Lignin. 18. Associative Effects among Organosolv Lignin Components

Organosolv lignins isolated under relatively mild conditions from angiosperms are composed of quite low molecular weight entities. The weight-average and number-average molecular weights of the preparations generally lie within the ranges M^w ~ 2000-5000 and Mⁿ ~ 800-1200. Varying degrees of association between the constituent species are characteristic of these samples both in solution and in the solid state. Profiles described by eluting Organosolv lignins from dextran gels with 0.10 M aqueous NaOH represent effective molecular weight distributions which approach those of the discrete components. Under aqueous conditions of pH 13-14, dissociation of Organosolv lignin species occurs in dilute solutions (~0.5 g L^-1), while a marked tendency to associate prevails at higher concentrations (~100 g L^-1). The extent to which an individual component may participate in association depends appreciably upon the relative proportions of the other species present. Within the bounds of this constraint, the association-dissociation process is reversible and involves at least two kinetically distinguishable steps. Certain counterions and zwitterions reduce the degree of association between Organosolv lignin components. The mechanism may involve the blocking of proton uptake from solution which presumably accompanies the formation of higher molecular weight associated complexes. The relationship between M^w and Mⁿ for Organosolv lignin samples during association implies that the process is dominated by preferential interactions between lower and higher molecular weight components.