Lignin. 19. Kraft Lignin Component Conformation and Associated Complex Configuration in Aqueous Alkaline Solution

Discrete kraft lignin components behave ideally when eluted with 0.10 M aqueous NaOH from a cross-linked dextran gel: fractions with apparent polydispersity indices of 1.04 ± 0.01 are described by elution profiles that are close to a Gaussian shape. Of those selected for the present study, the weight-average molecular weights (M̄_w) in aqueous solution at pH 9.5, ionic strength 0.10 M, were found by ultracentrifuge sedimentation equilibrium to vary between 1680 and ∼ 140 000. On the basis of a logarithmic-normal component distribution, the polydispersities of the fractions during chromatographic elution have been compared with those observed under the conditions imposed for the absolute molecular weight determinations. Below the excluded limit for the gel, the plot of log M̄_w vs. column retention volume is linear and parallel to that for poly(styrenesulfonates). The kraft lignin components adopt an appreciably expanded random coil conformation in aqueous alkaline solution; there is no indication of a hydrodynamic effect arising from long-chain branching. The hydrodynamic behavior, on the other hand, of associated kraft lignin complexes is in accord with a flexible lamellar configuration, which is distinct from the conformation of the discrete components.