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Oxidative sulfonation of kraft lignin

The effectiveness of two oxidative systems, sodium persulfate/ferric chloride and potassium permanganate, for the oxidative sulfonation of a softwood kraft lignin with sodium sulfite was compared. A large number of sulfonic acid groups were introduced with both systems. The content of aliphatic hydroxyl groups decreased and finally disappeared and the content of phenolic hydroxyl groups increased slightly during the sulfonation with both systems. The content of carboxylic groups increased when using sodium persulfate/ferric chloride as oxidant. The number of carboxylic groups remained constant when potassium permanganate was used as the oxidant. Interaction between the sodium salt of the sulfonated lignins and water was studied by differential scanning calorimetry (DSC) and isothermal sorption of water. In the DSc study, it was found that a highly concentrated aqueous solution of the sulfonated lignins characteristically behaved in a similar way to that of other polyelectrolytes. The sorption of water by the sulfonated samples was also investigated. The shape of the sorption isotherms was typical for hydrophilic polymers having a multilayer adsorption behaviour. The Brunauer, Emmet and Teller relation was used to calculate the adsorption of a monolayer of water on the sulfonated samples. From the comparison of the amount of monolayer water obtained using the BET equation with the number of water molecules bound to hydrophilic groups calculated using DSc results, it was concluded that only some of the hydrophilic groups interact with water under the experimental conditions of isothermal sorption.