Handbook of Size Exclusion Chromatography and Related ...

liquor is highest throughout the cooking process. The relative order of mav is then
black liquor .dissolved lignin ¼residual lignin.
Kraft lignin fractions leached from asoftwood pulp and fractionated by
ultrafiltration (89) were characterized with respect to phenolic group content,
MMDs, and self-diffusion coefficients. The self-diffusion coefficients obtained
from the 1H-pulsed field gradient (PFG) NMR self-diffusion measurements and
SEC analyses of the fractions were seen to correlate fairly well. From the selfdiffusion
measurements, the mass-weighted median hydrodynamic radii of the
diffusants in the fractions were calculated assuming spherical fragments.
Furthermore it was seen that the content of phenolic groups in the fractions
decreasedbyincreasinghydrodynamicradiusandMM,butthecalculatedmedian
surface charge densities of the macromolecules were in the range of oligomers of
phenylpropane units up to at least 65 structural units (Fig. 6).
The dissociation of phenolic groups in a polydisperse, low MM kraft
lignin (Indulin AT) was studied in alkaline aqueous solutions in the temperature
interval 21–708C, by a UV-spectrophotometric method (90). At a constant
concentration of OH ions, the degree of dissociation decreased when the
temperaturewas elevated. Dissociation curves and apparent pK 0values were also
calculated for the polydisperse sample at the same conditions, using the van’t
Hoff and the Poisson–Boltzmann equations. At dissociation degrees exceeding
approximately 0.4, the outcome of the theoretical approach was shown to be in
good agreement with the experimentally obtained results. Calculations were
Figure 5 Electropherogramofunderivatizeddissolvedlignin.Thesamplewasseparated
at pH 10.0 and 12.0. The increased mobility at pH 12.0 is due to an increased number
of ionized phenolic groups. (From Ref. 87.)
© 2004 by Marcel Dekker, Inc.