Hemicelluloses at Interfaces: Some Aspects of the Interactions - The Nanoscience and Technology of Renewable Biomaterials - page 167

as reported by Osterberg et al . (2001) and Claesson et al . (1995) for xylan-coated
surfaces. In our force measurements (Paananen 2007) no clear trend in the interaction
with increasing electrolyte concentration was observed (data not shown). This indicates
domination of steric forces, although the presence of electrostatic interaction cannot be
excluded. This is contradictory to the reported results by Claesson et al . (1995) and
Osterberg et al . (2001), where adsorption of xylan on mica and interaction between
xylan-coated mica surfaces has been investigated. The observed dominance of steric
or electrosteric repulsion in our results could partially be explained by the material
differences between cellulose and mica. The anionic charge of the cellulose surface
is relatively low and the the RMS roughness was approximately 30 nm whereas the
roughness of the highly charged anionic mica surface is less than 0.3 nm. As a result,
the adsorption of xylan on cellulose most likely differs from that on mica.
Claesson et al . (1995) explained the presence of steric forces by the existence of
long dangling tails in the adsorbed xylan layers due to prolonged times at elevated pH.
On the other hand, increase in electrolyte concentration decreases the steric interac-
tion, because the charges of the polymer molecules are screened out resulting in more
compact conformation of the molecules ( Osterberg et al . 2001). However, this trend
was not observed by our force and adsorption measurements and it seemed that elec-
trostatics had no strong effect on detected behavior. In order to further explain the
xylan-cellulose interactions the adsorbed xylan films were modeled with the Voigt-based
model. The Voigt model estimated relatively weakly bound xylan film on cellulose at
low ionic strength (Figure 6.5) and at high ionic strength the Voigt model failed. The
reason for the behavior detected may be the moderately limited solubility of the xylan
molecules. In solution xylan molecules probably take relatively coiled conformation and
polymer-polymer contacts are presumably more preferable than polymer-solution con-
tacts. At high ionic strength when the solubility is even more impaired, the xylan chains
may form soluble clusters which adsorbs on cellulose forming patches. The formation of
xylan clusters was supported by the AFM images in Figure 6.3. In the figure the xylan
is unevenly distributed as globular structures on the cellulose. This might be the reason
why the Voigt model could not estimate the xylan layer properties. The model fails if
the adsorbing material is not evenly distributed on the crystal surface as has been found
earlier when the Voigt model was used in an attempt to estimate the viscoelastic prop-
erties of extractive colloids on cellulose surface (unpublished results). Xylan assembly
studies on cellulose fibers (Linder et al . 2003) and on model cellulose (Henriksson and
Gatenholm 2001) suggest that xylan forms particle shaped, globular structures on these
surfaces in agreement with our observations. These investigations also suggest that the
assembly process is influenced by changes in xylan solubility and the affinities between
xylan and cellulose.
The magnitude of the adhesion between xylan coated cellulose surfaces was low in all
electrolyte concentrations (see Paananen 2007). The results showed, though, that repul-
sion was seen in the separation curves more often with the highest than with the lowest
electrolyte concentration. Adhesion between surfaces may originate from interpenetra-
tion of xylan chains from one surface to the other. When the electrolyte concentration is
increased, there are less protruding chains for interpenetration. In addition to this, when
xylan-coated surfaces are pushed together, more segments of xylan are forced to adsorb