SIZE EXCLUSION CHROMATOGRAPHY OF LIGNINS USING LITHIUM CHLORIDE/N,N-DIMETHYLACETAMIDE AS MOBILE PHASE. I. DISSOLVED AND RESIDUAL BIRCH KRAFT LIGNINS

Abstract

The molecular weight distributions (MWDs) of dissolved and residual lignins from different delignification phases during flow-through kraft cooking of birch have been studied. The fundamental characteristics of the isolated lignin samples were evaluated and the lignins were characterised by size exclusion chromatography using lithium chloride/N,N-dimethylacetamide (LiCl/DMAc) not used for isolated lignin samples before. The MWDs of underivatized and acetylated samples were compared with the MWDs of acetylated samples obtained by using tetrahydrofuran (THF) as mobile phase in a similar chromatographic system. The results indicate that removal of pulp lignin during the initial phase of kraft cooking of birch is restricted partly because of the large size of the lignin polymer. However, at the end of the cook no such restrictions remain. The apparently larger molecular size obtained with the LiCl/DMAc-system compared to the THF-system, may be due to interactions between the polystyrene standards and column matrix in combination with a more extensive conformation of the lignin polymer and/or a higher degree of swelling of the polystyrene-divinylbenzene matrix. Although differently accentuated, the residual lignin samples show a bimodal MWD in both solvents. Possible explanations for this phenomenon may be due to structural heterogeneity in the residual lignin, different molecular conformation due to the polarity of the solvent and/or different degrees of associations between lignin fragments in the two solvents.