Abstract
The profile of the mobility distribution of lignin-containing samples depends on type of sample. To facilitate comparison, a procedure for determining the average mobility (μav), i.e. the average charge density, of lignin is presented. The procedure is applied to black liquor (Sb), isolated dissolved lignin (Sd) and isolated residual (Sr) lignin samples, obtained from flow-through kraft cooks of softwood. The μav of the isolated lignin samples is compared with the concentration of phenol and carboxyl groups and relative molecular size. As the cook proceeds the μav for a particular type of lignin sample increases, reflecting an increase in average charge density. The μav, measured at pH 12, decreases in the order Sd>Sb>Sr, except at the end of the cook, when the average charge densities of the Sb and Sr samples are similar. Associations between lignin and carbohydrate fragments are proposed to cause the lower mobility of black liquor compared to isolated dissolved lignin. Characterisations performed at pH 10 indicate that the isolated dissolved lignin samples may have a higher pKa in the middle of the cook than the other samples.
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