DISTRIBUTION OF CHARGE IN WOOD PULPS

An analysis of titration curves obtained for synthetic ion-exchange resins, where the location of acid groups is therefore known, has shown that discontinuity points in the titration curve and/or in its first derivative are obtained when most of the acid groups are not on the surface but in the bulk of the resin beads. The intrinsic charge on wood pulp fibers, originating from the ionization of carboxylic acid groups during an alkalimetric titration, has most often been associated with surface charge. Titration curves with discontinuity points obtained for samples of a thermomechanical wood pulp have raised the question of whether the distribution of charge in wood pulps might have a mixture of contributions of bulk and surface charge. Specific surface areas determined by negative adsorption of a set of wood pulps have supported the concept that, for any wood pulp, bulk charge corresponds to a significant part of the total charge detected. Furthermore, for a much studied wood pulp, after the conversion of all carboxylic acid groups at the surface into carboxylate groups by ion exchange with concentrated sodium chloride solutions, it was found that this wood pulp still contained 30% of the initial acid content.