Lignin: Functional Biomaterial with Potential in Surface Chemistry and Nanoscience - The Nanoscience and Technology of Renewable Biomaterials - page 191

7.5.2 Use of Lignin Thin Films for the Investigation of Surface

Chemical Properties

Once lignin thin films can be prepared in a reproducible fashion, the possibility of
studying the physical and chemical properties of this material can be studied on a fun-
damental basis. Thus, a lignin surface which is continuous and smooth over a large area
lends itself to use in many surface specific analytical techniques and as such, a range of
interactions may be probed. Such studies have included the interaction of lignin with
other wood polymers such as cellulose as well as with other materials such as poly-
electrolytes, complexes and inorganic particles. As it has only been recently that such
well-defined surfaces have been prepared, the surface chemistry of lignin is still yet to
be fully investigated. Some of the recent studies are summarised below.
A key property of lignin that has been quantified by the preparation of lignin films
is its surface energy and wettability. The surface energy of lignin is highly dependent
on a number of factors, not least the method of evaluation, the isolation of the polymer
from the wood, the tree species and the surface preparation. Lee and Luner (1972)
studied six different lignin preparations and observed no significant differences in their
wetting characteristics. Recently, Notley and Norgren (manuscript in preparation) have
undertaken a similar study investigating the differences in the surface energy components
between kraft lignin films and milled wood lignin films determined by measuring contact
angles with test liquids of varying polar and dispersive components according to the
method of Fowkes. Table 7.5 shows that while there is not a significant variation
between the samples in terms of the total surface energy, the polar contribution is much
greater for the kraft lignin films. This is expected as the kraft pulping process is known
to introduce a large amount of polar functional groups such as carboxyl groups through
cleavage of the β -O-4 ether linkages.
Understanding the surface energy of lignin has a number of important implications,
particularly to the pulp and paper industry. A wood fibre that has a surface chemistry
rich in lignin will have a significantly different surface energy and hence wettability with
water to one that is predominantly cellulose thus having a dramatic effect on the devel-
opment of capillary forces during the drying and consolidation phases of paper-making
even though the total surface energies of lignin and cellulose are similar. The data for the
Figure 7.12 shows the contact angle that water makes with both the softwood kraft lignin
and softwood MWL films. It has long been suggested that one of the major functions
of lignin in the plant cell wall is to aid in waterproofing. However, for samples tested
in these studies, the contact angle is significantly less than 90 ◦ indicating that lignin is
far from being classified as hydrophobic. This fact is exploited by some plant species

Table7.5 Surfaceenergy,includingpolaranddispersivecomponents,ofmodel lignin
films.
Energy (mJm − 2 ) REAX 31 ∗

Softwood kraft Softwood MWL Hardwood MWL

γ T 57.1 58.8 57.0 52.5

γ d 33.7 44.5 43.9 43.5

γ p 23.4 14.3 13.1 9.0

∗ Data taken from Lee and Luner (1972).