Lignin: Functional Biomaterial with Potential in Surface Chemistry and Nanoscience - The Nanoscience and Technology of Renewable Biomaterials - page 193

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Figure 7.13 Poor solvent interaction of a synthetic lignin polymer as probed using single
moleculeforcespectroscopy.

of what may be expected for a single linear polymer chain. Kraft lignin films were
also investigated and found to have a radius of nearly 20 A which indicates its highly
branched nature.
The interaction of lignin with other materials has also been studied extensively. One
example is the surface forces between lignin and cellulose (Notley and Norgren 2006).
In this study, the measured potential energy of interaction was investigated as a func-
tion of the aqueous solution conditions. Softwood kraft lignin surface were used which
allowed the influence of a broad range of pH and ionic strength on the surface poten-
tial to be probed due to the stability of the films. The surface potential and forces of
interaction with cellulose were determined using the colloidal probe microscopy tech-
nique. In the pH range of 3.5 to 9 and for ionic strengths up to 0.01 M, the forces
could be well fit using the DLVO theory between the limits of constant charge and
constant potential as shown in Figure 7.14 (Deryagin and Landau 1941, Verwey and
Overbeek 1948, Chan and Horn 1985). As this theory could be applied to the experi-
mental data, the lignin films must behave similarly to solid state surface. Furthermore,
as a function of solution conditions, the surface potential of the kraft lignin films could
be determined. The surface potential increased as a function of pH, which may be
expected, through the successive ionisation of the carboxyl groups followed subse-
quently by dissociation of the phenolic functional groups on the lignin polymer. At

pH 8.5, the measured surface potential of the lignin films was 75 mV which corre-

sponds to a relative charge per area of 1 charge per 67 nm 2 . At pH greater than 9.5,
where the kraft lignin becomes significantly charged resulting in reduced stability of the

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