Lignin. 20. Associative Interactions Between Kraft Lignin Components

The profiles described by eluting gymnosperm kraft lignins from dextran gels with 0.10 M aqueous NaOH represent effective molecular weight distributions that approach those for the discrete components. Under aqueous conditions between pH 13 and 14, dissociation of kraft lignin complexes occurs in dilute solution (~0.5 g L^-1), while a marked tendency for kraft lignin components to associate prevails at higher concentrations (~20-150 g L^-1). These processes are, furthermore, reversible. The apparent molecular weight distributions of kraft lignins are affected by the presence of counterions and zwitterions in alkaline solution and are remarkably sensitive to the method employed for their isolation. The relative ratios of kraft lignin components with molecular weights below 3500 do not, however, vary with the degree of association for the sample as a whole. The relationship between the overall weight-average and number-average molecular weights indicates that the ensemble average product of the molecular weights of interacting species remains constant during association in aqueous alkaline solution. This implies that the associative processes occurring within kraft lignin samples are stoichiometrically constrained: each of the associated complexes possesses a locus which is respectively complementary to only one type of component.