Lignin: Functional Biomaterial with Potential in Surface Chemistry and Nanoscience - The Nanoscience and Technology of Renewable Biomaterials - page 186

500 nm

0.2 µ m

Figure7.7 To the left;Cryo-TEMof fractal clusterof kraft lignin(reprintedwithpermission
from Norgren etal. 2002. Copyright (2002), American Chemical Society). To the right;
self-aggregatedgoldcolloids (Weitzetal. 1987. ReprintedwithpermissionfromJohnWiley
&Sons, Inc.).

during KL aggregation, as can be viewed in Figure 7.7 (left). The resemblance between
the lignin aggregate to the left and the fractal gold colloid cluster to the right is striking.
In colloid science, the analysis of mass fractal dimensions of aggregates has shown to
be a good discriminator between different aggregation processes. For DLCA, the mass
fractal dimension often is found to be around d f

1 . 8, while in the case of RLCA
aggregate d f is usually situated around 2.1 (Weitz et al . 1991). Much concern has
been devoted to RLCA, due to the existence of a stability threshold in this regime.
The DLVO-theory, which divides the interaction forces into one attractive part (van
der Waals forces) and one repulsive part (the Columb forces), has been a great source
of understanding RLCA (Reerink and Overbeek 1954, Evans and Wennerstr om 1994).
Additional stabilising effects such as steric stabilisation might however also be attributed
(Napper 1983). Electrosteric stabilisation, which is a combination of both electrostatic
and configurational entropic repulsive forces between colloidal particles, gives sometimes
an explanation of why a colloidal dispersion still is stable at high ionic strengths and
elevated temperatures.
By fitting data from Figure 7.5 to R ∝ t − d f ,where R is the cluster radius at time t ,
the fractal dimensions, d f , of the clusters are obtained (Hoekstra et al . 1992). Figure 7.8
shows a plot of d f as a function of the W -ratio. According to Kim and Berg (2000), the
outcome suggests that it is reasonable to assume that the W -ratio can be thought as the
common denominator for fractal aggregation in KL systems.
Due to the chemical and physical heterogeneity of KL, self-aggregation in KL systems
is complex. The presence of larger (KL) macromolecules is proposed to determine the
onset (Leubner 2000). Depending on the KL sample composition, nuclei may either be
present from the beginning or are being formed due to changes in the solution conditions.
Figure 7.9 illustrates the probable modes of KL aggregation as proposed by Norgren
et al . (2002).

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