Comparison of molecular weight and molecular weight distributions of softwood and hardwood lignosulfonates
2003
Fredheim, G.E. | Braaten, S.M. | Christensen, B.E.
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 (Mw = 5.700-12.000 g/mol) as compared to softwood lignosulfonates (Mw = 36.000-61.000 g/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.
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