Handbook of Size Exclusion Chromatography and Related ...

values for lignins that are soluble in suitable eluents. However, association
phenomenabetweenligninmoleculesinorganicsolventsmakeanyresultdifficult
toevaluateiftheassociationphenomenaarenotconsidered.Thealkalineaqueous
solvent in which most lignins are soluble is difficult to use with some packing
materialsandsophisticateddetectors.Inorder tobetterunderstandthebehaviorof
lignin molecules with different structures and the interaction between lignin
molecules and between lignin molecules and solvent (eluent), abetter knowledge
about physicochemical phenomena should be obtained.
Detailed investigations of kraft lignins in alkali solutions have been
investigated by Sarkanen and colleagues (6,85). The association–dissociation
behaviorofligninmoleculeswasinvestigatedbyprecipitationofligninatdifferent
pH values and also by isolating paucidisperse fractions 0.1 M NaOH elution
profiles and determining the weight-average MM (Mw) of the fractions. At the
sametime physico-chemicalpropertiesofthelignin molecules inthesystem were
investigated using light-scattering measurements. Pulse-field-gradient NMR has
been used in order to obtain detailed information about the shape of lignin
molecules(13).Fromintrinsicviscositymeasurementsthesizeandshapeoflignin
molecules can be estimated. The coefficients in the Kuhn–Mark–Houwink–
Sakurada (KMHS) equations relate intrinsic viscosity data to the shape of
molecules in different solvents and were determined based on the absolute molar
mass determined with low-angle laser light scattering (86). The KMHS
exponential factors of kraft lignin were found to be 0.11, 0.13, and 0.23 in
DMFat318.2 KinDMFat350.7 K,andin0.5 MNaOHat302 K,respectively.It
was seen that the lignin molecules in solution were approximately spherical
particles and slightly solvated with solvent.
Anew method to characterize underivatized lignins in aqueous solutions is
capillary zone electrophoresis (CZE), by which separation is achieved due to
differencesinmobilities,whichdepend on differences inchargetomolecular size
ratios (87) (Fig. 5).
From flow-through kraft cooking of birch wood (88), a black liquor, an
isolated spent liquor lignin, and residual lignin from pulps obtained at different
cooking times were investigated by CZE. The average mobility (mav) of the lignincontaining
samples was determined. It was seen that the lignin samples had a
broad mobility distribution, which reflected the charge-to-size ratio of the
molecules. At pH 12, when lignin is completely dissociated, m av of each type of
sample increases during the cooking process, which is reflected as an increase in
charge density of the lignin. The lower charge density of black liquor compared to
dissolved lignin may be caused by association between lignin and carbohydrate
fragments dissolved in the black liquor. The decrease in mobility when lowering
the pH correlates with the degree of dissociation of the lignin phenol groups. At
pH 10, approximately the pKa of the phenolic groups in lignin, the mav of black
© 2004 by Marcel Dekker, Inc.