (E. globulus and E. grandis) were characterized by aqueous size exclusion chromatography (SEC) combined with in-line multi-angle laser light scattering (MALLS). In general, the hardwood lignosulfonates were shifted to lower molecular weights (M-w = 5.700-12.000 g/mol) as compared to soft-wood lignosulfonates (M-w=36.000-61.000g/mol). Lignosulfonates from E. grandis were further fractionated to obtain fractions of different molecular weights (3.500-30.000 g/mol). The degree of sulfonation increased with decreasing Mw for the fractions as previously found for fractions of spruce lignosulfonate (Fredheim, G.; Braaten S.M.; Christensen, B.E. Molecular weight determination of lignosulfonates by size exclusion chromatography and multi-angle laser lightscattering. J. Chromatogr. 2002, 942, 191-199). The relationship between the intrinsic. viscosity (in 0.1 M NaCl) and molecular weight was essentially the same for spruce and E. grandis lignosulfonate fractions, with an estimated Mark-Houwink-Sakurada (MHS) exponent of 0.36. This value, combined with the low intrinsic viscosities, confirms that lignosulfonates are compact structures in aqueous solution. Based on the SEC-MALLS results a SEC-method using a UV-detector (SEC-UV) was developed, where lignosulfonate fractions were used as broad molecular weight calibration standards.